Instructions
WebAssembly code consists of sequences of instructions.
Its computational model is based on a stack machine in that instructions manipulate values on an implicit operand stack,
consuming (popping) argument values and producing or returning (pushing) result values.
In addition to dynamic operands from the stack, some instructions also have static immediate arguments,
typically indices or type annotations,
which are part of the instruction itself.
Some instructions are structured in that they contain nested sequences of instructions.
The following sections group instructions into a number of different categories.
The syntax of instruction is further extended with additional forms for the purpose of specifying execution.
Parametric Instructions
Instructions in this group can operate on operands of any value type.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}} & ::= & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{nop}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{unreachable}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-parametric}{\mathsf{drop}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-parametric}{\mathsf{select}}~{({{\href{../syntax/types.html#syntax-valtype}{\mathit{valtype}}}^\ast})^?} \\
& & | & \dots \\
\end{array}\end{split}\]
The \(\mathsf{nop}\) instruction does nothing.
The \(\mathsf{unreachable}\) instruction causes an unconditional trap.
The \(\mathsf{drop}\) instruction simply throws away a single operand.
The \(\mathsf{select}\) instruction selects one of its first two operands based on whether its third operand is zero or not.
It may include a value type determining the type of these operands. If missing, the operands must be of numeric type.
Note
In future versions of WebAssembly, the type annotation on \(\mathsf{select}\) may allow for more than a single value being selected at the same time.
Numeric Instructions
Numeric instructions provide basic operations over numeric values of specific type.
These operations closely match respective operations available in hardware.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}} & ::= & \mathsf{{\scriptstyle 8}} ~|~ \mathsf{{\scriptstyle 16}} ~|~ \mathsf{{\scriptstyle 32}} ~|~ \mathsf{{\scriptstyle 64}} \\
& {\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}} & ::= & \href{../syntax/instructions.html#syntax-sx}{\mathsf{u}} ~|~ \href{../syntax/instructions.html#syntax-sx}{\mathsf{s}} \\[0.8ex]
& {{\href{../exec/runtime.html#syntax-num}{\mathit{num}}}}_{{\mathsf{i}}{N}} & ::= & {{\href{../syntax/values.html#syntax-int}{\mathit{i}\kern-0.1em}}}{N} \\
& {{\href{../exec/runtime.html#syntax-num}{\mathit{num}}}}_{{\mathsf{f}}{N}} & ::= & {{\href{../syntax/values.html#syntax-float}{\mathit{f}\kern-0.15em}}}{N} \\[0.8ex]
& {\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}} & ::= & \dots \\
& & | & {\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}{.}\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{const}}~{{\href{../exec/runtime.html#syntax-num}{\mathit{num}}}}_{{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}} \\
& & | & {\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}} {.} {{\href{../syntax/instructions.html#syntax-unop}{\mathit{unop}}}}_{{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}} \\
& & | & {\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}} {.} {{\href{../syntax/instructions.html#syntax-binop}{\mathit{binop}}}}_{{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}} \\
& & | & {\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}} {.} {{\href{../syntax/instructions.html#syntax-testop}{\mathit{testop}}}}_{{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}} \\
& & | & {\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}} {.} {{\href{../syntax/instructions.html#syntax-relop}{\mathit{relop}}}}_{{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}} \\
& & | & {\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}_1 {.} {{{\href{../syntax/instructions.html#syntax-cvtop}{\mathit{cvtop}}}}_{{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}_2, {\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}_1}}{\mathsf{\_}}{{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}_2} \\
& & | & \dots \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-unop}{\mathit{unop}}}}_{{\mathsf{i}}{N}} & ::= & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{clz}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{ctz}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{popcnt}} ~|~ {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{extend}}}{{\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}}}{\mathsf{\_}}{\href{../syntax/instructions.html#syntax-sx}{\mathsf{s}}} & \quad \mbox{if}~ {\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}} < N \\
& {{\href{../syntax/instructions.html#syntax-unop}{\mathit{unop}}}}_{{\mathsf{f}}{N}} & ::= & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{abs}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{neg}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{sqrt}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{ceil}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{floor}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{trunc}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{nearest}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-binop}{\mathit{binop}}}}_{{\mathsf{i}}{N}} & ::= & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{add}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{sub}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{mul}} ~|~ {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{div}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} ~|~ {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{rem}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{and}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{or}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{xor}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{shl}} ~|~ {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{shr}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{rotl}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{rotr}} \\
& {{\href{../syntax/instructions.html#syntax-binop}{\mathit{binop}}}}_{{\mathsf{f}}{N}} & ::= & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{add}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{sub}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{mul}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{div}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{min}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{max}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{copysign}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-testop}{\mathit{testop}}}}_{{\mathsf{i}}{N}} & ::= & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{eqz}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-relop}{\mathit{relop}}}}_{{\mathsf{i}}{N}} & ::= & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{eq}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{ne}} ~|~ {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{lt}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} ~|~ {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{gt}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} ~|~ {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{le}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} ~|~ {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{ge}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} \\
& {{\href{../syntax/instructions.html#syntax-relop}{\mathit{relop}}}}_{{\mathsf{f}}{N}} & ::= & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{eq}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{ne}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{lt}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{gt}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{le}} ~|~ \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{ge}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-cvtop}{\mathit{cvtop}}}}_{{{\mathsf{i}}{N}}_1, {{\mathsf{i}}{N}}_2} & ::= & {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{extend}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N_1 < N_2 \\
& & | & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{wrap}} & \quad \mbox{if}~ N_1 > N_2 \\
& {{\href{../syntax/instructions.html#syntax-cvtop}{\mathit{cvtop}}}}_{{{\mathsf{i}}{N}}_1, {{\mathsf{f}}{N}}_2} & ::= & {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{convert}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{reinterpret}} & \quad \mbox{if}~ N_1 = N_2 \\
& {{\href{../syntax/instructions.html#syntax-cvtop}{\mathit{cvtop}}}}_{{{\mathsf{f}}{N}}_1, {{\mathsf{i}}{N}}_2} & ::= & {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{trunc}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{trunc\_sat}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{reinterpret}} & \quad \mbox{if}~ N_1 = N_2 \\
& {{\href{../syntax/instructions.html#syntax-cvtop}{\mathit{cvtop}}}}_{{{\mathsf{f}}{N}}_1, {{\mathsf{f}}{N}}_2} & ::= & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{promote}} & \quad \mbox{if}~ N_1 < N_2 \\
& & | & \href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{demote}} & \quad \mbox{if}~ N_1 > N_2 \\
\end{array}\end{split}\]
Numeric instructions are divided by number type.
For each type, several subcategories can be distinguished:
Constants: return a static constant.
Unary Operations: consume one operand and produce one result of the respective type.
Binary Operations: consume two operands and produce one result of the respective type.
Tests: consume one operand of the respective type and produce a Boolean integer result.
Comparisons: consume two operands of the respective type and produce a Boolean integer result.
Conversions: consume a value of one type and produce a result of another
(the source type of the conversion is the one after the “\(\mathsf{\_}\)”).
Some integer instructions come in two flavors,
where a signedness annotation \({\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}\) distinguishes whether the operands are to be interpreted as unsigned or signed integers.
For the other integer instructions, the use of two’s complement for the signed interpretation means that they behave the same regardless of signedness.
Vector Instructions
Vector instructions (also known as SIMD instructions, single instruction multiple data) provide basic operations over values of vector type.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-lanetype}{\mathit{lanetype}}} & ::= & {\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}} ~|~ {\href{../syntax/types.html#syntax-packtype}{\mathit{packtype}}} \\
& {\href{../syntax/instructions.html#syntax-dim}{\mathit{dim}}} & ::= & \mathsf{{\scriptstyle 1}} ~|~ \mathsf{{\scriptstyle 2}} ~|~ \mathsf{{\scriptstyle 4}} ~|~ \mathsf{{\scriptstyle 8}} ~|~ \mathsf{{\scriptstyle 16}} \\
& {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}} & ::= & {{\href{../syntax/instructions.html#syntax-lanetype}{\mathit{lanetype}}}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{{\href{../syntax/instructions.html#syntax-dim}{\mathit{dim}}}} \\
& {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}} & ::= & {{\mathsf{i}}{N}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{{\href{../syntax/instructions.html#syntax-dim}{\mathit{dim}}}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-half}{\mathit{half}}}}_{{{{\mathsf{i}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{i}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{low}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{high}} & \quad \mbox{if}~ 2 \cdot N_1 = N_1 \\
& {{\href{../syntax/instructions.html#syntax-half}{\mathit{half}}}}_{{{{\mathsf{i}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{f}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{low}} & \quad \mbox{if}~ 2 \cdot N_1 = N_1 = \mathsf{{\scriptstyle 64}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-zero}{\mathit{zero}}}}_{{{{\mathsf{f}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{i}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{zero}} & \quad \mbox{if}~ 2 \cdot N_2 = N_1 = \mathsf{{\scriptstyle 64}} \\[0.8ex]
& {\href{../syntax/instructions.html#syntax-laneidx}{\mathit{laneidx}}} & ::= & {\href{../syntax/values.html#syntax-int}{\mathit{u\scriptstyle\kern-0.1em8}}} \\[0.8ex]
& {\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}} & ::= & \dots \\
& & | & {\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}}{.}\href{../syntax/instructions.html#syntax-instr-numeric}{\mathsf{const}}~{{\href{../exec/runtime.html#syntax-vec}{\mathit{vec}}}}_{{\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}}} \\
& & | & {\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}} {.} {\href{../syntax/instructions.html#syntax-vvunop}{\mathit{vvunop}}} \\
& & | & {\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}} {.} {\href{../syntax/instructions.html#syntax-vvbinop}{\mathit{vvbinop}}} \\
& & | & {\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}} {.} {\href{../syntax/instructions.html#syntax-vvternop}{\mathit{vvternop}}} \\
& & | & {\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}} {.} {\href{../syntax/instructions.html#syntax-vvtestop}{\mathit{vvtestop}}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}} {.} {{\href{../syntax/instructions.html#syntax-vunop}{\mathit{vunop}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}} {.} {{\href{../syntax/instructions.html#syntax-vbinop}{\mathit{vbinop}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}} {.} {{\href{../syntax/instructions.html#syntax-vrelop}{\mathit{vtestop}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}} {.} {{\href{../syntax/instructions.html#syntax-vrelop}{\mathit{vrelop}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}} {.} {{\href{../syntax/instructions.html#syntax-vshiftop}{\mathit{vshiftop}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}{.}\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{bitmask}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}{.}\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{swizzle}} & \quad \mbox{if}~ {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}} = {\href{../syntax/types.html#syntax-storagetype}{\mathsf{i\scriptstyle8}}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{\mathsf{{\scriptstyle 16}}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}{.}\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{shuffle}}~{{\href{../syntax/instructions.html#syntax-laneidx}{\mathit{laneidx}}}^\ast} & \quad \mbox{if}~ {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}} = {\href{../syntax/types.html#syntax-storagetype}{\mathsf{i\scriptstyle8}}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{\mathsf{{\scriptstyle 16}}} \land {|{{\href{../syntax/instructions.html#syntax-laneidx}{\mathit{laneidx}}}^\ast}|} = \mathsf{{\scriptstyle 16}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_1 {.} {{{\href{../syntax/instructions.html#syntax-vextunop}{\mathit{vextunop}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_2, {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_1}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_2} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_1 {.} {{{\href{../syntax/instructions.html#syntax-vextbinop}{\mathit{vextbinop}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_2, {\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_1}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_2} \\
& & | & {{\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_1{.}\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{narrow}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_2}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ {|{\href{../syntax/instructions.html#aux-lanetype}{\mathrm{lanetype}}}({\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_2)|} = 2 \cdot {|{\href{../syntax/instructions.html#aux-lanetype}{\mathrm{lanetype}}}({\href{../syntax/instructions.html#syntax-shape}{\mathit{ishape}}}_1)|} \leq \mathsf{{\scriptstyle 32}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_1 {.} {{{\href{../syntax/instructions.html#syntax-vcvtop}{\mathit{vcvtop}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_2, {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_1}}{\mathsf{\_}}{{{{\href{../syntax/instructions.html#syntax-zero}{\mathit{zero}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_2, {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_1}^?}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_2}{\mathsf{\_}}{{{{\href{../syntax/instructions.html#syntax-half}{\mathit{half}}}}_{{\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_2, {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_1}^?}} & \quad \mbox{if}~ {\href{../syntax/instructions.html#aux-lanetype}{\mathrm{lanetype}}}({\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_1) \neq {\href{../syntax/instructions.html#aux-lanetype}{\mathrm{lanetype}}}({\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}_2) \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}{.}\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{splat}} \\
& & | & {{\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}{.}\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{extract\_lane}}}{\mathsf{\_}}{{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}^?}}~{\href{../syntax/instructions.html#syntax-laneidx}{\mathit{laneidx}}} & \quad \mbox{if}~ {{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}^?} = \epsilon \Leftrightarrow {\href{../syntax/instructions.html#aux-lanetype}{\mathrm{lanetype}}}({\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}) = {\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}} \\
& & | & {\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}}{.}\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{replace\_lane}}~{\href{../syntax/instructions.html#syntax-laneidx}{\mathit{laneidx}}} \\
& & | & \dots \\
\end{array}\end{split}\]
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-vvunop}{\mathit{vvunop}}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{not}} \\[0.8ex]
& {\href{../syntax/instructions.html#syntax-vvbinop}{\mathit{vvbinop}}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{and}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{andnot}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{or}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{xor}} \\[0.8ex]
& {\href{../syntax/instructions.html#syntax-vvternop}{\mathit{vvternop}}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{bitselect}} \\[0.8ex]
& {\href{../syntax/instructions.html#syntax-vvtestop}{\mathit{vvtestop}}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{any\_true}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-vunop}{\mathit{vunop}}}}_{{{\mathsf{i}}{N}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{abs}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{neg}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{popcnt}} & \quad \mbox{if}~ N = \mathsf{{\scriptstyle 8}} \\
& {{\href{../syntax/instructions.html#syntax-vunop}{\mathit{vunop}}}}_{{{\mathsf{f}}{N}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{abs}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{neg}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{sqrt}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{ceil}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{floor}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{trunc}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{nearest}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-vbinop}{\mathit{vbinop}}}}_{{{\mathsf{i}}{N}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{add}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{sub}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{add\_sat}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N \leq \mathsf{{\scriptstyle 16}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{sub\_sat}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N \leq \mathsf{{\scriptstyle 16}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{mul}} & \quad \mbox{if}~ N \geq \mathsf{{\scriptstyle 16}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{avgr}}}{\mathsf{\_}}{\href{../syntax/instructions.html#syntax-sx}{\mathsf{u}}} & \quad \mbox{if}~ N \leq \mathsf{{\scriptstyle 16}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{q{\scriptstyle15}mulr\_sat}}}{\mathsf{\_}}{\href{../syntax/instructions.html#syntax-sx}{\mathsf{s}}} & \quad \mbox{if}~ N = \mathsf{{\scriptstyle 16}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{min}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N \leq \mathsf{{\scriptstyle 32}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{max}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N \leq \mathsf{{\scriptstyle 32}} \\
& {{\href{../syntax/instructions.html#syntax-vbinop}{\mathit{vbinop}}}}_{{{\mathsf{f}}{N}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{add}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{sub}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{mul}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{div}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{min}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{max}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{pmin}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{pmax}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-vrelop}{\mathit{vtestop}}}}_{{{\mathsf{i}}{N}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{all\_true}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-vrelop}{\mathit{vrelop}}}}_{{{\mathsf{i}}{N}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{eq}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{ne}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{lt}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N \neq \mathsf{{\scriptstyle 64}} \lor {\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}} = \href{../syntax/instructions.html#syntax-sx}{\mathsf{s}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{gt}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N \neq \mathsf{{\scriptstyle 64}} \lor {\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}} = \href{../syntax/instructions.html#syntax-sx}{\mathsf{s}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{le}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N \neq \mathsf{{\scriptstyle 64}} \lor {\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}} = \href{../syntax/instructions.html#syntax-sx}{\mathsf{s}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{ge}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N \neq \mathsf{{\scriptstyle 64}} \lor {\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}} = \href{../syntax/instructions.html#syntax-sx}{\mathsf{s}} \\
& {{\href{../syntax/instructions.html#syntax-vrelop}{\mathit{vrelop}}}}_{{{\mathsf{f}}{N}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{eq}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{ne}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{lt}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{gt}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{le}} ~|~ \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{ge}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-vshiftop}{\mathit{vshiftop}}}}_{{{\mathsf{i}}{N}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{shl}} ~|~ {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{shr}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-vextunop}{\mathit{vextunop}}}}_{{{{\mathsf{i}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{i}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}} & ::= & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{extadd\_pairwise}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ 16 \leq 2 \cdot N_1 = N_2 \leq \mathsf{{\scriptstyle 32}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-vextbinop}{\mathit{vextbinop}}}}_{{{{\mathsf{i}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{i}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}} & ::= & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{extmul}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}}{\mathsf{\_}}{{{\href{../syntax/instructions.html#syntax-half}{\mathit{half}}}}_{{{{\mathsf{i}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{i}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}}} & \quad \mbox{if}~ 2 \cdot N_1 = N_2 \geq \mathsf{{\scriptstyle 16}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{dot}}}{\mathsf{\_}}{\href{../syntax/instructions.html#syntax-sx}{\mathsf{s}}} & \quad \mbox{if}~ 2 \cdot N_1 = N_2 = \mathsf{{\scriptstyle 32}} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-vcvtop}{\mathit{vcvtop}}}}_{{{{\mathsf{i}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{i}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}} & ::= & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{extend}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N_2 = 2 \cdot N_1 \\
& {{\href{../syntax/instructions.html#syntax-vcvtop}{\mathit{vcvtop}}}}_{{{{\mathsf{i}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{f}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}} & ::= & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{convert}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N_2 \geq N_1 = \mathsf{{\scriptstyle 32}} \\
& {{\href{../syntax/instructions.html#syntax-vcvtop}{\mathit{vcvtop}}}}_{{{{\mathsf{f}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{i}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}} & ::= & {\href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{trunc\_sat}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ N_1 \geq N_2 = \mathsf{{\scriptstyle 32}} \\
& {{\href{../syntax/instructions.html#syntax-vcvtop}{\mathit{vcvtop}}}}_{{{{\mathsf{f}}{N}}_1}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_1}, {{{\mathsf{f}}{N}}_2}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M_2}} & ::= & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{demote}} & \quad \mbox{if}~ N_1 > N_2 \\
& & | & \href{../syntax/instructions.html#syntax-instr-vec}{\mathsf{promote}} & \quad \mbox{if}~ N_1 < N_2 \\
\end{array}\end{split}\]
Vector instructions have a naming convention involving a shape prefix that
determines how their operands will be interpreted,
written \({t}{\mathsf{x}}{N}\), and consisting of a lane type \(t\), a possibly packed numeric type, and the number of lanes \(N\) of that type.
Operations are performed point-wise on the values of each lane.
Note
For example, the shape \({\href{../syntax/types.html#syntax-numtype}{\mathsf{i\scriptstyle32}}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{\mathsf{{\scriptstyle 4}}}\) interprets the operand
as four \({\mathit{i{\kern-0.1em\scriptstyle 32}}}\) values, packed into an \({\mathit{i{\kern-0.1em\scriptstyle 128}}}\).
The bit width of the lane type \(t\) times \(N\) always is \(128\).
Instructions prefixed with \(\mathsf{v{\scriptstyle 128}}\) do not involve a specific interpretation, and treat the \(\mathsf{v{\scriptstyle 128}}\) as either an \({\mathit{i{\kern-0.1em\scriptstyle 128}}}\) value or a vector of \(128\) individual bits.
Vector instructions can be grouped into several subcategories:
Constants: return a static constant.
Unary Operations: consume one \(\mathsf{v{\scriptstyle 128}}\) operand and produce one \(\mathsf{v{\scriptstyle 128}}\) result.
Binary Operations: consume two \(\mathsf{v{\scriptstyle 128}}\) operands and produce one \(\mathsf{v{\scriptstyle 128}}\) result.
Ternary Operations: consume three \(\mathsf{v{\scriptstyle 128}}\) operands and produce one \(\mathsf{v{\scriptstyle 128}}\) result.
Tests: consume one \(\mathsf{v{\scriptstyle 128}}\) operand and produce a Boolean integer result.
Shifts: consume a \(\mathsf{v{\scriptstyle 128}}\) operand and an \(\mathsf{i{\scriptstyle 32}}\) operand, producing one \(\mathsf{v{\scriptstyle 128}}\) result.
Splats: consume a value of numeric type and produce a \(\mathsf{v{\scriptstyle 128}}\) result of a specified shape.
Extract lanes: consume a \(\mathsf{v{\scriptstyle 128}}\) operand and return the numeric value in a given lane.
Replace lanes: consume a \(\mathsf{v{\scriptstyle 128}}\) operand and a numeric value for a given lane, and produce a \(\mathsf{v{\scriptstyle 128}}\) result.
Some vector instructions have a signedness annotation \({\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}\) which distinguishes whether the elements in the operands are to be interpreted as unsigned or signed integers.
For the other vector instructions, the use of two’s complement for the signed interpretation means that they behave the same regardless of signedness.
Conventions
The function \({\href{../syntax/instructions.html#aux-lanetype}{\mathrm{lanetype}}}({\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}})\) extracts the lane type of a shape.
The function \({\href{../syntax/instructions.html#aux-dim}{\mathrm{dim}}}({\href{../syntax/instructions.html#syntax-shape}{\mathit{shape}}})\) extracts the dimension of a shape.
Reference Instructions
Instructions in this group are concerned with accessing references.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}} & ::= & \dots \\
& & | & \href{../syntax/instructions.html#syntax-instr-ref}{\mathsf{ref{.}func}}~{\href{../syntax/modules.html#syntax-funcidx}{\mathit{funcidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-ref}{\mathsf{ref{.}null}}~{\href{../syntax/types.html#syntax-heaptype}{\mathit{heaptype}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-ref}{\mathsf{ref{.}is\_null}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-ref}{\mathsf{ref{.}as\_non\_null}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-ref}{\mathsf{ref{.}eq}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-ref}{\mathsf{ref{.}test}}~{\href{../syntax/types.html#syntax-reftype}{\mathit{reftype}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-ref}{\mathsf{ref{.}cast}}~{\href{../syntax/types.html#syntax-reftype}{\mathit{reftype}}} \\
& & | & \dots \\
\end{array}\end{split}\]
The \(\mathsf{ref{.}null}\) and \(\mathsf{ref{.}func}\) instructions produce a null value or a reference to a given function, respectively.
The instruction \(\mathsf{ref{.}is\_null}\) checks for null,
while \(\mathsf{ref{.}as\_non\_null}\) converts a nullable to a non-null one, and traps if it encounters null.
The \(\mathsf{ref{.}eq}\) compares two references.
The instructions \(\mathsf{ref{.}test}\) and \(\mathsf{ref{.}cast}\) test the dynamic type of a reference operand.
The former merely returns the result of the test,
while the latter performs a downcast and traps if the operand’s type does not match.
Note
The \(\mathsf{br\_on\_null}\) and \(\mathsf{br\_on\_non\_null}\) instructions provide versions of \(\mathsf{ref{.}as\_null}\) that branch depending on the success of failure of a null test instead of trapping.
Similarly, the \(\mathsf{br\_on\_cast}\) and \(\mathsf{br\_on\_cast\_fail}\) instructions provides versions of \(\mathsf{ref{.}cast}\) that branch depending on the success of the downcast instead of trapping.
An additional instruction operating on function references is the control instruction \(\mathsf{call\_ref}\).
Aggregate Instructions
Instructions in this group are concerned with creating and accessing references to aggregate types.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}} & ::= & \dots \\
& & | & \href{../syntax/instructions.html#syntax-instr-struct}{\mathsf{struct{.}new}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-struct}{\mathsf{struct{.}new\_default}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-struct}{\mathsf{struct{.}get}}}{\mathsf{\_}}{{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}^?}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}}~{\href{../syntax/values.html#syntax-int}{\mathit{u\scriptstyle\kern-0.1em32}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-struct}{\mathsf{struct{.}set}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}}~{\href{../syntax/values.html#syntax-int}{\mathit{u\scriptstyle\kern-0.1em32}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}new}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}new\_default}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}new\_fixed}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}}~{\href{../syntax/values.html#syntax-int}{\mathit{u\scriptstyle\kern-0.1em32}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}new\_data}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}}~{\href{../syntax/modules.html#syntax-dataidx}{\mathit{dataidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}new\_elem}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}}~{\href{../syntax/modules.html#syntax-elemidx}{\mathit{elemidx}}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}get}}}{\mathsf{\_}}{{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}^?}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}set}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}len}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}fill}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}copy}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}init\_data}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}}~{\href{../syntax/modules.html#syntax-dataidx}{\mathit{dataidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-array}{\mathsf{array{.}init\_elem}}~{\href{../syntax/modules.html#syntax-typeidx}{\mathit{typeidx}}}~{\href{../syntax/modules.html#syntax-elemidx}{\mathit{elemidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-i31}{\mathsf{ref{.}i\scriptstyle31}} \\
& & | & {\href{../syntax/instructions.html#syntax-instr-i31}{\mathsf{i{\scriptstyle31}{.}get}}}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-extern}{\mathsf{extern{.}convert\_any}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-extern}{\mathsf{any{.}convert\_extern}} \\
& & | & \dots \\
\end{array}\end{split}\]
The instructions \(\mathsf{struct{.}new}\) and \(\mathsf{struct{.}new\_default}\) allocate a new structure, initializing them either with operands or with default values.
The remaining instructions on structs access individual fields,
allowing for different sign extension modes in the case of packed storage types.
Similarly, arrays can be allocated either with an explicit initialization operand or a default value.
Furthermore, \(\mathsf{array{.}new\_fixed}\) allocates an array with statically fixed size,
and \(\mathsf{array{.}new\_data}\) and \(\mathsf{array{.}new\_elem}\) allocate an array and initialize it from a data or element segment, respectively.
The instructions \(\mathsf{array{.}get}\), \(\mathsf{array{.}get}~{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}\), and \(\mathsf{array{.}set}\) access individual slots,
again allowing for different sign extension modes in the case of a packed storage type;
\(\mathsf{array{.}len}\) produces the length of an array;
\(\mathsf{array{.}fill}\) fills a specified slice of an array with a given value and \(\mathsf{array{.}copy}\), \(\mathsf{array{.}init\_data}\), and \(\mathsf{array{.}init\_elem}\) copy elements to a specified slice of an array from a given array, data segment, or element segment, respectively.
The instructions \(\mathsf{ref{.}i{\scriptstyle 31}}\) and \(\mathsf{i{\scriptstyle 31}{.}get}~{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}\) convert between type \(\mathsf{i{\scriptstyle 32}}\) and an unboxed scalar.
The instructions \(\mathsf{any{.}convert\_extern}\) and \(\mathsf{extern{.}convert\_any}\) allow lossless conversion between references represented as type \((\href{../syntax/types.html#syntax-reftype}{\mathsf{ref}}~\href{../syntax/types.html#syntax-reftype}{\mathsf{null}}~\href{../syntax/types.html#syntax-heaptype}{\mathsf{extern}})\) and as \((\href{../syntax/types.html#syntax-reftype}{\mathsf{ref}}~\href{../syntax/types.html#syntax-reftype}{\mathsf{null}}~\href{../syntax/types.html#syntax-heaptype}{\mathsf{any}})\).
Variable Instructions
Variable instructions are concerned with access to local or global variables.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}} & ::= & \dots \\
& & | & \href{../syntax/instructions.html#syntax-instr-variable}{\mathsf{local{.}get}}~{\href{../syntax/modules.html#syntax-localidx}{\mathit{localidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-variable}{\mathsf{local{.}set}}~{\href{../syntax/modules.html#syntax-localidx}{\mathit{localidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-variable}{\mathsf{local{.}tee}}~{\href{../syntax/modules.html#syntax-localidx}{\mathit{localidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-variable}{\mathsf{global{.}get}}~{\href{../syntax/modules.html#syntax-globalidx}{\mathit{globalidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-variable}{\mathsf{global{.}set}}~{\href{../syntax/modules.html#syntax-globalidx}{\mathit{globalidx}}} \\
& & | & \dots \\
\end{array}\end{split}\]
These instructions get or set the values of respective variables.
The \(\mathsf{local{.}tee}\) instruction is like \(\mathsf{local{.}set}\) but also returns its argument.
Table Instructions
Instructions in this group are concerned with tables table.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}} & ::= & \dots \\
& & | & \href{../syntax/instructions.html#syntax-instr-table}{\mathsf{table{.}get}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-table}{\mathsf{table{.}set}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-table}{\mathsf{table{.}size}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-table}{\mathsf{table{.}grow}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-table}{\mathsf{table{.}fill}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-table}{\mathsf{table{.}copy}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-table}{\mathsf{table{.}init}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}}~{\href{../syntax/modules.html#syntax-elemidx}{\mathit{elemidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-table}{\mathsf{elem{.}drop}}~{\href{../syntax/modules.html#syntax-elemidx}{\mathit{elemidx}}} \\
& & | & \dots \\
\end{array}\end{split}\]
The \(\mathsf{table{.}get}\) and \(\mathsf{table{.}set}\) instructions load or store an element in a table, respectively.
The \(\mathsf{table{.}size}\) instruction returns the current size of a table.
The \(\mathsf{table{.}grow}\) instruction grows table by a given delta and returns the previous size, or \({-1}\) if enough space cannot be allocated.
It also takes an initialization value for the newly allocated entries.
The \(\mathsf{table{.}fill}\) instruction sets all entries in a range to a given value.
The \(\mathsf{table{.}copy}\) instruction copies elements from a source table region to a possibly overlapping destination region; the first index denotes the destination.
The \(\mathsf{table{.}init}\) instruction copies elements from a passive element segment into a table.
The \(\mathsf{elem{.}drop}\) instruction prevents further use of a passive element segment. This instruction is intended to be used as an optimization hint. After an element segment is dropped its elements can no longer be retrieved, so the memory used by this segment may be freed.
Note
An additional instruction that accesses a table is the control instruction \(\mathsf{call\_indirect}\).
Memory Instructions
Instructions in this group are concerned with linear memory.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-memarg}{\mathit{memarg}}} & ::= & \{ \begin{array}[t]{@{}l@{}l@{}}
\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{align}}~{\href{../syntax/values.html#syntax-int}{\mathit{u\scriptstyle\kern-0.1em32}}} \href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{offset}}~{\href{../syntax/values.html#syntax-int}{\mathit{u\scriptstyle\kern-0.1em32}}} \} \\
\end{array} \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-loadop}{\mathit{loadop}}}}_{{\mathsf{i}}{N}} & ::= & {\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}}~{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}} & \quad \mbox{if}~ {\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}} < N \\[0.8ex]
& {{\href{../syntax/instructions.html#syntax-vloadop}{\mathit{vloadop}}}}_{{\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}}} & ::= & {{\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}}}{\href{../syntax/instructions.html#syntax-shape}{\mathsf{x}}}{M}{\mathsf{\_}}{{\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}} & \quad \mbox{if}~ {\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}} \cdot M = {|{\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}}|} / 2 \\
& & | & {{\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}}}{\mathsf{\_}}{\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{splat}}} \\
& & | & {{\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}}}{\mathsf{\_}}{\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{zero}}} & \quad \mbox{if}~ {\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}} \geq \mathsf{{\scriptstyle 32}} \\[0.8ex]
& {\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}} & ::= & \dots \\
& & | & {{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}{.}\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{load}}}{{{{\href{../syntax/instructions.html#syntax-loadop}{\mathit{loadop}}}}_{{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}}^?}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}}~{\href{../syntax/instructions.html#syntax-memarg}{\mathit{memarg}}} \\
& & | & {{\href{../syntax/types.html#syntax-numtype}{\mathit{numtype}}}{.}\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{store}}}{{{\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}}^?}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}}~{\href{../syntax/instructions.html#syntax-memarg}{\mathit{memarg}}} \\
& & | & {{\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}}{.}\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{load}}}{{{{\href{../syntax/instructions.html#syntax-vloadop}{\mathit{vloadop}}}}_{{\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}}}^?}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}}~{\href{../syntax/instructions.html#syntax-memarg}{\mathit{memarg}}} \\
& & | & {{\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}}{.}\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{load}}}{{\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}}}{\mathsf{\_}}{\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{lane}}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}}~{\href{../syntax/instructions.html#syntax-memarg}{\mathit{memarg}}}~{\href{../syntax/instructions.html#syntax-laneidx}{\mathit{laneidx}}} \\
& & | & {\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}}{.}\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{store}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}}~{\href{../syntax/instructions.html#syntax-memarg}{\mathit{memarg}}} \\
& & | & {{\href{../syntax/types.html#syntax-vectype}{\mathit{vectype}}}{.}\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{store}}}{{\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}}}{\mathsf{\_}}{\href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{lane}}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}}~{\href{../syntax/instructions.html#syntax-memarg}{\mathit{memarg}}}~{\href{../syntax/instructions.html#syntax-laneidx}{\mathit{laneidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{memory{.}size}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{memory{.}grow}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{memory{.}fill}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{memory{.}copy}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{memory{.}init}}~{\href{../syntax/modules.html#syntax-memidx}{\mathit{memidx}}}~{\href{../syntax/modules.html#syntax-dataidx}{\mathit{dataidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-memory}{\mathsf{data{.}drop}}~{\href{../syntax/modules.html#syntax-dataidx}{\mathit{dataidx}}} \\
& & | & \dots \\
\end{array}\end{split}\]
Memory is accessed with \(\mathsf{load}\) and \(\mathsf{store}\) instructions for the different number types and vector types <syntax-vectype>.
They all take a memory index and a memory argument \({\href{../syntax/instructions.html#syntax-memarg}{\mathit{memarg}}}\) that contains an address offset and the expected alignment (expressed as the exponent of a power of 2).
Integer loads and stores can optionally specify a storage size \({\href{../syntax/instructions.html#syntax-sz}{\mathit{sz}}}\) that is smaller than the bit width of the respective value type.
In the case of loads, a sign extension mode \({\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}\) is then required to select appropriate behavior.
Vector loads can specify a shape that is half the bit width of \(\mathsf{v{\scriptstyle 128}}\). Each lane is half its usual size, and the sign extension mode \({\href{../syntax/instructions.html#syntax-sx}{\mathit{sx}}}\) then specifies how the smaller lane is extended to the larger lane.
Alternatively, vector loads can perform a splat, such that only a single lane of the specified storage size is loaded, and the result is duplicated to all lanes.
The static address offset is added to the dynamic address operand, yielding a 33 bit effective address that is the zero-based index at which the memory is accessed.
All values are read and written in little endian byte order.
A trap results if any of the accessed memory bytes lies outside the address range implied by the memory’s current size.
Note
Future versions of WebAssembly might provide memory instructions with 64 bit address ranges.
The \(\mathsf{memory{.}size}\) instruction returns the current size of a memory.
The \(\mathsf{memory{.}grow}\) instruction grows a memory by a given delta and returns the previous size, or \({-1}\) if enough memory cannot be allocated.
Both instructions operate in units of page size.
The \(\mathsf{memory{.}fill}\) instruction sets all values in a regionof a memory to a given byte.
The \(\mathsf{memory{.}copy}\) instruction copies data from a source memory region to a possibly overlapping destination region in another or the same memory; the first index denotes the destination
The \(\mathsf{memory{.}init}\) instruction copies data from a passive data segment into a memory.
The \(\mathsf{data{.}drop}\) instruction prevents further use of a passive data segment. This instruction is intended to be used as an optimization hint. After a data segment is dropped its data can no longer be retrieved, so the memory used by this segment may be freed.
Note
In the current version of WebAssembly,
all memory instructions implicitly operate on memory index \(0\).
This restriction may be lifted in future versions.
Control Instructions
Instructions in this group affect the flow of control.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-blocktype}{\mathit{blocktype}}} & ::= & {{\href{../syntax/types.html#syntax-valtype}{\mathit{valtype}}}^?} \\
& & | & {\href{../syntax/modules.html#syntax-funcidx}{\mathit{funcidx}}} \\[0.8ex]
& {\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}} & ::= & \dots \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{block}}~{\href{../syntax/instructions.html#syntax-blocktype}{\mathit{blocktype}}}~{{\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}}^\ast} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{loop}}~{\href{../syntax/instructions.html#syntax-blocktype}{\mathit{blocktype}}}~{{\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}}^\ast} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{if}}~{\href{../syntax/instructions.html#syntax-blocktype}{\mathit{blocktype}}}~{{\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}}^\ast}~\href{../syntax/instructions.html#syntax-instr-control}{\mathsf{else}}~{{\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}}^\ast} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{br}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{br\_if}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{br\_table}}~{{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}}^\ast}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{br\_on\_null}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{br\_on\_non\_null}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{br\_on\_cast}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}}~{\href{../syntax/types.html#syntax-reftype}{\mathit{reftype}}}~{\href{../syntax/types.html#syntax-reftype}{\mathit{reftype}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{br\_on\_cast\_fail}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}}~{\href{../syntax/types.html#syntax-reftype}{\mathit{reftype}}}~{\href{../syntax/types.html#syntax-reftype}{\mathit{reftype}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{call}}~{\href{../syntax/modules.html#syntax-funcidx}{\mathit{funcidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{call\_ref}}~{\href{../syntax/types.html#syntax-typeuse}{\mathit{typeuse}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{call\_indirect}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}}~{\href{../syntax/types.html#syntax-typeuse}{\mathit{typeuse}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{return}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{return\_call}}~{\href{../syntax/modules.html#syntax-funcidx}{\mathit{funcidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{return\_call\_ref}}~{\href{../syntax/types.html#syntax-typeuse}{\mathit{typeuse}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{return\_call\_indirect}}~{\href{../syntax/modules.html#syntax-tableidx}{\mathit{tableidx}}}~{\href{../syntax/types.html#syntax-typeuse}{\mathit{typeuse}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{throw}}~{\href{../syntax/modules.html#syntax-tagidx}{\mathit{tagidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{throw\_ref}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{try\_table}}~{\href{../syntax/instructions.html#syntax-blocktype}{\mathit{blocktype}}}~{\href{../syntax/conventions.html#syntax-list}{\mathit{list}}}({\href{../syntax/instructions.html#syntax-catch}{\mathit{catch}}})~{{\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}}^\ast} \\
& & | & \dots \\[0.8ex]
& {\href{../syntax/instructions.html#syntax-catch}{\mathit{catch}}} & ::= & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{catch}}~{\href{../syntax/modules.html#syntax-tagidx}{\mathit{tagidx}}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{catch\_ref}}~{\href{../syntax/modules.html#syntax-tagidx}{\mathit{tagidx}}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{catch\_all}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}} \\
& & | & \href{../syntax/instructions.html#syntax-instr-control}{\mathsf{catch\_all\_ref}}~{\href{../syntax/modules.html#syntax-labelidx}{\mathit{labelidx}}} \\
\end{array}\end{split}\]
The \(\mathsf{block}\), \(\mathsf{loop}\), \(\mathsf{if}\) and \(\mathsf{try\_table}\) instructions are structured instructions.
They bracket nested sequences of instructions, called blocks, terminated with, or separated by, \(\mathsf{end}\) or \(\mathsf{else}\) pseudo-instructions.
As the grammar prescribes, they must be well-nested.
A structured instruction can consume input and produce output on the operand stack according to its annotated block type.
It is given either as a type index that refers to a suitable function type reinterpreted as an instruction type, or as an optional value type inline, which is a shorthand for the instruction type \(\epsilon \rightarrow {{\href{../syntax/types.html#syntax-valtype}{\mathit{valtype}}}^?}\).
Each structured control instruction introduces an implicit label.
Labels are targets for branch instructions that reference them with label indices.
Unlike with other index spaces, indexing of labels is relative by nesting depth,
that is, label \(0\) refers to the innermost structured control instruction enclosing the referring branch instruction,
while increasing indices refer to those farther out.
Consequently, labels can only be referenced from within the associated structured control instruction.
This also implies that branches can only be directed outwards,
“breaking” from the block of the control construct they target.
The exact effect depends on that control construct.
In case of \(\mathsf{block}\) or \(\mathsf{if}\) it is a forward jump,
resuming execution after the matching \(\mathsf{end}\).
In case of \(\mathsf{loop}\) it is a backward jump to the beginning of the loop.
Note
This enforces structured control flow.
Intuitively, a branch targeting a \(\mathsf{block}\) or \(\mathsf{if}\) behaves like a \(\mathsf{break}\) statement in most C-like languages,
while a branch targeting a \(\mathsf{loop}\) behaves like a \(\mathsf{continue}\) statement.
Branch instructions come in several flavors:
\(\mathsf{br}\) performs an unconditional branch,
\(\mathsf{br\_if}\) performs a conditional branch,
and \(\mathsf{br\_table}\) performs an indirect branch through an operand indexing into the label list that is an immediate to the instruction, or to a default target if the operand is out of bounds.
The \(\mathsf{br\_on\_null}\) and \(\mathsf{br\_on\_non\_null}\) instructions check whether a reference operand is null and branch if that is the case or not the case, respectively.
Similarly, \(\mathsf{br\_on\_cast}\) and \(\mathsf{br\_on\_cast\_fail}\) attempt a downcast on a reference operand and branch if that succeeds, or fails, respectively.
The \(\mathsf{return}\) instruction is a shortcut for an unconditional branch to the outermost block, which implicitly is the body of the current function.
Taking a branch unwinds the operand stack up to the height where the targeted structured control instruction was entered.
However, branches may additionally consume operands themselves, which they push back on the operand stack after unwinding.
Forward branches require operands according to the output of the targeted block’s type, i.e., represent the values produced by the terminated block.
Backward branches require operands according to the input of the targeted block’s type, i.e., represent the values consumed by the restarted block.
The \(\mathsf{call}\) instruction invokes another function, consuming the necessary arguments from the stack and returning the result values of the call.
The \(\mathsf{call\_ref}\) instruction invokes a function indirectly through a function reference operand.
The \(\mathsf{call\_indirect}\) instruction calls a function indirectly through an operand indexing into a table that is denoted by a table index and must contain function references.
Since it may contain functions of heterogeneous type,
the callee is dynamically checked against the function type indexed by the instruction’s second immediate, and the call is aborted with a trap if it does not match.
The \(\mathsf{return\_call}\), \(\mathsf{return\_call\_ref}\), and \(\mathsf{return\_call\_indirect}\) instructions are tail-call variants of the previous ones.
That is, they first return from the current function before actually performing the respective call.
It is guaranteed that no sequence of nested calls using only these instructions can cause resource exhaustion due to hitting an implementation’s limit on the number of active calls.
The instructions \(\mathsf{throw}\), \(\mathsf{throw\_ref}\), and \(\mathsf{try\_table}\) are concerned with exceptions.
The \(\mathsf{throw}\) and \(\mathsf{throw\_ref}\) instructions raise and reraise an exception, respectively, and transfers control to the innermost enclosing exception handler that has a matching catch clause.
The \(\mathsf{try\_table}\) instruction installs an exception handler that handles exceptions as specified by its catch clauses.
Expressions
Function bodies, initialization values for globals, elements and offsets of element segments, and offsets of data segments are given as expressions, which are sequences of instructions.
\[\begin{split}\begin{array}[t]{@{}l@{}rrl@{}l@{}}
& {\href{../syntax/instructions.html#syntax-expr}{\mathit{expr}}} & ::= & {{\href{../syntax/instructions.html#syntax-instr}{\mathit{instr}}}^\ast} \\
\end{array}\end{split}\]
In some places, validation restricts expressions to be constant, which limits the set of allowable instructions.
