select.md

select

Selects one of the first two operands based on the value of the third operand ^{1 2}.

The valtype immediate is optional, so long as the result type (and type of the first two operands) is a numeric type ¹.

Otherwise, the valtype immediate declares the result type ¹.

$$ \mathsf{select} \enspace \mathsf{valtype}(T)^? \enspace (a: T, b: T, c: \mathsf{i32}) \to \begin{cases} a &\text{if } c \not = 0 \\ b &\text{if } c = 0 \end{cases} $$

Instructions

Opcode	Instruction	Immediates	Stack Arity
0x1B	select	none	$[ \mathsf{numtype}_1, \mathsf{numtype}_1, \mathsf{i32} ] \to [ \mathsf{numtype}_1 ]$
0x1C	select	$\mathsf{valtype}(T)$	$[ T, T, \mathsf{i32} ] \to [ T ]$

WAT Examples

Without a valtype immediate

;; The first operand will be the result if the condition is not 0
i32.const 10

;; The second operand will be the result if the condition is 0
i32.const 20

;; The third operand is the condition
i32.const 1
select

With a valtype immediate

;; The first operand will be the result if the condition is not 0
ref.func $func1

;; The second operand will be the result if the condition is 0
ref.func $func2

;; The third operand is the condition
i32.const 1
select funcref

References

WebAssembly Core Specification

Footnotes

1. Structure, Parametric Instructions - https://www.w3.org/TR/wasm-core-2/syntax/instructions.html#parametric-instructions ↩ ↩² ↩³

2. Execution, Parametric Instructions, select(t*) - https://www.w3.org/TR/wasm-core-2/exec/instructions.html#exec-select ↩