Semantic of WebAssembly "select" instruction?

Stéphane Letz

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Nov 13, 2019, 2:09:19 PM11/13/19

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Does WebAssembly "select" instruction behaves like C/C++ (cond ? then : else), that is compute *only* one of the two branches depending of the "cond" value?

Thanks.

Thomas Lively

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Nov 13, 2019, 2:10:20 PM11/13/19

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No, it unconditionally evaluates all arguments.

On Wed, Nov 13, 2019 at 11:09 AM Stéphane Letz <le...@grame.fr> wrote:

Does WebAssembly "select" instruction behaves like C/C++ (cond ? then : else), that is compute *only* one of the two branches depending of the "cond" value?

Thanks.

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Stéphane Letz

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Nov 13, 2019, 2:18:50 PM11/13/19

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So, It means the (cond ? then : else) kind of semantic has to be implemented using "if" right ?

Le mercredi 13 novembre 2019 20:10:20 UTC+1, Thomas Lively a écrit :

No, it unconditionally evaluates all arguments.

On Wed, Nov 13, 2019 at 11:09 AM Stéphane Letz <le...@grame.fr> wrote:

Does WebAssembly "select" instruction behaves like C/C++ (cond ? then : else), that is compute *only* one of the two branches depending of the "cond" value?

Thanks.

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Thomas Lively

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Nov 13, 2019, 2:37:48 PM11/13/19

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Yes, that's right, unless the two operands have no side effects. In that case the compiler is free to use a select because the difference is not observable.

On Wed, Nov 13, 2019 at 11:18 AM Stéphane Letz <le...@grame.fr> wrote:

So, It means the (cond ? then : else) kind of semantic has to be implemented using "if" right ?

Le mercredi 13 novembre 2019 20:10:20 UTC+1, Thomas Lively a écrit :

No, it unconditionally evaluates all arguments.

On Wed, Nov 13, 2019 at 11:09 AM Stéphane Letz <le...@grame.fr> wrote:

Does WebAssembly "select" instruction behaves like C/C++ (cond ? then : else), that is compute *only* one of the two branches depending of the "cond" value?

Thanks.

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