Hi,

> In this code, if the compiler knew that accessing array[i + 1] on the
false path of conditionwas safe

You can tell the compiler that this is safe either by loading both values
before the if or by using std::assume (or various built-ins before C++23).
I am dubious, though, that a compiler would or could vectorize the example
you provided even with this information.

Cheers
Jeremy


On Thu, Oct 31, 2024 at 13:40 Matthew Kolbe via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I am proposing an attribute to aid compilers in vectorizing conditional
> code more effectively. In current scenarios, compilers sometimes hesitate
> to vectorize code containing conditional branches, as they cannot guarantee
> that accessing data on the “false path” won’t trigger undefined behavior
> (UB). This results in missed vectorization opportunities, particularly when
> working with SIMD operations, where executing both branches and blending
> the result is often optimal.
>
> Here's an example
>
> ```cpp
> int a;
> if (condition) {
> a = array[i];
> } else {
> a = array[i+1]
> }
> ```
>
> In this code, if the compiler knew that accessing array[i + 1] on the
> false path of condition was safe—even if that path isn’t executed—it
> could vectorize this operation by evaluating both paths and blending the
> results. However, the lack of clarity regarding UB in these cases can
> prevent this optimization.
>
> *Proposed Solution:*
>
> To resolve this, I propose a new attribute,
> [[false_path_is_defined_behavior]], which would guarantee that accessing
> the false path’s value does not invoke UB, thereby enabling compilers to
> vectorize more confidently.
>
> Example with the proposed attribute:
>
> ```cpp
> int a;
> if (condition) [[false_path_is_defined_behavior]] {
> a = array[i];
> } else {
> a = array[i+1]
> }
> ```
>
> This attribute would provide compilers the information needed to safely
> perform vectorization in SIMD-friendly loops, improving performance without
> additional programmer intervention.
>
> I would be grateful for any feedback from the committee on this concept
> and its feasibility.
>
> Thank you very much for your time and consideration.
>
> Best,
>
>
>
> --
> Matthew P. Kolbe
> (312) 218-6595
> matthew.kolbe_at_[hidden]
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
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>