On Sun, Nov 3, 2024 at 7:41 AM Andrew Pinski <quic_apin...@quicinc.com> wrote:
>
> I noticed that __builtin_assoc_barrier makes a differnce for FMAs formation
> but it was not documented. This adds that documentation even with a small
> example.
>
> Build the HTML documents to make sure everything looks correct.
OK.
Richard.
> gcc/ChangeLog:
>
> PR middle-end/115023
> * doc/extend.texi (__builtin_assoc_barrier): Document
> ffp-contract=fast
> and FMA usage.
>
> Signed-off-by: Andrew Pinski <quic_apin...@quicinc.com>
> ---
> gcc/doc/extend.texi | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index ad1821e3e23..5902e76f043 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -15555,8 +15555,7 @@ This built-in inhibits re-association of the
> floating-point expression
> @var{expr} with expressions consuming the return value of the built-in. The
> expression @var{expr} itself can be reordered, and the whole expression
> @var{expr} can be reordered with operands after the barrier. The barrier is
> -only relevant when @code{-fassociative-math} is active, since otherwise
> -floating-point is not treated as associative.
> +relevant when @code{-fassociative-math} is active.
>
> @smallexample
> float x0 = a + b - b;
> @@ -15566,6 +15565,19 @@ float x1 = __builtin_assoc_barrier(a + b) - b;
> @noindent
> means that, with @code{-fassociative-math}, @code{x0} can be optimized to
> @code{x0 = a} but @code{x1} cannot.
> +
> +It is also relevant when @code{-ffp-contract=fast} is active;
> +it will prevent contraction between expressions.
> +
> +@smallexample
> +float x0 = a * b + c;
> +float x1 = __builtin_assoc_barrier (a * b) + c;
> +@end smallexample
> +
> +@noindent
> +means that, with @code{-ffp-contract=fast}, @code{x0} may be optimized to
> +use a fused multiply-add instruction but @code{x1} cannot.
> +
> @enddefbuiltin
>
> @defbuiltin{{void *} __builtin_assume_aligned (const void *@var{exp}, size_t
> @var{align}, ...)}
> --
> 2.43.0
>
