| Message ID | 20241103064011.3662578-1-quic_apinski@quicinc.com |
|---|---|
| State | New |
| Headers | show |
| Series | docs: Document that __builtin_assoc_barrier also can be used for FMAs [PR115023] | expand |
On Sun, Nov 3, 2024 at 7:41 AM Andrew Pinski <quic_apinski@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_apinski@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 >
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}, ...)}
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. 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_apinski@quicinc.com> --- gcc/doc/extend.texi | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-)