On Fri, 27 Jun 2025, Richard Biener wrote:
> > I am testing partial fixes for these issues.
>
> Can you check again after r16-1731-g08bdb6b4a32f1f?
Certainly. There's no more fmaddsub-related testsuite failures, and only
three tests need adjustment on x86.
* gcc.target/i386/pr116979.c
Here we check that FMA is used for complex multiplication. This is a case
where use of FMA is detrimental for accuracy, but I guess we can choose
to preserve the status quo. I can look into teaching
expand_complex_multiplication to emit FMAs.
gcc.target/i386/vect-pr82426.c
Here we check that 2x2 matrix multiplication ends up using a specific mix
of plain mul/adds and FMAs, and SLP is having difficulties. Generally
we are doing poorer job compared to LLVM here, either with or without
early FMAs. Do you want a separate bugreport for this?
gcc.target/i386/intrinsics_opt-1.c
Here we check that SSE addss and mulss intrinsics are combined into an
FMA. Clearly a case where unrestricted contraction needs to be enabled
explicitly via -ffp-contract=fast or such (but also, I'd say it is a
questionable transform considering the source had intrinsics).
--- 8< ---
>From acf83683b03653e3317a30a549cecd3bde49a325 Mon Sep 17 00:00:00 2001
From: Alexander Monakov <amona...@ispras.ru>
Date: Mon, 12 May 2025 23:23:42 +0300
Subject: [PATCH] c-family: switch away from -ffp-contract=fast
Unrestricted FMA contraction, combined with optimizations that create
copies of expressions, is causing hard-to-debug issues such as PR106902.
Since we implement conformant contraction now, switch C and C++ from
-ffp-contract=fast to either =off (under -std=c[++]NN like before, also
for C++ now), or =on (under -std=gnu[++]NN). Keep -ffp-contract=fast
when -funsafe-math-optimizations (or -ffast-math, -Ofast) is active.
In other words,
- -ffast-math: no change, unrestricted contraction like before;
- standards compliant mode for C: no change, no contraction;
- ditto, for C++: align with C (no contraction);
- otherwise, switch C and C++ from -ffp-contract=fast to =on.
gcc/c-family/ChangeLog:
* c-opts.cc (c_common_post_options): Adjust handling of
flag_fp_contract_mode.
gcc/ChangeLog:
* doc/invoke.texi (-ffp-contract): Describe new defaults.
(-funsafe-math-optimizations): Add -ffp-contract=fast.
gcc/testsuite/ChangeLog:
* gcc.target/i386/pr116979.c: Add -ffp-contract=fast to flags.
* gcc.target/i386/vect-pr82426.c: Ditto.
* gcc.target/i386/intrinsics_opt-1.c: Ditto.
---
gcc/c-family/c-opts.cc | 11 ++---------
gcc/doc/invoke.texi | 10 ++++++----
gcc/testsuite/gcc.target/i386/intrinsics_opt-1.c | 2 +-
gcc/testsuite/gcc.target/i386/pr116979.c | 2 +-
gcc/testsuite/gcc.target/i386/vect-pr82426.c | 2 +-
5 files changed, 11 insertions(+), 16 deletions(-)
diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
index 3ee9444cbe..40aba4dcc1 100644
--- a/gcc/c-family/c-opts.cc
+++ b/gcc/c-family/c-opts.cc
@@ -877,15 +877,8 @@ c_common_post_options (const char **pfilename)
flag_excess_precision = (flag_iso ? EXCESS_PRECISION_STANDARD
: EXCESS_PRECISION_FAST);
- /* ISO C restricts floating-point expression contraction to within
- source-language expressions (-ffp-contract=on, currently an alias
- for -ffp-contract=off). */
- if (flag_iso
- && !c_dialect_cxx ()
- && (OPTION_SET_P (flag_fp_contract_mode)
- == (enum fp_contract_mode) 0)
- && flag_unsafe_math_optimizations == 0)
- flag_fp_contract_mode = FP_CONTRACT_OFF;
+ if (!flag_unsafe_math_optimizations && !OPTION_SET_P (flag_fp_contract_mode))
+ flag_fp_contract_mode = flag_iso ? FP_CONTRACT_OFF : FP_CONTRACT_ON;
/* C language modes before C99 enable -fpermissive by default, but
only if -pedantic-errors is not specified. Also treat
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index b83818337c..b03d2f16be 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -13111,16 +13111,17 @@ Disabled by default.
@opindex ffp-contract
@item -ffp-contract=@var{style}
@option{-ffp-contract=off} disables floating-point expression contraction.
+This is the default for C and C++ in a standards compliant mode
+(@option{-std=c11}, @option{-std=c++11} or similar).
@option{-ffp-contract=fast} enables floating-point expression contraction
such as forming of fused multiply-add operations if the target has
native support for them.
+This is the default for languages other than C and C++.
@option{-ffp-contract=on} enables floating-point expression contraction
if allowed by the language standard. This is implemented for C and C++,
where it enables contraction within one expression, but not across
different statements.
-
-The default is @option{-ffp-contract=off} for C in a standards compliant mode
-(@option{-std=c11} or similar), @option{-ffp-contract=fast} otherwise.
+This is the default when compiling GNU dialects of C or C++.
@opindex fomit-frame-pointer
@item -fomit-frame-pointer
@@ -15614,7 +15615,8 @@ or ISO rules/specifications for math functions. It may,
however,
yield faster code for programs that do not require the guarantees
of these specifications.
Enables @option{-fno-signed-zeros}, @option{-fno-trapping-math},
-@option{-fassociative-math} and @option{-freciprocal-math}.
+@option{-fassociative-math}, @option{-freciprocal-math} and
+@option{-ffp-contract=fast}.
The default is @option{-fno-unsafe-math-optimizations}.
diff --git a/gcc/testsuite/gcc.target/i386/intrinsics_opt-1.c
b/gcc/testsuite/gcc.target/i386/intrinsics_opt-1.c
index a75bf4e9ca..edd8a7d964 100644
--- a/gcc/testsuite/gcc.target/i386/intrinsics_opt-1.c
+++ b/gcc/testsuite/gcc.target/i386/intrinsics_opt-1.c
@@ -1,5 +1,5 @@
/* { dg-do compile } */
-/* { dg-options "-O2 -mfma" } */
+/* { dg-options "-O2 -mfma -ffp-contract=fast" } */
#include <emmintrin.h>
diff --git a/gcc/testsuite/gcc.target/i386/pr116979.c
b/gcc/testsuite/gcc.target/i386/pr116979.c
index 0d2a958af4..fa2022b8a9 100644
--- a/gcc/testsuite/gcc.target/i386/pr116979.c
+++ b/gcc/testsuite/gcc.target/i386/pr116979.c
@@ -1,6 +1,6 @@
/* PR target/116979 */
/* { dg-do compile } */
-/* { dg-options "-O2 -mfma -fvect-cost-model=unlimited" } */
+/* { dg-options "-O2 -mfma -fvect-cost-model=unlimited -ffp-contract=fast" } */
/* { dg-final { scan-assembler "vfmaddsub(?:132|213|231)pd" } } */
/* { dg-final { scan-assembler "vfmaddsub(?:132|213|231)ps" { target { ! ia32
} } } } */
diff --git a/gcc/testsuite/gcc.target/i386/vect-pr82426.c
b/gcc/testsuite/gcc.target/i386/vect-pr82426.c
index 03b10adff9..8ce8fe78a9 100644
--- a/gcc/testsuite/gcc.target/i386/vect-pr82426.c
+++ b/gcc/testsuite/gcc.target/i386/vect-pr82426.c
@@ -1,6 +1,6 @@
/* i?86 does not have V2SF, x32 does though. */
/* { dg-do compile { target { ! ia32 } } } */
-/* { dg-options "-O3 -mavx -mfma" } */
+/* { dg-options "-O3 -mavx -mfma -ffp-contract=fast" } */
struct Matrix
{
--
2.49.0
