On Fri, Jun 27, 2025 at 3:45 PM Alexander Monakov <amona...@ispras.ru> wrote:
>
>
> 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.
Yeah, I guess that's within -ffp-contract=on constraints. When you say it's
detrimental for accuracy, is that a general statement for complex multiply
lowering? If so we possibly do not want 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?
Yeah, it seems to be a lack of operand swizzling support for ternary
commutative ops and for swapping ops of calls. So yes, please open
a separate bugreport for this. I'll see what I can do there.
> 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).
Yep, I'd expect there are some cases where we would need to add
-ffp-contract=fast
when we switch the default.
Thanks,
Richard.
> --- 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
>
