On Fri, Dec 15, 2023 at 2:25 AM haochen.jiang
<haoch...@ecsmtp.sh.intel.com> wrote:
>
> On Linux/x86_64,
>
> 8afdbcdd7abe1e3c7a81e07f34c256e7f2dbc652 is the first bad commit
> commit 8afdbcdd7abe1e3c7a81e07f34c256e7f2dbc652
> Author: Di Zhao <diz...@os.amperecomputing.com>
> Date: Fri Dec 15 03:22:32 2023 +0800
>
> Consider fully pipelined FMA in get_reassociation_width
>
> caused
>
> FAIL: gcc.dg/guality/pr58791-4.c -O2 -DPREVENT_OPTIMIZATION line
> pr58791-4.c:32 i2 == 487
> FAIL: gcc.dg/guality/pr58791-4.c -O2 -DPREVENT_OPTIMIZATION line
> pr58791-4.c:32 i == 486
> FAIL: gcc.dg/guality/pr58791-4.c -O2 -flto -fno-use-linker-plugin
> -flto-partition=none -DPREVENT_OPTIMIZATION line pr58791-4.c:32 i2 == 487
> FAIL: gcc.dg/guality/pr58791-4.c -O2 -flto -fno-use-linker-plugin
> -flto-partition=none -DPREVENT_OPTIMIZATION line pr58791-4.c:32 i == 486
> FAIL: gcc.dg/guality/pr58791-4.c -O2 -flto -fuse-linker-plugin
> -fno-fat-lto-objects -DPREVENT_OPTIMIZATION line pr58791-4.c:32 i2 == 487
> FAIL: gcc.dg/guality/pr58791-4.c -O2 -flto -fuse-linker-plugin
> -fno-fat-lto-objects -DPREVENT_OPTIMIZATION line pr58791-4.c:32 i == 486
> FAIL: gcc.dg/guality/pr58791-4.c -O3 -g -DPREVENT_OPTIMIZATION line
> pr58791-4.c:32 i2 == 487
> FAIL: gcc.dg/guality/pr58791-4.c -O3 -g -DPREVENT_OPTIMIZATION line
> pr58791-4.c:32 i == 486
> FAIL: gcc.dg/guality/pr58791-4.c -Os -DPREVENT_OPTIMIZATION line
> pr58791-4.c:32 i2 == 487
> FAIL: gcc.dg/guality/pr58791-4.c -Os -DPREVENT_OPTIMIZATION line
> pr58791-4.c:32 i == 486
>
> with GCC configured with
>
> ../../gcc/configure
> --prefix=/export/users/haochenj/src/gcc-bisect/master/master/r14-6559/usr
> --enable-clocale=gnu --with-system-zlib --with-demangler-in-ld
> --with-fpmath=sse --enable-languages=c,c++,fortran --enable-cet --without-isl
> --enable-libmpx x86_64-linux --disable-bootstrap
>
> To reproduce:
>
> $ cd {build_dir}/gcc && make check
> RUNTESTFLAGS="guality.exp=gcc.dg/guality/pr58791-4.c
> --target_board='unix{-m64\ -march=cascadelake}'"
There's an extra intermediate stmt inserted (for much later use, but
reassoc inserts close to defs) that is
then also used for FMA forming. This disturbs things in some way:
g_5 = (double) f_4;
# DEBUG g => g_5
# DEBUG BEGIN_STMT
h_7 = (double) b_6(D);
# DEBUG h => h_7
# DEBUG BEGIN_STMT
_39 = h_7 * 3.25e+0;
# DEBUG D#5 => g_5 * h_7
# DEBUG i => D#5
# DEBUG BEGIN_STMT
# DEBUG i2 => D#5 + 1.0e+0
# DEBUG BEGIN_STMT
# DEBUG D#8 => g_5 * _39
_3 = .FMA (g_5, _39, h_7);
g_5 is dead after the FMA. Interestingly removing the
asm volatile (NOP : : : "memory");
asm volatile (NOP : : : "memory");
lines fixes the regression because then we can TER the FMA, keeping
g_5 live for longer.
Richard.
