On Wed, Apr 30, 2025 at 11:31 PM H.J. Lu <hjl.to...@gmail.com> wrote:
>
> On Wed, Apr 30, 2025 at 7:37 PM Uros Bizjak <ubiz...@gmail.com> wrote:
> >
> > On Tue, Apr 29, 2025 at 11:40 PM H.J. Lu <hjl.to...@gmail.com> wrote:
> > >
> > > AREG, DREG, CREG and AD_REGS are kept in ix86_class_likely_spilled_p to
> > > avoid the following regressions with
> > >
> > > $ make check RUNTESTFLAGS="--target_board='unix{-m32,}'"
> > >
> > > FAIL: gcc.dg/pr105911.c (internal compiler error: in
> > > lra_split_hard_reg_for, at lra-assigns.cc:1863)
> > > FAIL: gcc.dg/pr105911.c (test for excess errors)
> > > FAIL: gcc.target/i386/avx512vl-stv-rotatedi-1.c scan-assembler-times
> > > vpro[lr]q 29
> > > FAIL: gcc.target/i386/bt-7.c scan-assembler-not and[lq][ \t]
> > > FAIL: gcc.target/i386/naked-4.c scan-assembler-not %[re]bp
> > > FAIL: gcc.target/i386/pr107548-1.c scan-assembler-not addl
> > > FAIL: gcc.target/i386/pr107548-1.c scan-assembler-times \tv?movd\t 3
> > > FAIL: gcc.target/i386/pr107548-1.c scan-assembler-times v?paddd 6
> > > FAIL: gcc.target/i386/pr107548-2.c scan-assembler-not \taddq\t
> > > FAIL: gcc.target/i386/pr107548-2.c scan-assembler-times v?paddq 2
> > > FAIL: gcc.target/i386/pr119171-1.c (test for excess errors)
> > > FAIL: gcc.target/i386/pr57189.c scan-assembler-not movaps
> > > FAIL: gcc.target/i386/pr57189.c scan-assembler-not movaps
> > > FAIL: gcc.target/i386/pr78904-1b.c scan-assembler [ \t]andb
> > > FAIL: gcc.target/i386/pr78904-1b.c scan-assembler [ \t]orb
> > > FAIL: gcc.target/i386/pr78904-7b.c scan-assembler-not movzbl
> > > FAIL: gcc.target/i386/pr78904-7b.c scan-assembler [ \t]orb
> > > FAIL: gcc.target/i386/pr91188-2c.c scan-assembler [ \t]andw
> > >
> > > Tested with glibc master branch at
> > >
> > > commit ccdb68e829a31e4cda8339ea0d2dc3e51fb81ba5
> > > Author: Samuel Thibault <samuel.thiba...@ens-lyon.org>
> > > Date: Sun Mar 2 15:16:45 2025 +0100
> > >
> > > htl: move pthread_once into libc
> > >
> > > and built Linux kernel 6.13.5 on x86-64.
> > >
> > > PR target/119083
> > > * config/i386/i386.cc (ix86_class_likely_spilled_p): Remove CREG
> > > and BREG.
> >
> > The commit message doesn't reflect what the patch does.
> >
> > OTOH, this is a very delicate part of the compiler. You are risking RA
> > failures, the risk/benefit ratio is very high, so I wouldn't touch it
> > without clear benefits. Do you have a concrete example where declaring
> > BREG as spilled hurts?
> >
>
> I can find a testcase to show the improvement. But I am not sure if
> it is what you were asking for. ix86_class_likely_spilled_p was
> CLASS_LIKELY_SPILLED_P which was added by
>
> commit f5316dfe88b8d1b8d3012c1f75349edf2ba1bdde
> Author: Michael Meissner <meiss...@gcc.gnu.org>
> Date: Thu Sep 8 17:59:18 1994 +0000
>
> Add support for -mreg-alloc=<xxx>
>
> This option is long gone and there is no test in GCC testsuite to show
> that BX should be in ix86_class_likely_spilled_p. On x86-64, BX is just
> another callee-saved register, not different from R12. On i386, BX is
> used as the PIC register. But today RA may pick a different register if
> PLT isn't involved. This patch gives RA a little bit more freedom.
In the past *decades* CLASS_LIKELY_SPILLED_P was repurposed to signal
the compiler that some extra care is needed with listed classes. On
i386 and x86_64 these include single register classes that represent
"architectural" registers (registers with assigned role). The compiler
does take care to not extend life times of CLASS_LIKELY_SPILLED_P
classes too much to avoid reload failures in cases where instruction
with C_L_S_P class (e.g. shifts with %cl register) is emitted between
unrelated register def and use.
Registers in these classes won't disappear from the pool of available
registers and RA can still use them, but with some extra care. So,
without clear and noticeable benefits, single register classes remain
declared as CLASS_LIKELY_SPILLED_P to avoid possible reload failures.
Uros.
