On Thu, May 1, 2025 at 9:10 AM H.J. Lu <hjl.to...@gmail.com> wrote:
>
> On Thu, May 1, 2025 at 2:56 PM Uros Bizjak <ubiz...@gmail.com> wrote:
> >
> > 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.
>
> But there is no assigned role for RBX.
If no instruction uses "b" constraint and BREG class is not otherwise
used for x86_64, then CLASS_LIKELY_SPILLED_P change is irrelevant for
x86_64.
Uros.
