Re: [X86 PATCH] Implement doubleword shift left by 1 bit using add+adc.

[Uros Bizjak <ubizjak@gmail.com>]

On Thu, Oct 5, 2023 at 1:45 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
> Doh! ENOPATCH.
>
> > -----Original Message-----
> > From: Roger Sayle <roger@nextmovesoftware.com>
> > Sent: 05 October 2023 12:44
> > To: 'gcc-patches@gcc.gnu.org' <gcc-patches@gcc.gnu.org>
> > Cc: 'Uros Bizjak' <ubizjak@gmail.com>
> > Subject: [X86 PATCH] Implement doubleword shift left by 1 bit using
> add+adc.
> >
> >
> > This patch tweaks the i386 back-end's ix86_split_ashl to implement
> doubleword
> > left shifts by 1 bit, using an add followed by an add-with-carry (i.e. a
> doubleword
> > x+x) instead of using the x86's shld instruction.
> > The replacement sequence both requires fewer bytes and is faster on both
> Intel
> > and AMD architectures (from Agner Fog's latency tables and confirmed by my
> > own microbenchmarking).
> >
> > For the test case:
> > __int128 foo(__int128 x) { return x << 1; }
> >
> > with -O2 we previously generated:
> >
> > foo:    movq    %rdi, %rax
> >         movq    %rsi, %rdx
> >         shldq   $1, %rdi, %rdx
> >         addq    %rdi, %rax
> >         ret
> >
> > with this patch we now generate:
> >
> > foo:    movq    %rdi, %rax
> >         movq    %rsi, %rdx
> >         addq    %rdi, %rax
> >         adcq    %rsi, %rdx
> >         ret
> >
> >
> > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap and
> > make -k check, both with and without --target_board=unix{-m32} with no new
> > failures.  Ok for mainline?
> >
> >
> > 2023-10-05  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >         * config/i386/i386-expand.cc (ix86_split_ashl): Split shifts by
> >         one into add3_cc_overflow_1 followed by add3_carry.
> >         * config/i386/i386.md (@add<mode>3_cc_overflow_1): Renamed from
> >         "*add<mode>3_cc_overflow_1" to provide generator function.
> >
> > gcc/testsuite/ChangeLog
> >         * gcc.target/i386/ashldi3-2.c: New 32-bit test case.
> >         * gcc.target/i386/ashlti3-3.c: New 64-bit test case.

OK.

Thanks,
Uros.

> >
> >
> > Thanks in advance,
> > Roger
> > --
>