[x86_64 PATCH] Add post-reload splitter for extendditi2.

[Roger Sayle]

Hi Uros,
Happy New Year.
As requested here's a revised version of my patch to introduce a pattern for
extendditi2, but implementing your suggestion to re-use the existing
extendsidi2_1 splitters and peephole2 optimizations by using DWI/DWIH
mode iterators.

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?

2023-01-01  Roger Sayle  <roger@nextmovesoftware.com>
	    Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog
	* config/i386/i386.md (extendditi2): New define_insn.
	(define_split): Use DWIH mode iterator to treat new extendditi2
	identically to existing extendsidi2_1.
	(define_peephole2): Likewise.
	(define_peephole2): Likewise.
	(define_split): Likewise.

gcc/testsuite/ChangeLog
	* gcc.target/i386/extendditi2-1.c: New test case.
	* gcc.target/i386/extendditi2-2.c: Likewise.


Thanks in advance,
Roger
--

> -----Original Message-----
> From: Uros Bizjak <ubizjak@gmail.com>
> Sent: 28 December 2022 09:28
> To: Roger Sayle <roger@nextmovesoftware.com>
> Cc: GCC Patches <gcc-patches@gcc.gnu.org>
> Subject: Re: [x86_64 PATCH] Add post-reload splitter for extendditi2.
> 
> On Wed, Dec 28, 2022 at 1:32 AM Roger Sayle
> <roger@nextmovesoftware.com> wrote:
> >
> >
> > This is another step towards a possible solution for PR 105137.
> > This patch introduces a define_insn_and_split for extendditi2, that
> > allows DImode to TImode sign-extension to be represented in the early
> > RTL optimizers, before being split post-reload into the exact same
> > idiom as currently produced by RTL expansion.
> 
> Please see extendsidi2_1 insn pattern and follow-up splitters and
> peephole2 patterns that do exactly what you want to achieve, but they are
> currently handling only SImode to DImode on 32-bit targets. OTOH, these
> patterns handle several more cases (e.g. split to the memory
> output) and just have to be macroized with DWIH mode iterator to also handle
> DImode to TImode on 64-bit targets. Probably, an extendsidi expander will have
> to be slightly adjusted when macroized to signal middle end the availability of
> extendditi pattern.
> 
> Following macroization, any possible follow-up optimizations and improvements
> will then be automatically applied also to 32-bit targets.
> 
> Uros.
> 
> >
> > Typically this produces the identical code, so the first new test
> > case:
> > __int128 foo(long long x) { return (__int128)x; }
> >
> > continues to generate:
> > foo:    movq    %rdi, %rax
> >         cqto
> >         ret
> >
> > The "magic" is that this representation allows combine and the other
> > RTL optimizers to do a better job.  Hence, the second test case:
> >
> > __int128 foo(__int128 a, long long b) {
> >     a += ((__int128)b) << 70;
> >     return a;
> > }
> >
> > which mainline with -O2 currently generates as:
> >
> > foo:    movq    %rsi, %rax
> >         movq    %rdx, %rcx
> >         movq    %rdi, %rsi
> >         salq    $6, %rcx
> >         movq    %rax, %rdi
> >         xorl    %eax, %eax
> >         movq    %rcx, %rdx
> >         addq    %rsi, %rax
> >         adcq    %rdi, %rdx
> >         ret
> >
> > with this patch now becomes:
> > foo:    movl    $0, %eax
> >         salq    $6, %rdx
> >         addq    %rdi, %rax
> >         adcq    %rsi, %rdx
> >         ret
> >
> > i.e. the same code for the signed and unsigned extension variants.
> >
> > 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?
> >
> > 2022-12-28  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >         * config/i386/i386.md (extendditi2): New define_insn_and_split
> >         to split DImode to TImode sign-extension after reload.
> >
> > gcc/testsuite/ChangeLog
> >         * gcc.target/i386/extendditi2-1.c: New test case.
> >         * gcc.target/i386/extendditi2-2.c: Likewise.
> >
> >
> > Thanks in advance,
> > Roger
> > --
> >
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