On Wed, Dec 28, 2022 at 1:32 AM Roger Sayle <ro...@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 <ro...@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
> --
>
