On Wed, Dec 28, 2022 at 1:06 AM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
> This patch is a one line change, to call ix86_expand_clear instead of
> emit_move_insn with const0_rtx in ix86_split_ashl, allowing the backend
> to use an xor instruction to clear a register if appropriate.
>
> The effect is demonstrated with the following function.
> __int128 foo(__int128 x, unsigned long long b) {
> return ((__int128)b << 72) + x;
> }
>
> previously with -O2, GCC would generate
>
> foo: movl $0, %eax
> salq $8, %rdx
> addq %rdi, %rax
> adcq %rsi, %rdx
> ret
>
> with this patch, it now generates
>
> foo: xorl %eax, %eax
> salq $8, %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. It's an independent piece of a larger bug fix.
> Ok for mainline?
>
>
> 2022-12-28 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> * config/i386/i386-expand.cc (ix86_split_ashl): Call
> ix86_expand_clear to generate an xor instruction.
>
> gcc/testsuite/ChangeLog
> * gcc.target/i386/ashlti3-1.c: New test case.
OK.
Thanks,
Uros.
>
>
> Thanks in advance,
> Roger
> --
>
