On Mon, Feb 28, 2022 at 6:36 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> This patch is my proposed solution to PR tree-optimization/91384 which is
> a missed-optimization/code quality regression on x86_64. The problematic
> idiom is "if (r = -a)" which is equivalent to both "r = -a; if (r != 0)"
> and alternatively "r = -a; if (a != 0)". In this particular case, on
> x86_64, we prefer to use the condition codes from the negation, rather
> than require an explicit testl instruction.
>
> Unfortunately, combine can't help, as it doesn't attempt to merge pairs
> of instructions that share the same operand(s), only pairs/triples of
> instructions where the result of each instruction feeds the next. But
> I doubt there's sufficient benefit to attempt this kind of "combination"
> (that wouldn't already be caught by the tree-ssa passes).
>
> Fortunately, it's relatively easy to fix this up (addressing the
> regression) during peephole2 to eliminate the unnecessary testl in:
>
> movl %edi, %ebx
> negl %ebx
> testl %edi, %edi
> je .L2
>
> Tested on x86_64-pc-linux-gnu with make bootstrap and make -k check,
> both with and without --target_board='unix{-m32\ -march=cascadelake}'
> with no new failures. Ok for mainline?
>
>
> 2022-02-28 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> PR tree-optimization/91384
> * config/i386/i386.md (peephole2): Eliminate final testl insn
> from the sequence *movsi_internal, *negsi_1, *cmpsi_ccno_1 by
> transforming using *negsi_2 for the negation.
>
> gcc/testsuite/ChangeLog
> PR tree-optimization/91384
> * gcc.target/i386/pr91384.c: New test case.
OK.
Thanks,
Uros.
>
> Thanks in advance,
> Roger
> --
>
