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 > -- >