https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91384
--- Comment #16 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Roger Sayle <sa...@gcc.gnu.org>: https://gcc.gnu.org/g:28068d1115648adcc08ae57372170f3277915a0d commit r12-7417-g28068d1115648adcc08ae57372170f3277915a0d Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Mon Feb 28 22:30:27 2022 +0000 PR tree-optimization/91384: peephole2 to eliminate testl after negl. 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 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.
