https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98335
--- Comment #8 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:c5288df751f9ecd11898dec5f2a7b6b03267f79e commit r12-7615-gc5288df751f9ecd11898dec5f2a7b6b03267f79e Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Fri Mar 11 17:46:50 2022 +0000 PR tree-optimization/98335: Improvements to DSE's compute_trims. This patch is the main middle-end piece of a fix for PR tree-opt/98335, which is a code-quality regression affecting mainline. The issue occurs in DSE's (dead store elimination's) compute_trims function that determines where a store to memory can be trimmed. In the testcase given in the PR, this function notices that the first byte of a DImode store is dead, and replaces the 8-byte store at (aligned) offset zero, with a 7-byte store at (unaligned) offset one. Most architectures can store a power-of-two bytes (up to a maximum) in single instruction, so writing 7 bytes requires more instructions than writing 8 bytes. This patch follows Jakub Jelinek's suggestion in comment 5, that compute_trims needs improved heuristics. On x86_64-pc-linux-gnu with -O2 the new test case in the PR goes from: movl $0, -24(%rsp) movabsq $72057594037927935, %rdx movl $0, -21(%rsp) andq -24(%rsp), %rdx movq %rdx, %rax salq $8, %rax movb c(%rip), %al ret to xorl %eax, %eax movb c(%rip), %al ret 2022-03-11 Roger Sayle <ro...@nextmovesoftware.com> Richard Biener <rguent...@suse.de> gcc/ChangeLog PR tree-optimization/98335 * builtins.cc (get_object_alignment_2): Export. * builtins.h (get_object_alignment_2): Likewise. * tree-ssa-alias.cc (ao_ref_alignment): New. * tree-ssa-alias.h (ao_ref_alignment): Declare. * tree-ssa-dse.cc (compute_trims): Improve logic deciding whether to align head/tail, writing more bytes but using fewer store insns. gcc/testsuite/ChangeLog PR tree-optimization/98335 * g++.dg/pr98335.C: New test case. * gcc.dg/pr86010.c: New test case. * gcc.dg/pr86010-2.c: New test case.
