https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96789
Bug ID: 96789 Summary: x264: sub4x4_dct() improves when vectorization is disabled Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: linkw at gcc dot gnu.org Target Milestone: --- One of my workmates found that if we disable vectorization for SPEC2017 525.x264_r function sub4x4_dct in source file x264_src/common/dct.c with explicit function attribute __attribute__((optimize("no-tree-vectorize"))), it can speed up by 4%. The option used is: -O3 -mcpu=power9 -fcommon -fno-strict-aliasing -fgnu89-inline I confirmed this finding and it can further narrow down to SLP vectorization with attribute __attribute__((optimize("no-tree-slp-vectorize"))). I also checked with r11-0 commit for this particular file, the performance keep unchanged, with/without vectorization attribute. So I think it's a trunk regression, probably exposes one SLP flaw or one cost modeling issue.