https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108410
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Blocks| |53947
Last reconfirmed| |2023-01-16
Target| |x86_64-*-*
Keywords| |missed-optimization
CC| |rguenth at gcc dot gnu.org
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
One issue is that we at most perform one epilogue loop vectorization, so with
AVX512 we vectorize the epilogue with AVX2 but its epilogue remains
unvectorized. With AVX512 we'd want to use a fully masked epilogue using
AVX512 instead.
I started working on fully masked vectorization support for AVX512 but
got distracted.
Another option would be to use SSE vectorization for the epilogue
(note for SSE we vectorize the epilogue with 64bit half-SSE vectors!),
which would mean giving the target (some) control over the mode used
for vectorizing the epilogue. That is, in vect_analyze_loop change
/* For epilogues start the analysis from the first mode. The motivation
behind starting from the beginning comes from cases where the VECTOR_MODES
array may contain length-agnostic and length-specific modes. Their
ordering is not guaranteed, so we could end up picking a mode for the main
loop that is after the epilogue's optimal mode. */
vector_modes[0] = autodetected_vector_mode;
to go through a target hook (possibly first produce a "candidate mode" set
and allow the target to prune that). This might be an "easy" fix for the
AVX512 issue for low-trip loops.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53947
[Bug 53947] [meta-bug] vectorizer missed-optimizations
