https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102404
Bug ID: 102404
Summary: Loop vectorized with 32 byte vectors actually uses 16
byte vectors
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: freddie at witherden dot org
Target Milestone: ---
Created attachment 51480
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51480&action=edit
Test case
Consider the loop on L11 of the attached file. Compiling as:
❯ gcc -march=tigerlake -Ofast -mprefer-vector-width=512 -S -fopenmp test.c
-fopt-info
test.c:25:37: optimized: loop vectorized using 32 byte vectors
test.c:4:6: optimized: loop turned into non-loop; it never loops
which notes that (as requested) the loop has been vectorized using 32-byte
(zmm) vectors. Inspecting the resulting assembly (also attached) we observe
that has actually ben unrolled by a factor of two and then vectorized using
16-byte (ymm) vectors.
As a point of comparison recent versions of Clang use 32-byte vectors for this
loop, resulting in code which is half the size.
