https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114435
Bug ID: 114435
Summary: Bad code generated when SSA and PCOM are enabled.
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jchrist at linux dot ibm.com
Target Milestone: ---
Created attachment 57783
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57783&action=edit
Reproducer
While investigating a preformance difference between clang and gcc on
imagemagick I discovered that attached test case gets badly vectorized due to
pcom pass. If I disable pcom and set the vector cost model to unlimited, SLP
produces exactly what I would expect. With pcom, however, the code becomes
considerably bigger and, depending on the target, even mixes scalar and
vectorized operations while the whole body of the loop should be vectorizable
via SLP.
Difference is observed between the output of
```
gcc -O3 -fvect-cost-model=unlimited fma.c -c
```
and
```
gcc -O3 -fvect-cost-model=unlimited fma.c -c -fdisable-tree-pcom
```
I am wondering if the pcom pass should be after SLP vectorization?
