https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99728
Bug ID: 99728
Summary: code pessimization when using wrapper classes around
SIMD types
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: mar...@mpa-garching.mpg.de
Target Milestone: ---
Created attachment 50456
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50456&action=edit
test case
I originally reported this at
https://gcc.gnu.org/pipermail/gcc-help/2021-March/139976.html, but I'm now
fairly confident that this warrants a PR.
The test case needs to be processed on x86_64 with the command
g++ -mfma -O3 -std=c++17 -ffast-math -S testcase.cc
Code for two functions will be generated, and I would expect that the generated
assembler for both should be identical. However, for the version using the
wrapper class around __m256d, g++ does not seem to recognize the dead stores at
the end of the loop and leaves them inside the loop body instead of moving them
after the final jump instruction of the loop, which reduces performance
considerably.
clang++ generates nearly identical code for both functions and manages to
remove the dead stores, so I think that g++ might be able to do better here and
is not pessimizing the code due to some C++ intricacies.
