https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109601
Bug ID: 109601
Summary: Useless branch not eliminated when writing to a union
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: 67 at mniip dot com
Target Milestone: ---
Created attachment 54909
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54909&action=edit
Minimal testcase
On x86_64-pc-linux-gnu the attached code compiles (with -O3) to rather
unsatisfying assembly with a branch that does the same thing whether taken or
not:
> 10: 85 c0 test %eax,%eax
> 12: 74 04 je 18
> 14: c3 ret
> 15: 0f 1f 00 nopl (%rax)
> 18: c3 ret
I've tested on my machine in GCC 12.2.1, but Compiler Explorer (
https://godbolt.org/z/cW69rfrPc ) shows the same thing happens in a lot of
other versions, including trunk (6ab856aa49bef7c04efa6144a5048e129b3a058b).
Notably, this doesn't happen in GCC 4.5.3.
This seems to happen for a few other targets as well, e.g: arm, riscv32; but
not on some others, e.g. aarch64, mips.
