https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108298
Bug ID: 108298
Summary: Wrong optimization of volatile access from gcc 11 and
beyond
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: daniel.lundin.mail at gmail dot com
Target Milestone: ---
Created attachment 54194
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54194&action=edit
Example to reproduce incorrect optimizations
Before gcc 11.0.0, the attached example compiled under -O3 would result in the
whole function getting optimized into a single load instruction when volatile
was not used. Adding a volatile access would lead to instructions for fetching
the value from the stack getting generated.
This appears to be conforming behavior, references ISO/IEC 9899:2018 5.1.2.3,
particularly §2, §4 and §6. I believe gcc 10.4 or older behaves correctly.
After gcc 11.0.0, the function is optimized regardless of if volatile is used
or not. See attached example. With FREE_TO_OPTIMIZE defined, I would expect the
compiler to optimize the code under -O3. Without it, I would expect the
compiler to generate a read instruction since this is a "side effect".
For convenience, here is also an online compiler example for x86-64 Linux
https://godbolt.org/z/4qez1P746
