https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72835
Bug ID: 72835
Summary: [Regression 7] Incorrect arithmetic optimization
involving bitfield arguments
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: babokin at gmail dot com
Target Milestone: ---
Created attachment 39074
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39074&action=edit
reproducer
-O0 and -O2 produce different results, while the test doesn't contain undefined
behavior.
> g++ -O0 bit_field_math.cpp ; ./a.out
4098873984
> g++ -O2 bit_field_math.cpp ; ./a.out
0
> g++ --version
gcc (Revision=238966/svn-rev:238969/) 7.0.0 20160801 (experimental)
GCC version 6 works correctly.
The test case is attached.
One thing that might look suspicious in the test is negation of unsigned int.
But it's well defined in the standard (same bit pattern as if it's signed, but
the result is treated as unsigned).
