https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117927
Bug ID: 117927
Summary: Invalid rotate optimization
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kristerw at gcc dot gnu.org
Target Milestone: ---
The text about rotation in tree.def is not entirely clear, but the conclusion
in PR108440 is that it has the same restriction as shift: it is undefined
behavior to rotate more than the bit width.
"Now, it is unclear if we in the middle-end treat rotates with rotate count
equal or larger than precision as UB or not, unlike shifts there are less
reasons to do so, but e.g. expansion of X r<< Y if there is no rotate optab
for the mode is emitted as (X << Y) | (((unsigned) X) >> ((-Y) & (B - 1)))
and so with UB on Y == B."
This implies that the optimization implemented in PR109906 is invalid because
(for a 32-bit type)
x r<< (32 - y)
is valid for y = 32, while the optimized
x r>> y
is UB.
