https://bugs.llvm.org/show_bug.cgi?id=37793
Bug ID: 37793
Summary: Does SCEV have any problems with recognizing
shl+shr/shr+shl pairs turned into masks?
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Global Analyses
Assignee: unassignedb...@nondot.org
Reporter: lebedev...@gmail.com
CC: llvm-bugs@lists.llvm.org
I'm not sure this is a bug, more like a question.
In https://reviews.llvm.org/D46760#1123713, Sanjay Patel suggests transforming
lshr+shl into mask(and)+shl, which is not only a canonicalization, but
would also sidestep the problem which D46760 is trying to fix.
This canonicalization is already being done for the non-constants,
or equal constants, but not unequal constants.
The commit that introduced that restriction: https://reviews.llvm.org/rL155136
suggests that said transform negatively impacts backend (rotate instruction),
and SCEV.
The backend part will likely be resolved in https://reviews.llvm.org/D47681 or
followups.
But the state of SCEV is unknown.
Does it actually have a problem with the mask+shift,
or that comment is no longer correct?
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