https://bugs.llvm.org/show_bug.cgi?id=37872
Bug ID: 37872
Summary: Instcombine: transform shl(inexact shr(x)) with
different constant shifts.
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: lebedev...@gmail.com
CC: llvm-bugs@lists.llvm.org
IIn 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.
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