[llvm-bugs] [Bug 37872] New: Instcombine: transform shl(inexact shr(x)) with different constant shifts.
llvm-bugs at lists.llvm.org
Wed Jun 20 01:13:08 PDT 2018
Bug ID: 37872
Summary: Instcombine: transform shl(inexact shr(x)) with
different constant shifts.
Component: Scalar Optimizations
Assignee: unassignedbugs at nondot.org
Reporter: lebedev.ri at gmail.com
CC: llvm-bugs at 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),
The backend part will likely be resolved in https://reviews.llvm.org/D47681 or
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