https://bugs.llvm.org/show_bug.cgi?id=40194
Bug ID: 40194
Summary: BDCE clears poison flags too aggressively
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: nikita....@gmail.com
CC: llvm-bugs@lists.llvm.org
BDCE currently clears poison generating flags on all instructions that might
have been affected by a dead use. However, we don't actually have to do this
for all instructions.
In particular shl, lshr and ashr already take their nuw/nsw/exact flags into
account when computing demanded bits. Only add, sub and mul compute demanded
bits without respecting nuw/nsw.
As such, it should be sufficient to clear poison flags on add, sub and mul
only. This also matches what SimplifyDemandedBits in InstCombine does.
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