[llvm-bugs] [Bug 40194] New: BDCE clears poison flags too aggressively

via llvm-bugs llvm-bugs at lists.llvm.org
Tue Jan 1 03:43:38 PST 2019


            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: unassignedbugs at nondot.org
          Reporter: nikita.ppv at gmail.com
                CC: llvm-bugs at 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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