[llvm-bugs] [Bug 44802] New: InstCombine unsafely drops zero extensions when combining shifts

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44802

            Bug ID: 44802
           Summary: InstCombine unsafely drops zero extensions when
                    combining shifts
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: rspringer at google.com
                CC: llvm-bugs at lists.llvm.org

Reproducer: https://rise4fun.com/Alive/ne8y

In InstCombiner::reassociateShiftAmtsOfTwoSameDirectionShifts(), when
identifying two shifts to combine, it does so while ignoring any
zero-extensions of those values. In the problematic case above, the values were
both i1s zero-extended to i3s.

When their zero-extensions are dropped (taking them back to i1s) their sum
becomes an xor in SimplifyAddInst(), which causes the shift to basically
disappear (and to be dropped in a later pass).

I'm not familiar with the history of this code, particularly why the zero
extensions are ignored, but changing the code to leave them in place does fix
the issue.

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