Missed optimization: failure to remove dead edge #165179
Description
From discourse discussion: https://discourse.llvm.org/t/missed-optimization-due-to-overflow-check/88700
Godbolt link: https://godbolt.org/z/xqW3rcnKd.
Relevant IR code.
_ZN4FracC2Eii.exit.i: %0 = phi i32 [ 1, %if.else.i.i ], [ -1, %if.end10.i.i ] ... lor.lhs.false.i: switch i32 %0, label %if.end.i [ i32 1, label %_ZN7ReducedC2Eii.exit i32 -1, label %_ZN7ReducedC2Eii.exit ] if.end.i: call void @Reduced::reduce_in_place()(ptr noundef nonnull align 4 dereferenceable(8) %retval) br label %_ZN7ReducedC2Eii.exit _ZN7ReducedC2Eii.exit: %1 = load i64, ptr %retval, align 8 ret i64 %1 }From %0 = phi i32 [ 1, %if.else.i.i ], [ -1, %if.end10.i.i ] it should be obvious that %0 can only be 1 or -1 yet LLVM doesn't see it and fails to remove the dead edge.