[llvm-bugs] [Bug 40846] New: Missing redundancy elimination for uadd.with.overflow and manual overflow check

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40846

            Bug ID: 40846
           Summary: Missing redundancy elimination for uadd.with.overflow
                    and manual overflow check
           Product: libraries
           Version: trunk
          Hardware: All
                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

Originally reported at: https://github.com/rust-lang/rust/issues/58692

The following IR remains unchanged under opt -O3:

define i1 @test(i64 %x, i64 %y) nounwind {
  %a = call { i64, i1 } @llvm.uadd.with.overflow.i64(i64 %x, i64 %y)
  %b = extractvalue { i64, i1 } %a, 1
  br i1 %b, label %trap, label %bb

bb:
  %c = extractvalue { i64, i1 } %a, 0
  %d = icmp ult i64 %c, %x
  ret i1 %d

trap:
  call void @llvm.trap()
  unreachable
}

declare { i64, i1 } @llvm.uadd.with.overflow.i64(i64, i64)
declare void @llvm.trap()

Here %d is the same as %b, and must be false in %bb by implication.

I think this should be fairly easy to fix (at least for this specific case)
with an instcombine pattern that converts the icmp into an extract, and CSE/GVN
will take care of the rest.

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