[llvm-bugs] [Bug 40486] New: CodeGenPrepare::CombineUAddWithOverflow - improve icmp eq/ne matching

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40486

            Bug ID: 40486
           Summary: CodeGenPrepare::CombineUAddWithOverflow - improve icmp
                    eq/ne matching
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: arthur.j.odwyer at gmail.com, llvm-bugs at lists.llvm.org,
                    sanjoy at playingwithpointers.com,
                    spatel+llvm at rotateright.com

Reviewing the addcarry cases from [Bug #31754 Comment #3]:

https://godbolt.org/z/eZWlsG

The IR demonstrates that much of the problem is that while
CombineUAddWithOverflow can match the ult/ugt cases of the add result, it can't
handle ult/ugt canonicalizations (against 1/-1/signbit/etc) that result in a
icmp eq/ne instead:

define void @_Z4testILy1EEvRyS0_(i64*, i64*) {
  %3 = load i64, i64* %0, align 8
  %4 = add i64 %3, 1
  store i64 %4, i64* %0, align 8
  %5 = icmp eq i64 %4, 0 // <---- fails to match uaddo
  %6 = zext i1 %5 to i8
  %7 = load i64, i64* %1, align 8
  %8 = tail call { i8, i64 } @llvm.x86.addcarry.64(i8 %6, i64 %7, i64 0) #2
  %9 = extractvalue { i8, i64 } %8, 1
  store i64 %9, i64* %1, align 8
  ret void
}

define void @_Z4testILy2EEvRyS0_(i64*, i64*) {
  %3 = load i64, i64* %0, align 8
  %4 = add i64 %3, 2
  store i64 %4, i64* %0, align 8
  %5 = icmp ult i64 %4, 2 // <---- matches uaddo
  %6 = zext i1 %5 to i8
  %7 = load i64, i64* %1, align 8
  %8 = tail call { i8, i64 } @llvm.x86.addcarry.64(i8 %6, i64 %7, i64 0) #2
  %9 = extractvalue { i8, i64 } %8, 1
  store i64 %9, i64* %1, align 8
  ret void
}

declare { i8, i64 } @llvm.x86.addcarry.64(i8, i64, i64)

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