[llvm-bugs] [Bug 50028] New: SCEV ignores nuw/nsw when computing MaxBackedgeTakenCount

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50028

            Bug ID: 50028
           Summary: SCEV ignores nuw/nsw when computing
                    MaxBackedgeTakenCount
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Global Analyses
          Assignee: unassignedbugs at nondot.org
          Reporter: suc-daniil at yandex.ru
                CC: llvm-bugs at lists.llvm.org

define void @foo(i32 %n) {
entry:
  br label %loop

loop:
  %i = phi i32 [ 0, %entry ], [ %i.next, %loop ]
  %i.next = add nuw nsw i32 %i, 1
  %innercmp = icmp ult i32 %i, %n
  br i1 %innercmp, label %loop, label %exit

exit:
  ret void
}

Running opt -passes='print<scalar-evolution>' gives us the following result:

Printing analysis 'Scalar Evolution Analysis' for function 'foo':
Classifying expressions for: @foo
  %i = phi i32 [ 0, %entry ], [ %i.next, %loop ]
  -->  {0,+,1}<nuw><nsw><%loop> U: [0,-2147483648) S: [0,-2147483648)          
Exits: %n               LoopDispositions: { %loop: Computable }
  %i.next = add nuw nsw i32 %i, 1
  -->  {1,+,1}<nuw><%loop> U: [1,0) S: [1,0)            Exits: (1 + %n)        
LoopDispositions: { %loop: Computable }
Determining loop execution counts for: @foo
Loop %loop: backedge-taken count is %n
Loop %loop: max backedge-taken count is -1
Loop %loop: Predicated backedge-taken count is %n
 Predicates:

Loop %loop: Trip multiple is 1


SCEV understands that %i won't exceed 2^31, but says that the backedge can be
taken up to 2^32-1 times. Since it's an upper-bound estimate, it's correct, yet
suboptimal.

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