[llvm-bugs] [Bug 25115] New: An odd number can never be zero

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25115

            Bug ID: 25115
           Summary: An odd number can never be zero
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: listmail at philipreames.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

In the example below, we have a loop with an induction variable which
repeatedly wraps around in the <0,17> iteration space.  On each iteration, we
have two clamped decrements which reset the value to if 16 if need.  Given we
know the loop starts at an even number, we could prove away the second clamp
since a decrement of an even number can not be zero. 

declare i1 @unknown(i32)

define void @test() {
bci_21.preheader:
  br label %bci_21.us

bci_21.us:
  %v33 = phi i32 [ %storemerge.3.us, %bci_21.us ], [ 16, %bci_21.preheader ]
  %v46 = icmp eq i32 %v33, 0
  %v47 = add nsw i32 %v33, -1
  %storemerge.1.us = select i1 %v46, i32 16, i32 %v47 
  %v62 = icmp eq i32 %storemerge.1.us, 0 ;; <-- Always false
  %v63 = add nsw i32 %storemerge.1.us, -1
  %storemerge.3.us = select i1 %v62, i32 16, i32 %v63 ;; <-- Always %v63

  %cnd = call i1 @unknown(i32 %v33)
  br i1 %cnd, label %bci_21.us, label %loopexit

loopexit:
  ret void
}

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