[llvm-bugs] [Bug 43158] New: Failure to exploit assume to eliminate use of load

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43158

            Bug ID: 43158
           Summary: Failure to exploit assume to eliminate use of load
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: listmail at philipreames.com
                CC: llvm-bugs at lists.llvm.org

Test case:
define i32 @test(i32* %p, i32 %v) {
  %load = load i32, i32* %p
  %c = icmp eq i32 %load, %v
  call void @llvm.assume(i1 %c)
  ret i32 %load
}

define i32 @reverse(i32* %p, i32 %v) {
  %load = load i32, i32* %p
  %c = icmp eq i32 %load, %v
  call void @llvm.assume(i1 %c)
  ret i32 %v
}

declare void @llvm.assume(i1)

Both of these examples are stable under O3; we really should be canonicalizing
to one or the other.  In this particular example, reverse is the better of the
two, but a general heuristic requires more thought. 

GVN's propagateEquality has a "prefer early valno" heuristic which appears
reasonable.  GVN has a separate constant replacement in operands heuristic
which also seems reasonable.  I suspect these two need to be unified into
something of a broader scope.

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