[llvm-bugs] [Bug 41101] New: [EarlyCSE] recognize swapped form of selects

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41101

            Bug ID: 41101
           Summary: [EarlyCSE] recognize swapped form of selects
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org

%sel2 is the same value as %sel1:

define void @f(i8 %a, i8 %b, i8 %c, i8 %d, i8* %p1, i8* %p2) {
  %cmp1 = icmp ult i8 %a, %b
  %sel1 = select i1 %cmp1, i8 %c, i8 %d
  store i8 %sel1, i8* %p1
  %cmp2 = icmp uge i8 %a, %b
  %sel2 = select i1 %cmp2, i8 %d, i8 %c
  store i8 %sel2, i8* %p2
  ret void
}

This minimal example would be handled by instcombine canonicalizations, but
that can fail when the compares have multiple uses.

We should also match the pattern where the select condition is inverted from
the 1st cmp:

define void @f(i8 %a, i8 %b, i8 %c, i8 %d, i8* %p1, i8* %p2) {
  %cmp = icmp ult i8 %a, %b
  %sel1 = select i1 %cmp, i8 %c, i8 %d
  store i8 %sel1, i8* %p1
  %not = xor i1 %cmp, true
  %sel2 = select i1 %not, i8 %d, i8 %c
  store i8 %sel2, i8* %p2
  ret void
}

This should also work with fcmp.

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