[llvm-bugs] [Bug 30327] New: [InstCombine] should sub C, (zext i1) be canonicalized to a select?

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Sep 8 10:53:28 PDT 2016


            Bug ID: 30327
           Summary: [InstCombine] should sub C, (zext i1) be canonicalized
                    to a select?
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

These are logically equivalent:

unsigned sel(bool x) {
  if (x)
    return 54;
    return 55;

unsigned add(bool x) {
  return 55 - x;

But the optimized IR is:

define i32 @sel(i1 zeroext %x) {
  %s = select i1 %x, i32 54, i32 55
  ret i32 %s

define i32 @add(i1 zeroext %x) {
  %conv = zext i1 %x to i32
  %sub = sub nsw i32 55, %conv
  ret i32 %sub

Which one is canonical?

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