[InstCombine] narrow icmp with extended operands of different widths

An intermediate extend is used to widen the narrow operand to the width of
the other (wider) operand. At that point, we have the same logic as the
existing transform that was restricted to folds of equal width zext/sext.

This mostly solves PR42700:
https://bugs.llvm.org/show_bug.cgi?id=42700

llvm-svn: 369519

1 files changed, 17 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index b318d0b2de9..5e0a3c37979 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -4026,7 +4026,8 @@ Instruction *InstCombiner::foldICmpEquality(ICmpInst &I) {
   return nullptr;
 }
 
-static Instruction *foldICmpWithZextOrSext(ICmpInst &ICmp) {
+static Instruction *foldICmpWithZextOrSext(ICmpInst &ICmp,
+                                           InstCombiner::BuilderTy &Builder) {
   assert(isa<CastInst>(ICmp.getOperand(0)) && "Expected cast for operand 0");
   auto *CastOp0 = cast<CastInst>(ICmp.getOperand(0));
   Value *X;
@@ -4038,15 +4039,25 @@ static Instruction *foldICmpWithZextOrSext(ICmpInst &ICmp) {
   if (auto *CastOp1 = dyn_cast<CastInst>(ICmp.getOperand(1))) {
     // If the signedness of the two casts doesn't agree (i.e. one is a sext
     // and the other is a zext), then we can't handle this.
+    // TODO: This is too strict. We can handle some predicates (equality?).
     if (CastOp0->getOpcode() != CastOp1->getOpcode())
       return nullptr;
 
     // Not an extension from the same type?
-    // TODO: Handle this by extending the narrower operand to the type of
-    //       the wider operand.
     Value *Y = CastOp1->getOperand(0);
-    if (X->getType() != Y->getType())
-      return nullptr;
+    Type *XTy = X->getType(), *YTy = Y->getType();
+    if (XTy != YTy) {
+      // One of the casts must have one use because we are creating a new cast.
+      if (!CastOp0->hasOneUse() && !CastOp1->hasOneUse())
+        return nullptr;
+      // Extend the narrower operand to the type of the wider operand.
+      if (XTy->getScalarSizeInBits() < YTy->getScalarSizeInBits())
+        X = Builder.CreateCast(CastOp0->getOpcode(), X, YTy);
+      else if (YTy->getScalarSizeInBits() < XTy->getScalarSizeInBits())
+        Y = Builder.CreateCast(CastOp0->getOpcode(), Y, XTy);
+      else
+        return nullptr;
+    }
 
     // (zext X) == (zext Y) --> X == Y
     // (sext X) == (sext Y) --> X == Y
@@ -4148,7 +4159,7 @@ Instruction *InstCombiner::foldICmpWithCastOp(ICmpInst &ICmp) {
       return new ICmpInst(ICmp.getPredicate(), Op0Src, NewOp1);
   }
 
-  return foldICmpWithZextOrSext(ICmp);
+  return foldICmpWithZextOrSext(ICmp, Builder);
 }
 
 static bool isNeutralValue(Instruction::BinaryOps BinaryOp, Value *RHS) {