[InstCombine] add code comment for icmp transform; NFC

This was accidentally left out of cc88445
llvm · Jul 1, 2022 · ab372cd · ab372cd
1 parent ac8dab8
commit ab372cd
Showing 1 changed file with 2 additions and 0 deletions.
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1551,6 +1551,8 @@ Instruction *InstCombinerImpl::foldICmpTruncConstant(ICmpInst &Cmp,
   unsigned DstBits = Trunc->getType()->getScalarSizeInBits(),
            SrcBits = X->getType()->getScalarSizeInBits();
   if (Cmp.isEquality() && Trunc->hasOneUse()) {
+    // Canonicalize to a mask and wider compare if the wide type is suitable:
+    // (trunc X to i8) == C --> (X & 0xff) == (zext C)
     if (!X->getType()->isVectorTy() && shouldChangeType(DstBits, SrcBits)) {
       Constant *Mask = ConstantInt::get(X->getType(),
                                         APInt::getLowBitsSet(SrcBits, DstBits));