InstCombine: fold (A >> C) == (B >> C) --> (A^B) < (1 << C) for constant Cs.

This is common in bitfield code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194925 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 18 insertions, 0 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index e624572..226126b 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2948,6 +2948,24 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
                             Builder->CreateTrunc(B, A->getType()));
     }
 
+    // (A >> C) == (B >> C) --> (A^B) u< (1 << C)
+    // For lshr and ashr pairs.
+    if ((match(Op0, m_OneUse(m_LShr(m_Value(A), m_ConstantInt(Cst1)))) &&
+         match(Op1, m_OneUse(m_LShr(m_Value(B), m_Specific(Cst1))))) ||
+        (match(Op0, m_OneUse(m_AShr(m_Value(A), m_ConstantInt(Cst1)))) &&
+         match(Op1, m_OneUse(m_AShr(m_Value(B), m_Specific(Cst1)))))) {
+      unsigned TypeBits = Cst1->getBitWidth();
+      unsigned ShAmt = (unsigned)Cst1->getLimitedValue(TypeBits);
+      if (ShAmt < TypeBits && ShAmt != 0) {
+        ICmpInst::Predicate Pred = I.getPredicate() == ICmpInst::ICMP_NE
+                                       ? ICmpInst::ICMP_UGE
+                                       : ICmpInst::ICMP_ULT;
+        Value *Xor = Builder->CreateXor(A, B, I.getName() + ".unshifted");
+        APInt CmpVal = APInt::getOneBitSet(TypeBits, ShAmt);
+        return new ICmpInst(Pred, Xor, Builder->getInt(CmpVal));
+      }
+    }
+
     // Transform "icmp eq (trunc (lshr(X, cst1)), cst" to
     // "icmp (and X, mask), cst"
     uint64_t ShAmt = 0;