This patch implements optimization as mentioned in PR19753: Optimize comparisons with "ashr/lshr exact" of a constanst.

Suyog Sarda · Suyog Sarda · commit 3a8c2c1e6cb2 · 2014-07-22T19:19:36.000Z
It handles the errors which were seen in PR19958 where wrong code was being emitted due to earlier patch. Added code for lshr as well as non-exact right shifts. It implements : (icmp eq/ne (ashr/lshr const2, A), const1)" -> (icmp eq/ne A, Log2(const2/const1)) -> (icmp eq/ne A, Log2(const2) - Log2(const1)) Differential Revision: http://reviews.llvm.org/D4068 llvm-svn: 213678
diff --git a/llvm/lib/Transforms/InstCombine/InstCombine.h b/llvm/lib/Transforms/InstCombine/InstCombine.h
@@ -172,6 +172,8 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner
                               ConstantInt *DivRHS);
   Instruction *FoldICmpShrCst(ICmpInst &ICI, BinaryOperator *DivI,
                               ConstantInt *DivRHS);
+  Instruction *FoldICmpCstShrCst(ICmpInst &I, Value *Op, Value *A,
+                                 ConstantInt *CI1, ConstantInt *CI2);
   Instruction *FoldICmpAddOpCst(Instruction &ICI, Value *X, ConstantInt *CI,
                                 ICmpInst::Predicate Pred);
   Instruction *FoldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1044,6 +1044,90 @@ Instruction *InstCombiner::FoldICmpShrCst(ICmpInst &ICI, BinaryOperator *Shr,
   return nullptr;
 }
 
+/// FoldICmpCstShrCst - Handle "(icmp eq/ne (ashr/lshr const2, A), const1)" ->
+/// (icmp eq/ne A, Log2(const2/const1)) ->
+/// (icmp eq/ne A, Log2(const2) - Log2(const1)).
+Instruction *InstCombiner::FoldICmpCstShrCst(ICmpInst &I, Value *Op, Value *A,
+                                             ConstantInt *CI1,
+                                             ConstantInt *CI2) {
+  assert(I.isEquality() && "Cannot fold icmp gt/lt");
+
+  auto getConstant = [&I, this](bool IsTrue) {
+    if (I.getPredicate() == I.ICMP_NE)
+      IsTrue = !IsTrue;
+    return ReplaceInstUsesWith(I, ConstantInt::get(I.getType(), IsTrue));
+  };
+
+  auto getICmp = [&I](CmpInst::Predicate Pred, Value *LHS, Value *RHS) {
+    if (I.getPredicate() == I.ICMP_NE)
+      Pred = CmpInst::getInversePredicate(Pred);
+    return new ICmpInst(Pred, LHS, RHS);
+  };
+
+  APInt AP1 = CI1->getValue();
+  APInt AP2 = CI2->getValue();
+
+  if (!AP1) {
+    if (!AP2) {
+      // Both Constants are 0.
+      return getConstant(true);
+    }
+
+    if (cast<BinaryOperator>(Op)->isExact())
+      return getConstant(false);
+
+    if (AP2.isNegative()) {
+      // MSB is set, so a lshr with a large enough 'A' would be undefined.
+      return getConstant(false);
+    }
+
+    // 'A' must be large enough to shift out the highest set bit.
+    return getICmp(I.ICMP_UGT, A,
+                   ConstantInt::get(A->getType(), AP2.logBase2()));
+  }
+
+  if (!AP2) {
+    // Shifting 0 by any value gives 0.
+    return getConstant(false);
+  }
+
+  bool IsAShr = isa<AShrOperator>(Op);
+  if (AP1 == AP2) {
+    if (AP1.isAllOnesValue() && IsAShr) {
+      // Arithmatic shift of -1 is always -1.
+      return getConstant(true);
+    }
+    return getICmp(I.ICMP_EQ, A, ConstantInt::getNullValue(A->getType()));
+  }
+
+  if (IsAShr) {
+    if (AP1.isNegative() != AP2.isNegative()) {
+      // Arithmetic shift will never change the sign.
+      return getConstant(false);
+    }
+    // Both the constants are negative, take their positive to calculate
+    // log.
+    if (AP1.isNegative()) {
+      AP1 = -AP1;
+      AP2 = -AP2;
+    }
+  }
+
+  if (AP1.ugt(AP2)) {
+    // Right-shifting will not increase the value.
+    return getConstant(false);
+  }
+
+  // Get the distance between the highest bit that's set.
+  int Shift = AP2.logBase2() - AP1.logBase2();
+
+  // Use lshr here, since we've canonicalized to +ve numbers.
+  if (AP1 == AP2.lshr(Shift))
+    return getICmp(I.ICMP_EQ, A, ConstantInt::get(A->getType(), Shift));
+
+  // Shifting const2 will never be equal to const1.
+  return getConstant(false);
+}
 
 /// visitICmpInstWithInstAndIntCst - Handle "icmp (instr, intcst)".
 ///
@@ -2469,6 +2553,15 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
                           Builder->getInt(CI->getValue()-1));
     }
 
+    // (icmp eq/ne (ashr/lshr const2, A), const1)
+    if (I.isEquality()) {
+      ConstantInt *CI2;
+      if (match(Op0, m_AShr(m_ConstantInt(CI2), m_Value(A))) ||
+          match(Op0, m_LShr(m_ConstantInt(CI2), m_Value(A)))) {
+        return FoldICmpCstShrCst(I, Op0, A, CI, CI2);
+      }
+    }
+
     // If this comparison is a normal comparison, it demands all
     // bits, if it is a sign bit comparison, it only demands the sign bit.
     bool UnusedBit;
diff --git a/llvm/test/Transforms/InstCombine/icmp-shr.ll b/llvm/test/Transforms/InstCombine/icmp-shr.ll
