Added support to optimize comparisons with "lshr exact" of a constant.

Patch by Rahul Jain.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210040 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -2318,6 +2318,32 @@ static bool swapMayExposeCSEOpportunities(const Value * Op0,
   return GlobalSwapBenefits > 0;
 }
 
+// Helper function to check whether Op represents a lshr/ashr exact
+// instruction. For example:
+// (icmp (ashr exact const2, A), const1) -> icmp A, Log2(const2/const1)
+// Here if Op represents -> (ashr exact const2, A), and CI represents
+// const1, we compute Quotient as const2/const1.
+
+static bool checkShrExact(Value *Op, APInt &Quotient, const ConstantInt *CI,
+                          Value *&A) {
+
+  ConstantInt *CI2;
+  if (match(Op, m_AShr(m_ConstantInt(CI2), m_Value(A))) &&
+      (cast<BinaryOperator>(Op)->isExact())) {
+    Quotient = CI2->getValue().sdiv(CI->getValue());
+    return true;
+  }
+
+  // Handle the case for lhsr.
+  if (match(Op, m_LShr(m_ConstantInt(CI2), m_Value(A))) &&
+      (cast<BinaryOperator>(Op)->isExact())) {
+    Quotient = CI2->getValue().udiv(CI->getValue());
+    return true;
+  }
+
+  return false;
+}
+
 Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
   bool Changed = false;
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
@@ -2443,13 +2469,10 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
     // (icmp (ashr exact const2, A), const1) -> icmp A, Log2(const2/const1)
     // Cases where const1 doesn't divide const2 exactly or Quotient is not
     // exact of log2 are handled by SimplifyICmpInst call above where we
-    // return false.
-    // TODO: Handle this for lshr exact with udiv.
+    // return false. Similar for lshr.
     {
-      ConstantInt *CI2;
-      if (match(Op0, m_AShr(m_ConstantInt(CI2), m_Value(A))) &&
-          (cast<BinaryOperator>(Op0)->isExact())) {
-        APInt Quotient = CI2->getValue().sdiv(CI->getValue());
+      APInt Quotient;
+      if (checkShrExact(Op0, Quotient, CI, A)) {
         unsigned shift = Quotient.logBase2();
         return new ICmpInst(I.getPredicate(), A,
                             ConstantInt::get(A->getType(), shift));

test/Transforms/InstCombine/icmp.ll

@@ -1390,3 +1390,11 @@ define i1 @exact_ashr_eq_false(i32 %a) {
   %cmp = icmp eq i32 %shr, -15
   ret i1 %cmp
 }
+
+; CHECK-LABEL: @exact_lhsr
+; CHECK-NEXT: icmp eq i32 %a, 3
+define i1 @exact_lhsr(i32 %a) {
+  %shr = lshr exact i32 80, %a
+  %cmp = icmp eq i32 %shr, 10
+  ret i1 %cmp
+}