[PATCH] [InstCombine] Constant comparison involving "lshr exact"

[Rahul Jain]

Hi Rafael,

Thanks for your valuable comments.
As per your suggestion, I implemented a helper function.

Please help review the same.

Thanks,
Rahul

http://reviews.llvm.org/D3987

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/icmp.ll

Index: test/Transforms/InstCombine/icmp.ll
===================================================================
--- test/Transforms/InstCombine/icmp.ll
+++ test/Transforms/InstCombine/icmp.ll
@@ -1373,3 +1373,11 @@
   %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
+}
Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2318,6 +2318,32 @@
   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.
+  else 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 @@
     // (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));
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