[InstCombine] FoldICmpCstShrCst didn't handle icmps of -1 in the ashr case correctly

llvm-svn: 248073

1 files changed, 10 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 0d56ce5ebe8..ac94cdb93b0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1074,18 +1074,22 @@ Instruction *InstCombiner::FoldICmpCstShrCst(ICmpInst &I, Value *Op, Value *A,
   if (AP1 == AP2)
     return getICmp(I.ICMP_EQ, A, ConstantInt::getNullValue(A->getType()));
 
-  // Get the distance between the highest bit that's set.
   int Shift;
-  // Both the constants are negative, take their positive to calculate log.
   if (IsAShr && AP1.isNegative())
-    // Get the ones' complement of AP2 and AP1 when computing the distance.
-    Shift = (~AP2).logBase2() - (~AP1).logBase2();
+    Shift = AP1.countLeadingOnes() - AP2.countLeadingOnes();
   else
-    Shift = AP2.logBase2() - AP1.logBase2();
+    Shift = AP1.countLeadingZeros() - AP2.countLeadingZeros();
 
   if (Shift > 0) {
-    if (IsAShr ? AP1 == AP2.ashr(Shift) : AP1 == AP2.lshr(Shift))
+    if (IsAShr && AP1 == AP2.ashr(Shift)) {
+      // There are multiple solutions if we are comparing against -1 and the LHS
+      // of the ashr is not a power of two..
+      if (AP1.isAllOnesValue() && !AP2.isPowerOf2())
+        return getICmp(I.ICMP_UGE, A, ConstantInt::get(A->getType(), Shift));
       return getICmp(I.ICMP_EQ, A, ConstantInt::get(A->getType(), Shift));
+    } else 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);