Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1085,6 +1085,7 @@
     return getICmp(I.ICMP_EQ, A, ConstantInt::getNullValue(A->getType()));
   }
 
+  bool AreBothNegative = false;
   if (IsAShr) {
     if (AP1.isNegative() != AP2.isNegative()) {
       // Arithmetic shift will never change the sign.
@@ -1095,6 +1096,7 @@
     if (AP1.isNegative()) {
       AP1 = -AP1;
       AP2 = -AP2;
+      AreBothNegative = true;
     }
   }
 
@@ -1106,8 +1108,17 @@
   // 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))
+  // Revert the canonicalization if both AP1 and AP2 are negative,
+  // as (-AP2 >> Shift == -AP1) doesn't imply (AP2 >> Shift == AP1).
+  bool CanFoldCompare = false;
+  if (AreBothNegative) {
+    AP1 = -AP1;
+    AP2 = -AP2;
+    CanFoldCompare = AP1 == AP2.ashr(Shift);
+  } else
+    CanFoldCompare = AP1 == AP2.lshr(Shift); 
+
+  if (CanFoldCompare)
     return getICmp(I.ICMP_EQ, A, ConstantInt::get(A->getType(), Shift));
 
   // Shifting const2 will never be equal to const1.
Index: test/Transforms/InstCombine/icmp-shr.ll
===================================================================
--- test/Transforms/InstCombine/icmp-shr.ll
+++ test/Transforms/InstCombine/icmp-shr.ll
@@ -675,3 +675,19 @@
  %cmp = icmp ne i8 %shr, -30
  ret i1 %cmp
 }
+
+; Don't try to fold the entire body of function @PR20945 into a
+; single `ret i1 true` statement.
+; If %B is equal to 1, then this function would return false.
+; As a consequence, the instruction combiner is not allowed to fold %cmp
+; to 'true'. Instead, it should replace %cmp with a simpler comparison
+; between %B and 1.
+
+; CHECK-LABEL: @PR20945(
+; CHECK: icmp ne i32 %B, 1
+define i1 @PR20945(i32 %B) {
+  %shr = ashr i32 -9, %B
+  %cmp = icmp ne i32 %shr, -5
+  ret i1 %cmp
+}
+