[InstCombine] Fix wrong folding of constant comparisons involving ashr and negative values.

This patch fixes a bug in method InstCombiner::FoldCmpCstShrCst where we
wrongly computed the distance between the highest bits set of two negative
values.

This fixes PR21222.

Differential Revision: http://reviews.llvm.org/D5700

llvm-svn: 219406

2 files changed, 10 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 00623b1cbf6..2918f031b3f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1107,7 +1107,8 @@ Instruction *InstCombiner::FoldICmpCstShrCst(ICmpInst &I, Value *Op, Value *A,
   // Get the distance between the highest bit that's set.
   int Shift;
   if (IsNegative)
-    Shift = (-AP2).logBase2() - (-AP1).logBase2();
+    // Get the ones' complement of AP2 and AP1 when computing the distance.
+    Shift = (~AP2).logBase2() - (~AP1).logBase2();
   else
     Shift = AP2.logBase2() - AP1.logBase2();
 
diff --git a/llvm/test/Transforms/InstCombine/icmp-shr.ll b/llvm/test/Transforms/InstCombine/icmp-shr.ll
index 8e7c906a545..7562f24a35d 100644
--- a/llvm/test/Transforms/InstCombine/icmp-shr.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-shr.ll
@@ -688,3 +688,11 @@ define i1 @PR20945(i32 %B) {
   %cmp = icmp ne i32 %shr, -5
   ret i1 %cmp
 }
+
+; CHECK-LABEL: @PR21222
+; CHECK: icmp eq i32 %B, 6
+define i1 @PR21222(i32 %B) {
+  %shr = ashr i32 -93, %B
+  %cmp = icmp eq i32 %shr, -2
+  ret i1 %cmp
+}