Fixes PR2395. Looking for a constant in a GEP tail (when the first GEP

is longer than the second one) should stop after finding one. Added break 
instruction guarantees it. It also changes difference between offsets to 
absolute value of this difference in the condition.

llvm-svn: 51875

2 files changed, 22 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 21108c64535..599a6f650fd 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -687,7 +687,7 @@ BasicAliasAnalysis::CheckGEPInstructions(
         if (isa<ConstantInt>(GEP1Ops[i]) && 
             !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
           // Yup, there's a constant in the tail.  Set all variables to
-          // constants in the GEP instruction to make it suiteable for
+          // constants in the GEP instruction to make it suitable for
           // TargetData::getIndexedOffset.
           for (i = 0; i != MaxOperands; ++i)
             if (!isa<ConstantInt>(GEP1Ops[i]))
@@ -702,9 +702,15 @@ BasicAliasAnalysis::CheckGEPInstructions(
           int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
                                                 MinOperands);
 
+          // Make sure we compare the absolute difference.
+          if (Offset1 > Offset2)
+            std::swap(Offset1, Offset2);
+
           // If the tail provided a bit enough offset, return noalias!
           if ((uint64_t)(Offset2-Offset1) >= SizeMax)
             return NoAlias;
+          // Otherwise break - we don't look for another constant in the tail.
+          break;
         }
     }
 
diff --git a/llvm/test/Analysis/BasicAA/2008-06-02-GEPTailCrash.ll b/llvm/test/Analysis/BasicAA/2008-06-02-GEPTailCrash.ll
new file mode 100644
index 00000000000..40d1e32d673
--- /dev/null
+++ b/llvm/test/Analysis/BasicAA/2008-06-02-GEPTailCrash.ll
@@ -0,0 +1,15 @@
+; RUN: llvm-as < %s | opt -gvn -disable-output
+; PR2395
+
+target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32"
+target triple = "i686-pc-linux-gnu"
+	%struct.S291 = type <{ %union.anon, i32 }>
+	%union.anon = type {  }
+@a291 = external global [5 x %struct.S291]		; <[5 x %struct.S291]*> [#uses=2]
+
+define void @test291() nounwind  {
+entry:
+	store i32 1138410269, i32* getelementptr ([5 x %struct.S291]* @a291, i32 0, i32 2, i32 1)
+	%tmp54 = load i32* bitcast (%struct.S291* getelementptr ([5 x %struct.S291]* @a291, i32 0, i32 2) to i32*), align 4		; <i32> [#uses=0]
+	unreachable
+}