Index: lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- lib/Analysis/BasicAliasAnalysis.cpp
+++ lib/Analysis/BasicAliasAnalysis.cpp
@@ -42,11 +42,10 @@
 /// Enable analysis of recursive PHI nodes.
 static cl::opt<bool> EnableRecPhiAnalysis("basicaa-recphi", cl::Hidden,
                                           cl::init(false));
-
+#define DEBUG_TYPE "basicaa"
 /// SearchLimitReached / SearchTimes shows how often the limit of
 /// to decompose GEPs is reached. It will affect the precision
 /// of basic alias analysis.
-#define DEBUG_TYPE "basicaa"
 STATISTIC(SearchLimitReached, "Number of times the limit to "
                               "decompose GEPs is reached");
 STATISTIC(SearchTimes, "Number of times a GEP is decomposed");
@@ -319,6 +318,17 @@
   return V;
 }
 
+/// To ensure a pointer offset fits in an integer of size PointerSize
+/// (in bits) when that size is smaller than 64. This is an issue in
+/// particular for 32b programs with negative indices that rely on two's
+/// complement wrap-arounds for correct alias information.
+static int64_t adjustToPointerSize(int64_t Offset, unsigned PointerSize) {
+  assert(PointerSize <= 64 && "Invalide PointerSize!");
+  unsigned ShiftBits = 64 - PointerSize;
+  Offset <<= ShiftBits;
+  return Offset >> ShiftBits;
+}
+
 /// If V is a symbolic pointer expression, decompose it into a base pointer
 /// with a constant offset and a number of scaled symbolic offsets.
 ///
@@ -387,6 +397,7 @@
     unsigned AS = GEPOp->getPointerAddressSpace();
     // Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
     gep_type_iterator GTI = gep_type_begin(GEPOp);
+    unsigned PointerSize = DL.getPointerSizeInBits(AS);
     for (User::const_op_iterator I = GEPOp->op_begin() + 1, E = GEPOp->op_end();
          I != E; ++I) {
       const Value *Index = *I;
@@ -415,7 +426,6 @@
       // If the integer type is smaller than the pointer size, it is implicitly
       // sign extended to pointer size.
       unsigned Width = Index->getType()->getIntegerBitWidth();
-      unsigned PointerSize = DL.getPointerSizeInBits(AS);
       if (PointerSize > Width)
         SExtBits += PointerSize - Width;
 
@@ -457,6 +467,8 @@
       }
     }
 
+    // Take care of wrap-arounds
+    BaseOffs = adjustToPointerSize(BaseOffs, PointerSize);
     // Analyze the base pointer next.
     V = GEPOp->getOperand(0);
   } while (--MaxLookup);
Index: test/Analysis/BasicAA/noalias-wraparound-bug.ll
===================================================================
--- /dev/null
+++ test/Analysis/BasicAA/noalias-wraparound-bug.ll
@@ -0,0 +1,24 @@
+; RUN: opt -S -basicaa -gvn < %s | FileCheck %s
+
+target datalayout = "e-m:o-p:32:32-f64:32:64-f80:128-n8:16:32-S128"
+target triple = "i386-apple-macosx10.6.0"
+
+; We incorrectly returned noalias in the example below for "tmp5" and
+; "tmp12" returning i32 32, since basicaa converted the offsets to 64b
+; and missed the wrap-around
+
+define i32 @foo(i8* %buffer) {
+entry:
+  %tmp2 = getelementptr i8, i8* %buffer, i32 -2071408432
+  %tmp3 = bitcast i8* %tmp2 to i32*
+  %tmp4 = getelementptr i8, i8* %buffer, i32 128
+  %tmp5 = bitcast i8* %tmp4 to i32*
+  store i32 32, i32* %tmp5, align 4
+  %tmp12 = getelementptr i32, i32* %tmp3, i32 -1629631508
+  store i32 28, i32* %tmp12, align 4
+  %tmp13 = getelementptr i8, i8* %buffer, i32 128
+  %tmp14 = bitcast i8* %tmp13 to i32*
+  %tmp2083 = load i32, i32* %tmp14, align 4
+; CHECK: ret i32 28
+  ret i32 %tmp2083
+}