Index: lib/Analysis/MemoryDependenceAnalysis.cpp
===================================================================
--- lib/Analysis/MemoryDependenceAnalysis.cpp
+++ lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -283,6 +283,12 @@
                                 const DataLayout &TD) {
   // We can only extend simple integer loads.
   if (!isa<IntegerType>(LI->getType()) || !LI->isSimple()) return 0;
+
+  // Load widening is hostile to ThreadSanitizer: it may cause false positives
+  // or make the reports more cryptic (access sizes are wrong).
+  if (LI->getParent()->getParent()->getAttributes().
+      hasAttribute(AttributeSet::FunctionIndex, Attribute::ThreadSafety))
+    return 0;
   
   // Get the base of this load.
   int64_t LIOffs = 0;
Index: test/Instrumentation/ThreadSanitizer/tsan-vs-gvn.ll
===================================================================
--- test/Instrumentation/ThreadSanitizer/tsan-vs-gvn.ll
+++ test/Instrumentation/ThreadSanitizer/tsan-vs-gvn.ll
@@ -0,0 +1,26 @@
+; RUN: opt < %s -basicaa -gvn -tsan -S | FileCheck %s
+; TSAN conflicts with load widening. Make sure the load widening is off with -tsan.
+
+; 32-bit little endian target.
+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-f80:128:128-n8:16:32"
+
+%struct_of_8_bytes_4_aligned = type { i32, i8, i8, i8, i8}
+
+@f = global %struct_of_8_bytes_4_aligned zeroinitializer, align 4
+
+; Accessing bytes 4 and 6, not ok to widen to i32 if thread_safety is set.
+
+define i32 @test_widening_bad(i8* %P) nounwind ssp noredzone thread_safety {
+entry:
+  %tmp = load i8* getelementptr inbounds (%struct_of_8_bytes_4_aligned* @f, i64 0, i32 1), align 4
+  %conv = zext i8 %tmp to i32
+  %tmp1 = load i8* getelementptr inbounds (%struct_of_8_bytes_4_aligned* @f, i64 0, i32 3), align 1
+  %conv2 = zext i8 %tmp1 to i32
+  %add = add nsw i32 %conv, %conv2
+  ret i32 %add
+; CHECK: @test_widening_bad
+; CHECK: call void @__tsan_read1
+; CHECK: call void @__tsan_read1
+; CHECK-NOT: call void @__tsan_read4
+; CHECK: ret i32
+}