[msan] Fix select instrumentation.

Select condition shadow was being ignored resulting in false negatives.
This change OR-s sign-extended condition shadow into the result shadow.

llvm-svn: 189785

1 files changed, 6 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index ae73a2e9c9e..f2cf7a76e54 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -1743,9 +1743,12 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
 
   void visitSelectInst(SelectInst& I) {
     IRBuilder<> IRB(&I);
-    setShadow(&I,  IRB.CreateSelect(I.getCondition(),
-              getShadow(I.getTrueValue()), getShadow(I.getFalseValue()),
-              "_msprop"));
+    // a = select b, c, d
+    // Sa = (sext Sb) | (select b, Sc, Sd)
+    Value *S = IRB.CreateSelect(I.getCondition(), getShadow(I.getTrueValue()),
+                                getShadow(I.getFalseValue()));
+    Value *S2 = IRB.CreateSExt(getShadow(I.getCondition()), S->getType());
+    setShadow(&I, IRB.CreateOr(S, S2, "_msprop"));
     if (MS.TrackOrigins) {
       // Origins are always i32, so any vector conditions must be flattened.
       // FIXME: consider tracking vector origins for app vectors?