[msan] Change va_start/va_copy shadow memset alignment to 8.

This fixes va_start/va_copy of a va_list field which happens to not
be laid out at a 16-byte boundary.

Differential Revision: http://llvm-reviews.chandlerc.com/D276

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172128 91177308-0d34-0410-b5e6-96231b3b80d8

Author: pcc

lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -1774,7 +1774,7 @@ struct VarArgAMD64Helper : public VarArgHelper {
     // Unpoison the whole __va_list_tag.
     // FIXME: magic ABI constants.
     IRB.CreateMemSet(ShadowPtr, Constant::getNullValue(IRB.getInt8Ty()),
-                     /* size */24, /* alignment */16, false);
+                     /* size */24, /* alignment */8, false);
   }
 
   void visitVACopyInst(VACopyInst &I) {
@@ -1785,7 +1785,7 @@ struct VarArgAMD64Helper : public VarArgHelper {
     // Unpoison the whole __va_list_tag.
     // FIXME: magic ABI constants.
     IRB.CreateMemSet(ShadowPtr, Constant::getNullValue(IRB.getInt8Ty()),
-                     /* size */ 24, /* alignment */ 16, false);
+                     /* size */24, /* alignment */8, false);
   }
 
   void finalizeInstrumentation() {

test/Instrumentation/MemorySanitizer/msan_basic.ll

@@ -534,3 +534,16 @@ define <8 x i8*> @VectorOfPointers(<8 x i8*>* %p) nounwind uwtable {
 ; CHECK: load <8 x i8*>*
 ; CHECK: store <8 x i64> {{.*}} @__msan_retval_tls
 ; CHECK: ret <8 x i8*>
+
+; Test handling of va_copy.
+
+declare void @llvm.va_copy(i8*, i8*) nounwind
+
+define void @VACopy(i8* %p1, i8* %p2) nounwind uwtable {
+  call void @llvm.va_copy(i8* %p1, i8* %p2) nounwind
+  ret void
+}
+
+; CHECK: @VACopy
+; CHECK: call void @llvm.memset.p0i8.i64({{.*}}, i8 0, i64 24, i32 8, i1 false)
+; CHECK: ret void