diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -3726,10 +3726,14 @@
               getShadowOriginPtr(A, IRB, IRB.getInt8Ty(), Alignment,
                                  /*isStore*/ false)
                   .first;
-
-          Store = IRB.CreateMemCpy(ArgShadowBase, Alignment, AShadowPtr,
-                                   Alignment, Size);
-          // TODO(glider): need to copy origins.
+          if (!PropagateShadow) {
+            Store = IRB.CreateMemSet(ArgShadowBase,
+                                     Constant::getNullValue(IRB.getInt8Ty()),
+                                     Size, Alignment);
+          } else {
+            Store = IRB.CreateMemCpy(ArgShadowBase, Alignment, AShadowPtr,
+                                     Alignment, Size);
+          }
         } else {
           // Any other parameters mean we need bit-grained tracking of uninit
           // data
diff --git a/llvm/test/Instrumentation/MemorySanitizer/byval.ll b/llvm/test/Instrumentation/MemorySanitizer/byval.ll
--- a/llvm/test/Instrumentation/MemorySanitizer/byval.ll
+++ b/llvm/test/Instrumentation/MemorySanitizer/byval.ll
@@ -79,12 +79,11 @@
   ret void
 }
 
-; FIXME: Shadow for byval should be reset not copied before the call.
 define void @ByValForwardByValNoSanitize(i32, i128* byval(i128) %p) {
 ; CHECK-LABEL: @ByValForwardByValNoSanitize(
 ; CHECK-NEXT:  entry:
 ; CHECK:         call void @llvm.memset.p0i8.i64(i8* align 8 {{.*}}, i8 0, i64 16, i1 false)
-; CHECK:         call void @llvm.memcpy.p0i8.p0i8.i64(i8* bitcast ([100 x i64]* @__msan_param_tls to i8*), i8* {{.*}}, i64 16, i1 false) 
+; CHECK:         call void @llvm.memset.p0i8.i64(i8* bitcast ([100 x i64]* @__msan_param_tls to i8*), i8 0, i64 16, i1 false)
 ; CHECK:         store i32 0, i32* getelementptr inbounds ([200 x i32], [200 x i32]* @__msan_param_origin_tls, i32 0, i32 0)
 ; CHECK-NEXT:    call void @FnByVal(
 ; CHECK-NEXT:    ret void