Index: clang/lib/CodeGen/TargetInfo.cpp
===================================================================
--- clang/lib/CodeGen/TargetInfo.cpp
+++ clang/lib/CodeGen/TargetInfo.cpp
@@ -2883,6 +2883,7 @@
 
     // Classify the fields one at a time, merging the results.
     unsigned idx = 0;
+    const RecordType *IsUnion = RT->getAsUnionType();
     for (RecordDecl::field_iterator i = RD->field_begin(), e = RD->field_end();
            i != e; ++i, ++idx) {
       uint64_t Offset = OffsetBase + Layout.getFieldOffset(idx);
@@ -2893,14 +2894,16 @@
         continue;
 
       // AMD64-ABI 3.2.3p2: Rule 1. If the size of an object is larger than
-      // four eightbytes, or it contains unaligned fields, it has class MEMORY.
+      // eight eightbytes, or it contains unaligned fields, it has class MEMORY.
       //
-      // The only case a 256-bit wide vector could be used is when the struct
-      // contains a single 256-bit element. Since Lo and Hi logic isn't extended
-      // to work for sizes wider than 128, early check and fallback to memory.
+      // The cases a 256-bit or a 512-bit wide vector could be used is when the
+      // struct contains a single 256-bit or 512-bit element. Since Lo and Hi
+      // logic isn't extended to work for sizes wider than 128, early check and
+      // fallback to memory.
       //
-      if (Size > 128 && (Size != getContext().getTypeSize(i->getType()) ||
-                         Size > getNativeVectorSizeForAVXABI(AVXLevel))) {
+      if (Size > 128 &&
+          ((!IsUnion && Size != getContext().getTypeSize(i->getType())) ||
+           Size > getNativeVectorSizeForAVXABI(AVXLevel))) {
         Lo = Memory;
         postMerge(Size, Lo, Hi);
         return;
Index: clang/test/CodeGen/passing-union.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/passing-union.c
@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -w -fblocks -ffreestanding -triple x86_64-linux-gnu -target-feature +avx -emit-llvm -o %t %s || FileCheck < %t %s --check-prefix=CHECK, AVX
+// RUN: %clang_cc1 -w -fblocks -ffreestanding -triple x86_64-linux-gnu -target-feature +avx512f -emit-llvm -o %t %s || FileCheck < %t %s --check-prefix=CHECK, AVX512
+// This test verifies that passing union parameters through register.
+
+#include <immintrin.h>
+
+union M256 {
+  double d;
+  __m256 m;
+};
+
+union M512 {
+  double d;
+  __m512 m;
+};
+
+extern void foo1(union M256 A);
+extern void foo2(union M512 A);
+union M256 m1;
+union M512 m2;
+// CHECK-LABEL: define dso_local void @test()
+// CHECK:       void @foo1(<4 x double>
+// AVX:         call void @foo2(%union.M512* byval(%union.M512) align 64
+// AVX512:      call void @foo2(<8 x double>
+void test() {
+  foo1(m1);
+  foo2(m2);
+}