diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -3520,6 +3520,8 @@
   // Add the default headers if dxc_no_stdinc is not set.
   if (!Args.hasArg(options::OPT_dxc_no_stdinc))
     CmdArgs.push_back("-finclude-default-header");
+  // Preserve vec3 for HLSL.
+  CmdArgs.push_back("-fpreserve-vec3-type");
 }
 
 static void RenderARCMigrateToolOptions(const Driver &D, const ArgList &Args,
diff --git a/clang/test/CodeGenHLSL/float3.hlsl b/clang/test/CodeGenHLSL/float3.hlsl
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenHLSL/float3.hlsl
@@ -0,0 +1,11 @@
+// RUN: %clang --driver-mode=dxc -Tlib_6_7 -fcgl  -Fo - %s | FileCheck %s
+
+// Make sure float3 is not changed into float4.
+// CHECK:<3 x float> @"?foo@@YAT?$__vector@M$02@__clang@@T12@@Z"(<3 x float> noundef %a)
+// CHECK:%[[A_ADDR:.+]] = alloca <3 x float>, align 16
+// CHECK-NEXT:store <3 x float> %a, ptr %[[A_ADDR]], align 16
+// CHECK-NEXT:%[[V:[0-9]+]] = load <3 x float>, ptr %[[A_ADDR]], align 16
+// CHECK-NEXT:ret <3 x float> %[[V]]
+float3 foo(float3 a) {
+  return a;
+}