diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -14,6 +14,7 @@
 #include "CGCXXABI.h"
 #include "CGCleanup.h"
 #include "CGDebugInfo.h"
+#include "CGHLSLRuntime.h"
 #include "CGOpenCLRuntime.h"
 #include "CGOpenMPRuntime.h"
 #include "CodeGenFunction.h"
@@ -2588,6 +2589,10 @@
     }
   }
 
+  // Emit HLSL specific initialization for parameters with attributes
+  if (getLangOpts().HLSL && D.hasAttrs())
+    DoStore = DoStore && !CGM.getHLSLRuntime().emitParamAttrs(*this, D, lv);
+  
   // Store the initial value into the alloca.
   if (DoStore)
     EmitStoreOfScalar(ArgVal, lv, /* isInitialization */ true);
diff --git a/clang/lib/CodeGen/CGHLSLRuntime.h b/clang/lib/CodeGen/CGHLSLRuntime.h
--- a/clang/lib/CodeGen/CGHLSLRuntime.h
+++ b/clang/lib/CodeGen/CGHLSLRuntime.h
@@ -26,9 +26,14 @@
 class Type;
 class VarDecl;
 
+namespace CodeGen {
+class LValue;
+}
+
 namespace CodeGen {
 
 class CodeGenModule;
+class CodeGenFunction;
 
 class CGHLSLRuntime {
 protected:
@@ -42,6 +47,9 @@
 
   void annotateHLSLResource(const VarDecl *D, llvm::GlobalVariable *GV);
 
+  /// Returns true if an attribute was handled, false otherwise.
+  bool emitParamAttrs(CodeGenFunction &CGF, const VarDecl &D, LValue &LV);
+
   void finishCodeGen();
 };
 
diff --git a/clang/lib/CodeGen/CGHLSLRuntime.cpp b/clang/lib/CodeGen/CGHLSLRuntime.cpp
--- a/clang/lib/CodeGen/CGHLSLRuntime.cpp
+++ b/clang/lib/CodeGen/CGHLSLRuntime.cpp
@@ -13,8 +13,11 @@
 //===----------------------------------------------------------------------===//
 
 #include "CGHLSLRuntime.h"
+#include "CGValue.h"
+#include "CodeGenFunction.h"
 #include "CodeGenModule.h"
 #include "clang/Basic/TargetOptions.h"
+#include "llvm/IR/IntrinsicsDirectX.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
 
@@ -86,3 +89,15 @@
       Ctx, {ValueAsMetadata::get(GV), MDString::get(Ctx, QT.getAsString()),
             ConstantAsMetadata::get(B.getInt32(Counter))}));
 }
+
+bool CGHLSLRuntime::emitParamAttrs(CodeGenFunction &CGF, const VarDecl &D,
+                                   LValue &LV) {
+  if (D.hasAttr<HLSLSV_GroupIndexAttr>()) {
+    llvm::Function *DxGroupIndex =
+        CGM.getIntrinsic(Intrinsic::dx_flattened_thread_id_in_group);
+    CallInst *CI = CGF.Builder.CreateCall(FunctionCallee(DxGroupIndex));
+    CGF.Builder.CreateStore(CI, LV.getAddress(CGF));
+    return true;
+  }
+  return false;
+}
diff --git a/clang/test/CodeGenHLSL/semantics/GroupIndex-codegen.hlsl b/clang/test/CodeGenHLSL/semantics/GroupIndex-codegen.hlsl
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGenHLSL/semantics/GroupIndex-codegen.hlsl
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-compute -x hlsl -emit-llvm -disable-llvm-passes -o - %s
+
+[numthreads(1,1,1)]
+void main(unsigned GI : SV_GroupIndex) {
+}
+
+// CHECK: %GI.addr = alloca i32, align 4
+// CHECK-NEXT: %0 = call i32 @llvm.dx.flattened.thread.id.in.group()
+// CHECK-NEXT: store i32 %0, ptr %GI.addr, align 4