diff --git a/include/llvm/IR/CallingConv.h b/include/llvm/IR/CallingConv.h
index 585fc5ea841..35c7df9b2ac 100644
--- a/include/llvm/IR/CallingConv.h
+++ b/include/llvm/IR/CallingConv.h
@@ -51,6 +51,9 @@ namespace CallingConv {
     // (HiPE).
     HiPE = 11,
 
+    // WebKit JS - Calling convention for stack based JavaScript calls
+    WebKit_JS = 12,
+
     // Target - This is the start of the target-specific calling conventions,
     // e.g. fastcall and thiscall on X86.
     FirstTargetCC = 64,
diff --git a/lib/AsmParser/LLLexer.cpp b/lib/AsmParser/LLLexer.cpp
index 99bff45792c..1f81800053c 100644
--- a/lib/AsmParser/LLLexer.cpp
+++ b/lib/AsmParser/LLLexer.cpp
@@ -559,6 +559,7 @@ lltok::Kind LLLexer::LexIdentifier() {
   KEYWORD(intel_ocl_bicc);
   KEYWORD(x86_64_sysvcc);
   KEYWORD(x86_64_win64cc);
+  KEYWORD(webkit_jscc);
 
   KEYWORD(cc);
   KEYWORD(c);
diff --git a/lib/AsmParser/LLParser.cpp b/lib/AsmParser/LLParser.cpp
index 74c0ea4af52..66136521159 100644
--- a/lib/AsmParser/LLParser.cpp
+++ b/lib/AsmParser/LLParser.cpp
@@ -1349,6 +1349,7 @@ bool LLParser::ParseOptionalVisibility(unsigned &Res) {
 ///   ::= 'spir_kernel'
 ///   ::= 'x86_64_sysvcc'
 ///   ::= 'x86_64_win64cc'
+///   ::= 'webkit_jscc'
 ///   ::= 'cc' UINT
 ///
 bool LLParser::ParseOptionalCallingConv(CallingConv::ID &CC) {
@@ -1371,6 +1372,7 @@ bool LLParser::ParseOptionalCallingConv(CallingConv::ID &CC) {
   case lltok::kw_intel_ocl_bicc: CC = CallingConv::Intel_OCL_BI; break;
   case lltok::kw_x86_64_sysvcc:  CC = CallingConv::X86_64_SysV; break;
   case lltok::kw_x86_64_win64cc: CC = CallingConv::X86_64_Win64; break;
+  case lltok::kw_webkit_jscc:    CC = CallingConv::WebKit_JS; break;
   case lltok::kw_cc: {
       unsigned ArbitraryCC;
       Lex.Lex();
diff --git a/lib/AsmParser/LLToken.h b/lib/AsmParser/LLToken.h
index e1382fdb597..c31883cfc02 100644
--- a/lib/AsmParser/LLToken.h
+++ b/lib/AsmParser/LLToken.h
@@ -92,6 +92,7 @@ namespace lltok {
     kw_ptx_kernel, kw_ptx_device,
     kw_spir_kernel, kw_spir_func,
     kw_x86_64_sysvcc, kw_x86_64_win64cc,
+    kw_webkit_jscc,
 
     // Attributes:
     kw_attributes,
diff --git a/lib/Target/X86/X86CallingConv.td b/lib/Target/X86/X86CallingConv.td
index 2795b4cdd8f..f5c8d9fda43 100644
--- a/lib/Target/X86/X86CallingConv.td
+++ b/lib/Target/X86/X86CallingConv.td
@@ -151,6 +151,15 @@ def RetCC_X86_64_HiPE : CallingConv<[
   CCIfType<[i64], CCAssignToReg<[R15, RBP, RAX, RDX]>>
 ]>;
 
+// X86-64 WebKit_JS return-value convention.
+def RetCC_X86_64_WebKit_JS : CallingConv<[
+  // Promote all types to i64
+  CCIfType<[i8, i16, i32], CCPromoteToType<i64>>,
+
+  // Return: RAX
+  CCIfType<[i64], CCAssignToReg<[RAX]>>
+]>;
+
 // This is the root return-value convention for the X86-32 backend.
 def RetCC_X86_32 : CallingConv<[
   // If FastCC, use RetCC_X86_32_Fast.
@@ -167,6 +176,9 @@ def RetCC_X86_64 : CallingConv<[
   // HiPE uses RetCC_X86_64_HiPE
   CCIfCC<"CallingConv::HiPE", CCDelegateTo<RetCC_X86_64_HiPE>>,
 
+  // Handle JavaScript calls.
+  CCIfCC<"CallingConv::WebKit_JS", CCDelegateTo<RetCC_X86_64_WebKit_JS>>,
+
   // Handle explicit CC selection
   CCIfCC<"CallingConv::X86_64_Win64", CCDelegateTo<RetCC_X86_Win64_C>>,
   CCIfCC<"CallingConv::X86_64_SysV", CCDelegateTo<RetCC_X86_64_C>>,
@@ -329,6 +341,15 @@ def CC_X86_64_HiPE : CallingConv<[
   CCIfType<[i32, i64, f32, f64], CCAssignToStack<8, 8>>
 ]>;
 
+def CC_X86_64_WebKit_JS : CallingConv<[
+  // Promote i8/i16 arguments to i32.
+  CCIfType<[i8, i16], CCPromoteToType<i32>>,
+
+  // Integer/FP values are always stored in stack slots that are 8 bytes in size
+  // and 8-byte aligned.
+  CCIfType<[i32, i64, f32, f64], CCAssignToStack<8, 8>>
+]>;
+
 //===----------------------------------------------------------------------===//
 // X86 C Calling Convention
 //===----------------------------------------------------------------------===//
@@ -520,6 +541,7 @@ def CC_X86_32 : CallingConv<[
 def CC_X86_64 : CallingConv<[
   CCIfCC<"CallingConv::GHC", CCDelegateTo<CC_X86_64_GHC>>,
   CCIfCC<"CallingConv::HiPE", CCDelegateTo<CC_X86_64_HiPE>>,
+  CCIfCC<"CallingConv::WebKit_JS", CCDelegateTo<CC_X86_64_WebKit_JS>>,
   CCIfCC<"CallingConv::X86_64_Win64", CCDelegateTo<CC_X86_Win64_C>>,
   CCIfCC<"CallingConv::X86_64_SysV", CCDelegateTo<CC_X86_64_C>>,
 
diff --git a/test/CodeGen/X86/patchpoint.ll b/test/CodeGen/X86/patchpoint.ll
index 9226adfdaef..36cca84932d 100644
--- a/test/CodeGen/X86/patchpoint.ll
+++ b/test/CodeGen/X86/patchpoint.ll
@@ -42,6 +42,26 @@ entry:
   ret void
 }
 
+; Test the webkit_jscc calling convention.
+; Two arguments will be pushed on the stack.
+; Return value in $rax.
+define void @jscall_patchpoint_codegen(i64 %p1, i64 %p2, i64 %p3, i64 %p4) {
+entry:
+; CHECK-LABEL: _jscall_patchpoint_codegen:
+; CHECK: Ltmp
+; CHECK: movq %r{{.+}}, 8(%rsp)
+; CHECK: movq %r{{.+}}, (%rsp)
+; CHECK: movq $-559038736, %rax
+; CHECK: Ltmp
+; CHECK: callq *%rax
+; CHECK: movq %rax, 8(%rsp)
+; CHECK: callq
+  %resolveCall2 = inttoptr i64 -559038736 to i8*
+  %result = tail call webkit_jscc i64 (i32, i32, i8*, i32, ...)* @llvm.experimental.patchpoint.i64(i32 5, i32 12, i8* %resolveCall2, i32 2, i64 %p1, i64 %p2)
+  %resolveCall3 = inttoptr i64 -559038737 to i8*
+  tail call webkit_jscc void (i32, i32, i8*, i32, ...)* @llvm.experimental.patchpoint.void(i32 6, i32 12, i8* %resolveCall3, i32 2, i64 %p1, i64 %result)
+  ret void
+}
 
 declare void @llvm.experimental.stackmap(i32, i32, ...)
 declare void @llvm.experimental.patchpoint.void(i32, i32, i8*, i32, ...)