diff --git a/lib/Target/X86/X86Subtarget.h b/lib/Target/X86/X86Subtarget.h
index d420abbe143..f026d4295f7 100644
--- a/lib/Target/X86/X86Subtarget.h
+++ b/lib/Target/X86/X86Subtarget.h
@@ -447,8 +447,26 @@ public:
   }
 
   bool isCallingConvWin64(CallingConv::ID CC) const {
-    return (isTargetWin64() && CC != CallingConv::X86_64_SysV) ||
-           CC == CallingConv::X86_64_Win64;
+    switch (CC) {
+    // On Win64, all these conventions just use the default convention.
+    case CallingConv::C:
+    case CallingConv::Fast:
+    case CallingConv::X86_FastCall:
+    case CallingConv::X86_StdCall:
+    case CallingConv::X86_ThisCall:
+    case CallingConv::X86_VectorCall:
+    case CallingConv::Intel_OCL_BI:
+      return isTargetWin64();
+    // This convention allows using the Win64 convention on other targets.
+    case CallingConv::X86_64_Win64:
+      return true;
+    // This convention allows using the SysV convention on Windows targets.
+    case CallingConv::X86_64_SysV:
+      return false;
+    // Otherwise, who knows what this is.
+    default:
+      return false;
+    }
   }
 
   /// ClassifyGlobalReference - Classify a global variable reference for the
diff --git a/test/CodeGen/X86/webkit-jscc.ll b/test/CodeGen/X86/webkit-jscc.ll
new file mode 100644
index 00000000000..a58c53e024e
--- /dev/null
+++ b/test/CodeGen/X86/webkit-jscc.ll
@@ -0,0 +1,18 @@
+; RUN: llc -mtriple=x86_64-apple-darwin -mcpu=corei7 < %s | FileCheck %s
+; RUN: llc -mtriple=x86_64-linux-gnu -mcpu=corei7 < %s | FileCheck %s
+; RUN: llc -mtriple=x86_64-windows-gnu -mcpu=corei7 < %s | FileCheck %s
+; RUN: llc -mtriple=x86_64-windows-msvc -mcpu=corei7 < %s | FileCheck %s
+
+define webkit_jscc i32 @simple_jscall(i32 %a, i32 %b, i32 %c) {
+  %ab = add i32 %a, %b
+  %abc = add i32 %ab, %c
+  ret i32 %abc
+}
+
+; 32-bit integers are only aligned to 4 bytes, even on x64. They are *not*
+; promoted to i64.
+
+; CHECK: simple_jscall:
+; CHECK: addl 8(%rsp), %eax
+; CHECK-NEXT: addl 12(%rsp), %eax
+; CHECK-NEXT: retq