diff --git a/llvm/lib/Target/X86/X86CallingConv.td b/llvm/lib/Target/X86/X86CallingConv.td
index 05dd537ec4280..e50e74aafe301 100644
--- a/llvm/lib/Target/X86/X86CallingConv.td
+++ b/llvm/lib/Target/X86/X86CallingConv.td
@@ -444,8 +444,9 @@ def RetCC_X86_64_OCaml : CallingConv<[
   // Promote all types to i64
   CCIfType<[i8, i16, i32], CCPromoteToType<i64>>,
 
-  // Every function preserves the necessary function arguments
-  CCIfType<[i64], CCAssignToReg<[R14, RAX, RBX, RDI, RSI, RDX, RCX, R8, R9, R12, R13]>>,
+  // Runtime registers (R14, R15) are threaded through function calls
+  // See the argument calling conventions for OCaml for more details
+  CCIfType<[i64], CCAssignToReg<[R14, R15, RAX, RBX, RDI, RSI, RDX, RCX, R8, R9, R12, R13]>>,
 
   CCIfType<[f32, f64], CCAssignToReg<[
     XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6, XMM7,
@@ -453,6 +454,11 @@ def RetCC_X86_64_OCaml : CallingConv<[
   ]>>
 ]>;
 
+def RetCC_X86_64_OCaml_C_Call : CallingConv<[
+  CCIfType<[i64], CCAssignToReg<[R14, R15]>>,
+
+  CCDelegateTo<RetCC_X86_64_C>
+]>;
 
 defm X86_32_RegCall :
 	 X86_RegCall_base<RC_X86_32_RegCall>;
@@ -509,6 +515,7 @@ def RetCC_X86_64 : CallingConv<[
 
   // Handle OCaml calls
   CCIfCC<"CallingConv::OCaml", CCDelegateTo<RetCC_X86_64_OCaml>>,
+  CCIfCC<"CallingConv::OCaml_C_Call", CCDelegateTo<RetCC_X86_64_OCaml_C_Call>>,
 
   // Otherwise, drop to normal X86-64 CC
   CCDelegateTo<RetCC_X86_64_C>
@@ -725,9 +732,8 @@ def CC_X86_64_OCaml : CallingConv<[
   // (registers containing pointers to data structures maintained by the
   // runtime) must be preserved through the run of the program. To ensure LLVM
   // doesn't mess with them, they are explicitly threaded through function calls
-  // and returns. These are R14 and R15. Note that R15 isn't listed since it is
-  // not implemented yet.
-  CCIfType<[i64], CCAssignToReg<[R14, RAX, RBX, RDI, RSI, RDX, RCX, R8, R9, R12, R13]>>,
+  // and returns. These are R14 and R15.
+  CCIfType<[i64], CCAssignToReg<[R14, R15, RAX, RBX, RDI, RSI, RDX, RCX, R8, R9, R12, R13]>>,
 
   CCIfType<[f32, f64], CCAssignToReg<[
     XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6, XMM7,
@@ -738,7 +744,7 @@ def CC_X86_64_OCaml : CallingConv<[
 def CC_X86_64_OCaml_C_Call : CallingConv<[
   // OCaml wraps C calls with another function that transfers stack arguments.
   // RAX contains the function address, and R12 contains the size of the stack arguments.
-  CCIfType<[i64], CCAssignToReg<[RAX, R12]>>,
+  CCIfType<[i64], CCAssignToReg<[R14, R15, RAX, R12]>>,
 
   // Follow C convention normally otherwise
   CCDelegateTo<CC_X86_64_C>
@@ -1173,6 +1179,9 @@ def CSR_NoRegs : CalleeSavedRegs<(add)>;
 def CSR_32 : CalleeSavedRegs<(add ESI, EDI, EBX, EBP)>;
 def CSR_64 : CalleeSavedRegs<(add RBX, R12, R13, R14, R15, RBP)>;
 
+// R14 and R15 are used as return registers, so they aren't callee saved.
+def CSR_64_OCaml_C_Call : CalleeSavedRegs<(sub CSR_64, R14, R15)>;
+
 def CSR_64_SwiftError : CalleeSavedRegs<(sub CSR_64, R12)>;
 def CSR_64_SwiftTail : CalleeSavedRegs<(sub CSR_64, R13, R14)>;
 
diff --git a/llvm/lib/Target/X86/X86RegisterInfo.cpp b/llvm/lib/Target/X86/X86RegisterInfo.cpp
index 3ca9a0b09a24a..07872da230769 100644
--- a/llvm/lib/Target/X86/X86RegisterInfo.cpp
+++ b/llvm/lib/Target/X86/X86RegisterInfo.cpp
@@ -302,6 +302,8 @@ X86RegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
   case CallingConv::HiPE:
   case CallingConv::OCaml:
     return CSR_NoRegs_SaveList;
+  case CallingConv::OCaml_C_Call:
+    return CSR_64_OCaml_C_Call_SaveList;
   case CallingConv::AnyReg:
     if (HasAVX)
       return CSR_64_AllRegs_AVX_SaveList;
@@ -426,6 +428,8 @@ X86RegisterInfo::getCallPreservedMask(const MachineFunction &MF,
   case CallingConv::HiPE:
   case CallingConv::OCaml:
     return CSR_NoRegs_RegMask;
+  case CallingConv::OCaml_C_Call:
+    return CSR_64_OCaml_C_Call_RegMask;
   case CallingConv::AnyReg:
     if (HasAVX)
       return CSR_64_AllRegs_AVX_RegMask;