Index: include/llvm/ADT/Triple.h
===================================================================
--- include/llvm/ADT/Triple.h
+++ include/llvm/ADT/Triple.h
@@ -174,7 +174,8 @@
     Cygnus,
     AMDOpenCL,
     CoreCLR,
-    LastEnvironmentType = CoreCLR
+    ELFIAMCU,
+    LastEnvironmentType = ELFIAMCU
   };
   enum ObjectFormatType {
     UnknownObjectFormat,
@@ -431,6 +432,10 @@
     return getOS() == Triple::Bitrig;
   }
 
+  bool isEnvironmentIAMCU() const {
+    return getEnvironment() == Triple::ELFIAMCU;
+  }
+
   bool isWindowsMSVCEnvironment() const {
     return getOS() == Triple::Win32 &&
            (getEnvironment() == Triple::UnknownEnvironment ||
Index: include/llvm/Target/TargetLowering.h
===================================================================
--- include/llvm/Target/TargetLowering.h
+++ include/llvm/Target/TargetLowering.h
@@ -2435,6 +2435,13 @@
 
   };
 
+  // Mark inreg arguments for lib-calls. For normal calls this is done by
+  // the frontend ABI code.
+  virtual void markInRegArguments(SelectionDAG &DAG, 
+                 TargetLowering::ArgListTy &Args) const {
+    return;
+  }
+
   /// This function lowers an abstract call to a function into an actual call.
   /// This returns a pair of operands.  The first element is the return value
   /// for the function (if RetTy is not VoidTy).  The second element is the
Index: lib/CodeGen/SelectionDAG/TargetLowering.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -100,6 +100,9 @@
     Entry.isZExt = !shouldSignExtendTypeInLibCall(Ops[i].getValueType(), isSigned);
     Args.push_back(Entry);
   }
+
+  markInRegArguments(DAG, Args);
+
   if (LC == RTLIB::UNKNOWN_LIBCALL)
     report_fatal_error("Unsupported library call operation!");
   SDValue Callee = DAG.getExternalSymbol(getLibcallName(LC),
Index: lib/Support/Triple.cpp
===================================================================
--- lib/Support/Triple.cpp
+++ lib/Support/Triple.cpp
@@ -202,6 +202,7 @@
   case Cygnus: return "cygnus";
   case AMDOpenCL: return "amdopencl";
   case CoreCLR: return "coreclr";
+  case ELFIAMCU: return "elfiamcu";
   }
 
   llvm_unreachable("Invalid EnvironmentType!");
@@ -453,6 +454,7 @@
     .StartsWith("cygnus", Triple::Cygnus)
     .StartsWith("amdopencl", Triple::AMDOpenCL)
     .StartsWith("coreclr", Triple::CoreCLR)
+    .StartsWith("elfiamcu", Triple::ELFIAMCU)
     .Default(Triple::UnknownEnvironment);
 }
 
Index: lib/Target/X86/X86ISelLowering.h
===================================================================
--- lib/Target/X86/X86ISelLowering.h
+++ lib/Target/X86/X86ISelLowering.h
@@ -917,6 +917,9 @@
 
     bool isIntDivCheap(EVT VT, AttributeSet Attr) const override;
 
+    void markInRegArguments(SelectionDAG &DAG, TargetLowering::ArgListTy& Args)
+      const override;
+
   protected:
     std::pair<const TargetRegisterClass *, uint8_t>
     findRepresentativeClass(const TargetRegisterInfo *TRI,
Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -27508,3 +27508,27 @@
                                    Attribute::MinSize);
   return OptSize && !VT.isVector();
 }
+
+void X86TargetLowering::markInRegArguments(SelectionDAG &DAG,
+       TargetLowering::ArgListTy& Args) const {
+  // The MCU psABI requires some arguments to be passed in-register.
+  // For regular calls, the inreg arguments are marked by the front-end.
+  // However, for compiler generated library calls, we have to patch this
+  // up here.
+  if (!Subtarget->isTargetMCU() || !Args.size())
+    return;
+
+  unsigned FreeRegs = 3;
+  for (auto &Arg : Args) {
+    // For library functions, we do not expect any fancy types.
+    unsigned Size = DAG.getDataLayout().getTypeSizeInBits(Arg.Ty);
+    unsigned SizeInRegs = (Size + 31) / 32;
+    if (SizeInRegs > 2 || SizeInRegs > FreeRegs)
+      continue;
+
+    Arg.isInReg = true;
+    FreeRegs -= SizeInRegs;
+    if (!FreeRegs)
+      break;
+  }
+}
Index: lib/Target/X86/X86Subtarget.h
===================================================================
--- lib/Target/X86/X86Subtarget.h
+++ lib/Target/X86/X86Subtarget.h
@@ -418,6 +418,7 @@
   bool isTargetNaCl() const { return TargetTriple.isOSNaCl(); }
   bool isTargetNaCl32() const { return isTargetNaCl() && !is64Bit(); }
   bool isTargetNaCl64() const { return isTargetNaCl() && is64Bit(); }
+  bool isTargetMCU() const { return TargetTriple.isEnvironmentIAMCU(); }
 
   bool isTargetWindowsMSVC() const {
     return TargetTriple.isWindowsMSVCEnvironment();
Index: test/CodeGen/X86/mcu-abi.ll
===================================================================
--- test/CodeGen/X86/mcu-abi.ll
+++ test/CodeGen/X86/mcu-abi.ll
@@ -0,0 +1,11 @@
+; RUN: llc < %s -mtriple=i686-pc-linux-elfiamcu | FileCheck %s
+
+; CHECK-LABEL: test_lib_args:
+; CHECK: movl %edx, %eax
+; CHECK: calll __fixsfsi
+define i32 @test_lib_args(float inreg %a, float inreg %b) #0 {
+  %ret = fptosi float %b to i32
+  ret i32 %ret
+}
+
+attributes #0 = { nounwind "use-soft-float"="true"}
Index: unittests/ADT/TripleTest.cpp
===================================================================
--- unittests/ADT/TripleTest.cpp
+++ unittests/ADT/TripleTest.cpp
@@ -81,6 +81,12 @@
   EXPECT_EQ(Triple::Darwin, T.getOS());
   EXPECT_EQ(Triple::UnknownEnvironment, T.getEnvironment());
 
+  T = Triple("i386-pc-linux-elfiamcu");
+  EXPECT_EQ(Triple::x86, T.getArch());
+  EXPECT_EQ(Triple::PC, T.getVendor());
+  EXPECT_EQ(Triple::Linux, T.getOS());
+  EXPECT_EQ(Triple::ELFIAMCU, T.getEnvironment());
+
   T = Triple("x86_64-pc-linux-gnu");
   EXPECT_EQ(Triple::x86_64, T.getArch());
   EXPECT_EQ(Triple::PC, T.getVendor());