MIPS: Fix compilation under clang.

src/mips/assembler-mips.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 300 matching lines @@
 const FPURegister f25 = { 25 };
 const FPURegister f26 = { 26 };
 const FPURegister f27 = { 27 };
 const FPURegister f28 = { 28 };
 const FPURegister f29 = { 29 };
 const FPURegister f30 = { 30 };
 const FPURegister f31 = { 31 };
 
 // Register aliases.
 // cp is assumed to be a callee saved register.
-static const Register& kLithiumScratchReg = s3;  // Scratch register.
-static const Register& kLithiumScratchReg2 = s4;  // Scratch register.
-static const Register& kRootRegister = s6;  // Roots array pointer.
-static const Register& cp = s7;     // JavaScript context pointer.
-static const DoubleRegister& kLithiumScratchDouble = f30;
-static const FPURegister& kDoubleRegZero = f28;
+// Defined using #define instead of "static const Register&" because Clang
+// complains otherwise when a compilation unit that includes this header
+// doesn't use the variables.
+#define kRootRegister s6
+#define cp s7
+#define kLithiumScratchReg s3
+#define kLithiumScratchReg2 s4
+#define kLithiumScratchDouble f30
+#define kDoubleRegZero f28
 
 // FPU (coprocessor 1) control registers.
 // Currently only FCSR (#31) is implemented.
 struct FPUControlRegister {
   bool is_valid() const { return code_ == kFCSRRegister; }
   bool is(FPUControlRegister creg) const { return code_ == creg.code_; }
   int code() const {
     ASSERT(is_valid());
     return code_;
   }
@@ skipping 948 matching lines @@
 class EnsureSpace BASE_EMBEDDED {
  public:
   explicit EnsureSpace(Assembler* assembler) {
     assembler->CheckBuffer();
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_ARM_ASSEMBLER_MIPS_H_