diff --git a/gcm.cpp b/gcm.cpp
index 690f2002..edb11766 100644
--- a/gcm.cpp
+++ b/gcm.cpp
@@ -28,6 +28,15 @@
 
 NAMESPACE_BEGIN(CryptoPP)
 
+// Different assemblers accept different mnemonic: 'movd eax, xmm0' vs 'movd rax, xmm0' vs 'mov eax, xmm0' vs 'mov rax, xmm0'
+#if (CRYPTOPP_LLVM_CLANG_VERSION >= 30600) || (CRYPTOPP_APPLE_CLANG_VERSION >= 70000) || defined(CRYPTOPP_CLANG_INTEGRATED_ASSEMBLER)
+# define USE_MOVD_EXX_REG 1
+#elif (defined(CRYPTOPP_LLVM_CLANG_VERSION) || defined(CRYPTOPP_APPLE_CLANG_VERSION)) && defined(CRYPTOPP_X64_ASM_AVAILABLE)
+# define USE_MOVD_EXX_OR_RXX_REG 1
+#else	// GNU Assembler
+# define USE_MOV_EXX_OR_RXX_REG 1
+#endif
+
 word16 GCM_Base::s_reductionTable[256];
 volatile bool GCM_Base::s_reductionTableInitialized = false;
 
@@ -887,9 +896,9 @@ size_t GCM_Base::AuthenticateBlocks(const byte *data, size_t len)
 		AS2(	pxor	xmm5, xmm2						)
 
 		AS2(	psrldq	xmm0, 15						)
-#if (CRYPTOPP_LLVM_CLANG_VERSION >= 30600) || (CRYPTOPP_APPLE_CLANG_VERSION >= 70000) || defined(CRYPTOPP_CLANG_INTEGRATED_ASSEMBLER)
+#if USE_MOVD_EXX_REG
 		AS2(	movd	edi, xmm0						)
-#elif (defined(CRYPTOPP_LLVM_CLANG_VERSION) || defined(CRYPTOPP_APPLE_CLANG_VERSION)) && defined(CRYPTOPP_X64_ASM_AVAILABLE)
+#elif USE_MOVD_EXX_OR_RXX_REG
 		AS2(	mov		WORD_REG(di), xmm0				)
 #else	// GNU Assembler
 		AS2(	movd	WORD_REG(di), xmm0				)
@@ -902,9 +911,9 @@ size_t GCM_Base::AuthenticateBlocks(const byte *data, size_t len)
 		AS2(	pxor	xmm4, xmm5						)
 
 		AS2(	psrldq	xmm1, 15						)
-#if (CRYPTOPP_LLVM_CLANG_VERSION >= 30600) || (CRYPTOPP_APPLE_CLANG_VERSION >= 70000) || defined(CRYPTOPP_CLANG_INTEGRATED_ASSEMBLER)
+#if USE_MOVD_EXX_REG
 		AS2(	movd	edi, xmm1						)
-#elif (defined(CRYPTOPP_LLVM_CLANG_VERSION) || defined(CRYPTOPP_APPLE_CLANG_VERSION)) && defined(CRYPTOPP_X64_ASM_AVAILABLE)
+#elif USE_MOVD_EXX_OR_RXX_REG
 		AS2(	mov		WORD_REG(di), xmm1				)
 #else
 		AS2(	movd	WORD_REG(di), xmm1				)
@@ -913,9 +922,9 @@ size_t GCM_Base::AuthenticateBlocks(const byte *data, size_t len)
 		AS2(	shl		eax, 8							)
 
 		AS2(	psrldq	xmm0, 15						)
-#if (CRYPTOPP_LLVM_CLANG_VERSION >= 30600) || (CRYPTOPP_APPLE_CLANG_VERSION >= 70000) || defined(CRYPTOPP_CLANG_INTEGRATED_ASSEMBLER)
+#if USE_MOVD_EXX_REG
 		AS2(	movd	edi, xmm0						)
-#elif (defined(CRYPTOPP_LLVM_CLANG_VERSION) || defined(CRYPTOPP_APPLE_CLANG_VERSION)) && defined(CRYPTOPP_X64_ASM_AVAILABLE)
+#elif USE_MOVD_EXX_OR_RXX_REG
 		AS2(	mov		WORD_REG(di), xmm0				)
 #else
 		AS2(	movd	WORD_REG(di), xmm0				)