Review intrinsics includes and macros on gf256.h

- Define the LINUX_ARM macro automatically based on ARM flags set by the
  compiler.
- Detect ARM Neon based on compiler flags.
- Fix the condition for inclusion of AVX2, as discussed in Issue catid#31.
- Reorganize some macro definitions for better readability. For example,
  use an isolated if-elif-else conditional to define GF256_M128.
- Throw error if SSE2 is not available when building for a non-mobile
  target.
- Remove the unused GF256_ALIGNED_ACCESSES macro.

CMakeLists.txt

@@ -59,9 +59,6 @@ else()
     set(CMAKE_CXX_FLAGS "-Wall -Wextra")
     set(CMAKE_CXX_FLAGS_DEBUG "-g -O0 -march=native")
     set(CMAKE_CXX_FLAGS_RELEASE "-O3 -march=native")
-    if (CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")
-        add_definitions(-DLINUX_ARM=1)
-    endif()
 endif()
 
 add_library(wirehair ${LIB_SOURCE_FILES})

gf256.h

@@ -53,50 +53,46 @@
 //------------------------------------------------------------------------------
 // Platform/Architecture
 
+#if defined(__ARM_ARCH) || defined(__ARM_NEON) || defined(__ARM_NEON__)
+    #define LINUX_ARM
+#endif
+
 #if defined(ANDROID) || defined(IOS) || defined(LINUX_ARM) || defined(__powerpc__) || defined(__s390__)
     #define GF256_TARGET_MOBILE
 #endif // ANDROID
 
-#if defined(__AVX2__) || (defined (_MSC_VER) && _MSC_VER >= 1900) && !defined(GF256_TARGET_MOBILE)
+#if defined(__AVX2__) && (!defined (_MSC_VER) || _MSC_VER >= 1900)
     #define GF256_TRY_AVX2 /* 256-bit */
     #include <immintrin.h>
     #define GF256_ALIGN_BYTES 32
+    #define GF256_M256 __m256i
 #else // __AVX2__
     #define GF256_ALIGN_BYTES 16
 #endif // __AVX2__
 
-#if !defined(GF256_TARGET_MOBILE)
-    // Note: MSVC currently only supports SSSE3 but not AVX2
-    #include <tmmintrin.h> // SSSE3: _mm_shuffle_epi8
+#if defined(__SSE2__)
     #include <emmintrin.h> // SSE2
-#endif // GF256_TARGET_MOBILE
+#elif !defined(GF256_TARGET_MOBILE)
+    #error "SSE2 is required for non-mobile target"
+#endif // __SSE2__
 
-#if defined(HAVE_ARM_NEON_H)
-    #include <arm_neon.h>
-#endif // HAVE_ARM_NEON_H
-
-#if defined(GF256_TARGET_MOBILE)
+#if defined(__SSE3__)
+    #define GF256_TRY_SSE3
+    #include <tmmintrin.h> // SSSE3: _mm_shuffle_epi8
+#endif // __SSE3__
 
-    #define GF256_ALIGNED_ACCESSES /* Inputs must be aligned to GF256_ALIGN_BYTES */
+#if defined(__ARM_NEON) || defined(__ARM_NEON__)
+    #include <arm_neon.h>
+    #define GF256_TRY_NEON
+#endif // __ARM_NEON__
 
-# if defined(HAVE_ARM_NEON_H)
-    // Compiler-specific 128-bit SIMD register keyword
+// Compiler-specific 128-bit SIMD register keyword
+#if defined(GF256_TARGET_MOBILE) && defined(GF256_TRY_NEON)
     #define GF256_M128 uint8x16_t
-    #define GF256_TRY_NEON
+#elif defined(__SSE2__) || defined(GF256_TRY_SSE3)
+    #define GF256_M128 __m128i
 #else
     #define GF256_M128 uint64_t
-# endif
-
-#else // GF256_TARGET_MOBILE
-
-    // Compiler-specific 128-bit SIMD register keyword
-    #define GF256_M128 __m128i
-
-#endif // GF256_TARGET_MOBILE
-
-#ifdef GF256_TRY_AVX2
-    // Compiler-specific 256-bit SIMD register keyword
-    #define GF256_M256 __m256i
 #endif
 
 // Compiler-specific C++11 restrict keyword