add NEON support to blake3_dispatch.c

Currently this requires setting the BLAKE3_USE_NEON preprocessor flag.
In the future we may enable this automatically on AArch32/64 or include
some kind of dynamic feature detection. (Though ARM makes this harder
than x86.)

As part of this, get rid of the IS_ARM flag. It wasn't being set
properly when I tried it on a Raspberry Pi.

Closes BLAKE3-team#30.

README.md

@@ -88,9 +88,28 @@ gcc -shared -O3 -DBLAKE3_NO_SSE41 -DBLAKE3_NO_AVX2 -DBLAKE3_NO_AVX512 \
     blake3.c blake3_dispatch.c blake3_portable.c -o libblake3.so
 ```
 
-### ARM
+### ARM NEON
 
-TODO: add NEON support to `blake3_dispatch.c`.
+The NEON implementation is not enabled by default on ARM, since not all
+ARM targets support it. To enable it, set `BLAKE3_USE_NEON=1`. Here's an
+example of building a shared library on ARM Linux with NEON support:
+
+```bash
+gcc -shared -O3 -DBLAKE3_USE_NEON blake3.c blake3_dispatch.c \
+    blake3_portable.c blake3_neon.c -o libblake3.so
+```
+
+Note that on some targets (ARMv7 in particular), extra flags may be
+required to activate NEON support in the compiler. If you see an error
+like...
+
+```
+/usr/lib/gcc/armv7l-unknown-linux-gnueabihf/9.2.0/include/arm_neon.h:635:1: error: inlining failed
+in call to always_inline ‘vaddq_u32’: target specific option mismatch
+```
+
+...then you may need to add something like `-mfpu=neon-vfpv4
+-mfloat-abi=hard`.
 
 ### Other Platforms
 

blake3_dispatch.c

@@ -73,7 +73,7 @@ void blake3_hash_many_avx512(const uint8_t *const *inputs, size_t num_inputs,
 #endif
 #endif
 
-#if defined(IS_ARM) && defined(BLAKE3_USE_NEON)
+#if defined(BLAKE3_USE_NEON)
 void blake3_hash_many_neon(const uint8_t *const *inputs, size_t num_inputs,
                            size_t blocks, const uint32_t key[8],
                            uint64_t counter, bool increment_counter,
@@ -191,10 +191,8 @@ static
     }
     g_cpu_features = features;
     return features;
-#elif defined(IS_ARM)
-    /* How to detect NEON? */
-    return 0;
 #else
+    /* How to detect NEON? */
     return 0;
 #endif
   }
@@ -275,6 +273,13 @@ void blake3_hash_many(const uint8_t *const *inputs, size_t num_inputs,
   }
 #endif
 #endif
+
+#if defined(BLAKE3_USE_NEON)
+  blake3_hash_many_neon(inputs, num_inputs, blocks, key, counter,
+                        increment_counter, flags, flags_start, flags_end, out);
+  return;
+#endif
+
   blake3_hash_many_portable(inputs, num_inputs, blocks, key, counter,
                             increment_counter, flags, flags_start, flags_end,
                             out);

blake3_impl.h

@@ -38,10 +38,6 @@ enum blake3_flags {
 #define IS_X86_32
 #endif
 
-#if defined(__arm__)
-#define IS_ARM
-#endif
-
 #if defined(IS_X86)
 #if defined(_MSC_VER)
 #include <intrin.h>