Fix for macOS 10.13 (error: instruction requires: AVX-512 ISA) (#400)

* Fix for macOS 10.13 and clang-6 issues with AVX-512 instructions.

* Renamed USE_AVX512 macro to more correct COMPILER_SUPPORTS_AVX512.

Author: vshabanov

cbits/measure_off.c

@@ -17,8 +17,27 @@
 #include <stdatomic.h>
 #endif
 
+/*
+  Clang-6 does not enable proper -march flags for assembly modules
+  which leads to "error: instruction requires: AVX-512 ISA"
+  at the assembler phase.
+
+  Apple LLVM version 10.0.0 (clang-1000.11.45.5) is based on clang-6
+  https://en.wikipedia.org/wiki/Xcode#Toolchain_versions
+  and it's the latest available version on macOS 10.13.
+
+  Disable AVX-512 instructions as they are most likely not supported
+  on the hardware running clang-6.
+*/
+#if !((defined(__apple_build_version__) && __apple_build_version__ <= 10001145) \
+      || (defined(__clang_major__) && __clang_major__ <= 6))
+#define COMPILER_SUPPORTS_AVX512
+#endif
+
+
+#if defined(__x86_64__) && defined(COMPILER_SUPPORTS_AVX512)
 bool has_avx512_vl_bw() {
-#if defined(__x86_64__) && (__GNUC__ >= 6 || defined(__clang_major__))
+#if __GNUC__ >= 6 || defined(__clang_major__)
   uint32_t eax = 0, ebx = 0, ecx = 0, edx = 0;
   __get_cpuid_count(7, 0, &eax, &ebx, &ecx, &edx);
   // https://en.wikipedia.org/wiki/CPUID#EAX=7,_ECX=0:_Extended_Features
@@ -30,6 +49,7 @@ bool has_avx512_vl_bw() {
   return false;
 #endif
 }
+#endif
 
 /*
   measure_off_naive / measure_off_avx / measure_off_sse
@@ -68,7 +88,7 @@ static inline const ssize_t measure_off_naive(const uint8_t *src, const uint8_t
   return cnt == 0 ? (ssize_t)(srcend - src) : (ssize_t)(- cnt);
 }
 
-#ifdef __x86_64__
+#if defined(__x86_64__) && defined(COMPILER_SUPPORTS_AVX512)
 __attribute__((target("avx512vl,avx512bw")))
 static const ssize_t measure_off_avx(const uint8_t *src, const uint8_t *srcend, size_t cnt)
 {
@@ -146,7 +166,7 @@ ssize_t _hs_text_measure_off(const uint8_t *src, size_t off, size_t len, size_t
   static _Atomic measure_off_t s_impl = (measure_off_t)NULL;
   measure_off_t impl = atomic_load_explicit(&s_impl, memory_order_relaxed);
   if (!impl) {
-#ifdef __x86_64__
+#if defined(__x86_64__) && defined(COMPILER_SUPPORTS_AVX512)
     impl = has_avx512_vl_bw() ? measure_off_avx : measure_off_sse;
 #else
     impl = measure_off_sse;