util/crc32c: always compile FastCRC32 with SSE4.2 instructions

The use of this function is guarded by a CPUID check. That is,
compiling this function with SSE4.2 crc32q instructions will never cause
a SIGILL, even on CPUs that don't support SSE4.2, because the function
can never be reached.

The previous solution to enable hardware support for CRC32C required
compiling all of RocksDB with `-msse4.2` or the equivalent. This gave
the compiler permission to emit SSE4.2 instructions in functions that
were not guarded by a CPUID check, resulting in SIGILLS on non-SSE4.2
CPUs.

util/crc32c.cc

@@ -15,14 +15,7 @@
 #include "util/crc32c.h"
 
 #include <stdint.h>
-#ifdef __SSE4_2__
-#include <nmmintrin.h>
-#endif
-#if defined(_WIN64)
-#ifdef __AVX2__
 #include <nmmintrin.h>
-#endif
-#endif
 #include "util/coding.h"
 
 namespace rocksdb {
@@ -298,21 +291,12 @@ static inline uint32_t LE_LOAD32(const uint8_t *p) {
   return DecodeFixed32(reinterpret_cast<const char*>(p));
 }
 
-#ifdef __SSE4_2__
-#ifdef __LP64__
+#if defined(__LP64__) || defined(_WIN64)
 static inline uint64_t LE_LOAD64(const uint8_t *p) {
   return DecodeFixed64(reinterpret_cast<const char*>(p));
 }
 #endif
-#endif
 
-#if defined(_WIN64)
-#ifdef __AVX2__
-static inline uint64_t LE_LOAD64(const uint8_t *p) {
-  return DecodeFixed64(reinterpret_cast<const char*>(p));
-}
-#endif
-#endif
 static inline void Slow_CRC32(uint64_t* l, uint8_t const **p) {
   uint32_t c = static_cast<uint32_t>(*l ^ LE_LOAD32(*p));
   *p += 4;
@@ -329,8 +313,8 @@ static inline void Slow_CRC32(uint64_t* l, uint8_t const **p) {
   table0_[c >> 24];
 }
 
+__attribute__((target("sse4.2")))
 static inline void Fast_CRC32(uint64_t* l, uint8_t const **p) {
-#ifdef __SSE4_2__
 #ifdef __LP64__
   *l = _mm_crc32_u64(*l, LE_LOAD64(*p));
   *p += 8;
@@ -340,16 +324,6 @@ static inline void Fast_CRC32(uint64_t* l, uint8_t const **p) {
   *l = _mm_crc32_u32(static_cast<unsigned int>(*l), LE_LOAD32(*p));
   *p += 4;
 #endif
-#elif defined(_WIN64)
-#ifdef __AVX2__
-  *l = _mm_crc32_u64(*l, LE_LOAD64(*p));
-  *p += 8;
-#else
-  Slow_CRC32(l, p);
-#endif
-#else
-  Slow_CRC32(l, p);
-#endif
 }
 
 template<void (*CRC32)(uint64_t*, uint8_t const**)>
@@ -411,6 +385,9 @@ static bool isSSE42() {
 #endif
 }
 
+template __attribute__((target("sse4.2")))
+uint32_t ExtendImpl<Fast_CRC32>(uint32_t, const char*, size_t);
+
 typedef uint32_t (*Function)(uint32_t, const char*, size_t);
 
 static inline Function Choose_Extend() {