Dynamically choose SSE 4.2

Summary: Otherwise, if we compile on machine with SSE4.2 support and run it on machine without the support, we will fail.

Test Plan: compiles, verified that isSse42() gets called.

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17505

util/crc32c.cc

@@ -313,12 +313,14 @@ static inline void Slow_CRC32(uint64_t* l, uint8_t const **p) {
   table0_[c >> 24];
 }
 
-#ifdef __SSE4_2__
 static inline void Fast_CRC32(uint64_t* l, uint8_t const **p) {
+#ifdef __SSE4_2__
   *l = _mm_crc32_u64(*l, LE_LOAD64(*p));
   *p += 8;
-}
+#else
+  Slow_CRC32(l, p);
 #endif
+}
 
 template<void (*CRC32)(uint64_t*, uint8_t const**)>
 uint32_t ExtendImpl(uint32_t crc, const char* buf, size_t size) {
@@ -363,14 +365,22 @@ uint32_t ExtendImpl(uint32_t crc, const char* buf, size_t size) {
   return l ^ 0xffffffffu;
 }
 
+// Detect if SS42 or not.
+static bool isSSE42() {
+#if defined(__GNUC__) && defined(__x86_64__) && !defined(IOS_CROSS_COMPILE)
+  uint32_t c_;
+  uint32_t d_;
+  __asm__("cpuid" : "=c"(c_), "=d"(d_) : "a"(1) : "ebx");
+  return c_ & (1U << 20);  // copied from CpuId.h in Folly.
+#else
+  return false;
+#endif
+}
+
 typedef uint32_t (*Function)(uint32_t, const char*, size_t);
 
 static inline Function Choose_Extend() {
-#ifdef __SSE4_2__
+  return isSSE42() ? ExtendImpl<Fast_CRC32> : ExtendImpl<Slow_CRC32>;
-  return ExtendImpl<Fast_CRC32>;
-#else
-  return ExtendImpl<Slow_CRC32>;
-#endif
 }
 
 Function ChosenExtend = Choose_Extend();