blake3_dispatch: Fix race condition initializing g_cpu_features.

If multiple threads try to compute a hash simultaneously before the library has been used for the first time,
the logic in get_cpu_features that detects CPU features will write to g_cpu_features without synchronization,
which is a race condition and flagged by ThreadSanitizer.

This change marks g_cpu_features as an atomic variable to address the race condition.

c/blake3_dispatch.c

@@ -6,6 +6,7 @@
 
 #if defined(IS_X86)
 #if defined(_MSC_VER)
+#include <Windows.h>
 #include <intrin.h>
 #elif defined(__GNUC__)
 #include <immintrin.h>
@@ -14,6 +15,32 @@
 #endif
 #endif
 
+#if !defined(BLAKE3_ATOMICS)
+#if defined(__has_include)
+#if __has_include(<stdatomic.h>) && !defined(_MSC_VER)
+#define BLAKE3_ATOMICS 1
+#else
+#define BLAKE3_ATOMICS 0
+#endif /* __has_include(<stdatomic.h>) && !defined(_MSC_VER) */
+#else
+#define BLAKE3_ATOMICS 0
+#endif /* defined(__has_include) */
+#endif /* BLAKE3_ATOMICS */
+
+#if BLAKE3_ATOMICS
+#define ATOMIC_INT _Atomic int
+#define ATOMIC_LOAD(x) x
+#define ATOMIC_STORE(x, y) x = y
+#elif defined(_MSC_VER)
+#define ATOMIC_INT LONG
+#define ATOMIC_LOAD(x) InterlockedOr(&x, 0)
+#define ATOMIC_STORE(x, y) InterlockedExchange(&x, y)
+#else
+#define ATOMIC_INT int
+#define ATOMIC_LOAD(x) x
+#define ATOMIC_STORE(x, y) x = y
+#endif
+
 #define MAYBE_UNUSED(x) (void)((x))
 
 #if defined(IS_X86)
@@ -76,22 +103,24 @@ enum cpu_feature {
 #if !defined(BLAKE3_TESTING)
 static /* Allow the variable to be controlled manually for testing */
 #endif
-    enum cpu_feature g_cpu_features = UNDEFINED;
+    ATOMIC_INT g_cpu_features = UNDEFINED;
 
 #if !defined(BLAKE3_TESTING)
 static
 #endif
     enum cpu_feature
     get_cpu_features(void) {
 
-  if (g_cpu_features != UNDEFINED) {
-    return g_cpu_features;
+  /* If TSAN detects a data race here, try compiling with -DBLAKE3_ATOMICS=1 */
+  enum cpu_feature features = ATOMIC_LOAD(g_cpu_features);
+  if (features != UNDEFINED) {
+    return features;
   } else {
 #if defined(IS_X86)
     uint32_t regs[4] = {0};
     uint32_t *eax = &regs[0], *ebx = &regs[1], *ecx = &regs[2], *edx = &regs[3];
     (void)edx;
-    enum cpu_feature features = 0;
+    features = 0;
     cpuid(regs, 0);
     const int max_id = *eax;
     cpuid(regs, 1);
@@ -124,7 +153,7 @@ static
         }
       }
     }
-    g_cpu_features = features;
+    ATOMIC_STORE(g_cpu_features, features);
     return features;
 #else
     /* How to detect NEON? */