Fix data race in CPU-feature global, not just the dispatch pointers

The previous fix (PR #4) made the per-codec dispatch function pointers
(crc32_impl, adler32_impl, ...) atomic, but each dispatcher still calls
get_x86_cpu_features() / get_arm_cpu_features(), which lazily initialize the
global libdeflate_x86_cpu_features / libdeflate_arm_cpu_features bitmask on
the first call. That global was only 'volatile', so a plain load racing with
the store in libdeflate_init_*_cpu_features() is undefined behavior and was
still flagged by ThreadSanitizer:

  WARNING: ThreadSanitizer: data race
    Write ... libdeflate_init_x86_cpu_features cpu_features.c
    Read  ... get_x86_cpu_features cpu_features.h
    Location is global 'libdeflate_x86_cpu_features'

This reproduced as a "Server died" failure when two HTTP connections first
compressed a gzip response concurrently under TSan.

Access the global with relaxed __atomic_load_n / __atomic_store_n, matching
the dispatch-pointer fix. The first-call initialization is a benign race
(every thread computes the same bitmask, a pure function of the CPU), and
relaxed ordering suffices because no other memory is published through it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: alexey-milovidov

lib/arm/cpu_features.c

@@ -202,7 +202,7 @@ static const struct cpu_feature arm_cpu_feature_table[] = {
 	{ARM_CPU_FEATURE_DOTPROD,	"dotprod"},
 };
 
-volatile u32 libdeflate_arm_cpu_features = 0;
+u32 libdeflate_arm_cpu_features = 0;
 
 void libdeflate_init_arm_cpu_features(void)
 {
@@ -224,7 +224,8 @@ void libdeflate_init_arm_cpu_features(void)
 	disable_cpu_features_for_testing(&features, arm_cpu_feature_table,
 					 ARRAY_LEN(arm_cpu_feature_table));
 
-	libdeflate_arm_cpu_features = features | ARM_CPU_FEATURES_KNOWN;
+	__atomic_store_n(&libdeflate_arm_cpu_features,
+			 features | ARM_CPU_FEATURES_KNOWN, __ATOMIC_RELAXED);
 }
 
 #endif /* ARM_CPU_FEATURES_KNOWN */

lib/arm/cpu_features.h

@@ -51,15 +51,25 @@
      (defined(_WIN32) && defined(ARCH_ARM64)))
 /* Runtime ARM CPU feature detection is supported. */
 #  define ARM_CPU_FEATURES_KNOWN	(1U << 31)
-extern volatile u32 libdeflate_arm_cpu_features;
+extern u32 libdeflate_arm_cpu_features;
 
 void libdeflate_init_arm_cpu_features(void);
 
+/*
+ * Resolved on the first call. Accessed with relaxed atomics: the first-call initialization is a
+ * benign race (every thread computes the same features bitmask, a pure function of the CPU), but
+ * a plain load racing with the store in libdeflate_init_arm_cpu_features() is undefined behavior
+ * and is flagged by ThreadSanitizer. Relaxed ordering suffices because no other memory is
+ * published through it.
+ */
 static inline u32 get_arm_cpu_features(void)
 {
-	if (libdeflate_arm_cpu_features == 0)
+	u32 features = __atomic_load_n(&libdeflate_arm_cpu_features, __ATOMIC_RELAXED);
+	if (features == 0) {
 		libdeflate_init_arm_cpu_features();
-	return libdeflate_arm_cpu_features;
+		features = __atomic_load_n(&libdeflate_arm_cpu_features, __ATOMIC_RELAXED);
+	}
+	return features;
 }
 #else
 static inline u32 get_arm_cpu_features(void) { return 0; }

lib/x86/cpu_features.c

@@ -86,7 +86,7 @@ static const struct cpu_feature x86_cpu_feature_table[] = {
 	{X86_CPU_FEATURE_AVXVNNI,	"avx_vnni"},
 };
 
-volatile u32 libdeflate_x86_cpu_features = 0;
+u32 libdeflate_x86_cpu_features = 0;
 
 static inline bool
 os_supports_avx512(u64 xcr0)
@@ -207,7 +207,8 @@ void libdeflate_init_x86_cpu_features(void)
 	disable_cpu_features_for_testing(&features, x86_cpu_feature_table,
 					 ARRAY_LEN(x86_cpu_feature_table));
 
-	libdeflate_x86_cpu_features = features | X86_CPU_FEATURES_KNOWN;
+	__atomic_store_n(&libdeflate_x86_cpu_features,
+			 features | X86_CPU_FEATURES_KNOWN, __ATOMIC_RELAXED);
 }
 
 #endif /* X86_CPU_FEATURES_KNOWN */

lib/x86/cpu_features.h

@@ -53,15 +53,25 @@
 #if defined(__GNUC__) || defined(__clang__) || defined(_MSC_VER)
 /* Runtime x86 CPU feature detection is supported. */
 #  define X86_CPU_FEATURES_KNOWN	(1U << 31)
-extern volatile u32 libdeflate_x86_cpu_features;
+extern u32 libdeflate_x86_cpu_features;
 
 void libdeflate_init_x86_cpu_features(void);
 
+/*
+ * Resolved on the first call. Accessed with relaxed atomics: the first-call initialization is a
+ * benign race (every thread computes the same features bitmask, a pure function of the CPU), but
+ * a plain load racing with the store in libdeflate_init_x86_cpu_features() is undefined behavior
+ * and is flagged by ThreadSanitizer. Relaxed ordering suffices because no other memory is
+ * published through it.
+ */
 static inline u32 get_x86_cpu_features(void)
 {
-	if (libdeflate_x86_cpu_features == 0)
+	u32 features = __atomic_load_n(&libdeflate_x86_cpu_features, __ATOMIC_RELAXED);
+	if (features == 0) {
 		libdeflate_init_x86_cpu_features();
-	return libdeflate_x86_cpu_features;
+		features = __atomic_load_n(&libdeflate_x86_cpu_features, __ATOMIC_RELAXED);
+	}
+	return features;
 }
 /*
  * x86 intrinsics are also supported.  Include the headers needed to use them.