diff --git a/llvm/lib/Support/BLAKE3/blake3_dispatch.c b/llvm/lib/Support/BLAKE3/blake3_dispatch.c
index e96e714225f41..41d013495cb5b 100644
--- a/llvm/lib/Support/BLAKE3/blake3_dispatch.c
+++ b/llvm/lib/Support/BLAKE3/blake3_dispatch.c
@@ -4,16 +4,46 @@
 
 #include "blake3_impl.h"
 
+#if defined(_MSC_VER)
+#include <Windows.h>
+#endif
+
 #if defined(IS_X86)
 #if defined(_MSC_VER)
 #include <intrin.h>
 #elif defined(__GNUC__)
 #include <immintrin.h>
 #else
-#error "Unimplemented!"
+#undef IS_X86 /* Unimplemented! */
 #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)
@@ -59,7 +89,6 @@ static void cpuidex(uint32_t out[4], uint32_t id, uint32_t sid) {
 #endif
 }
 
-#endif
 
 enum cpu_feature {
   SSE2 = 1 << 0,
@@ -76,7 +105,7 @@ 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;
 
 LLVM_ATTRIBUTE_USED
 #if !defined(BLAKE3_TESTING)
@@ -85,14 +114,16 @@ static
     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);
@@ -102,7 +133,7 @@ static
     if (*edx & (1UL << 26))
       features |= SSE2;
 #endif
-    if (*ecx & (1UL << 0))
+    if (*ecx & (1UL << 9))
       features |= SSSE3;
     if (*ecx & (1UL << 19))
       features |= SSE41;
@@ -125,7 +156,7 @@ static
         }
       }
     }
-    g_cpu_features = features;
+    ATOMIC_STORE(g_cpu_features, features);
     return features;
 #else
     /* How to detect NEON? */
@@ -133,6 +164,7 @@ static
 #endif
   }
 }
+#endif
 
 void blake3_compress_in_place(uint32_t cv[8],
                               const uint8_t block[BLAKE3_BLOCK_LEN],