[Refactor] CPU feature detection logic for SHA256

Author: sipa

src/crypto/sha256.cpp

@@ -552,32 +552,50 @@ std::string SHA256AutoDetect()
 {
     std::string ret = "standard";
 #if defined(USE_ASM) && (defined(__x86_64__) || defined(__amd64__) || defined(__i386__))
-    (void)AVXEnabled; // Silence unused warning (in case ENABLE_AVX2 is not defined)
+    bool have_sse4 = false;
+    bool have_xsave = false;
+    bool have_avx = false;
+    bool have_avx2 = false;
+    bool enabled_avx = false;
+
+    (void)AVXEnabled;
+    (void)have_sse4;
+    (void)have_avx;
+    (void)have_xsave;
+    (void)have_avx2;
+    (void)enabled_avx;
+
     uint32_t eax, ebx, ecx, edx;
     cpuid(1, 0, eax, ebx, ecx, edx);
-    if ((ecx >> 19) & 1) {
+    have_sse4 = (ecx >> 19) & 1;
+    have_xsave = (ecx >> 27) & 1;
+    have_avx = (ecx >> 28) & 1;
+    if (have_xsave && have_avx) {
+        enabled_avx = AVXEnabled();
+    }
+    if (have_sse4) {
+        cpuid(7, 0, eax, ebx, ecx, edx);
+        have_avx2 = (ebx >> 5) & 1;
+    }
+
+    if (have_sse4) {
 #if defined(__x86_64__) || defined(__amd64__)
         Transform = sha256_sse4::Transform;
         TransformD64 = TransformD64Wrapper<sha256_sse4::Transform>;
+        ret = "sse4(1way)";
 #endif
 #if defined(ENABLE_SSE41) && !defined(BUILD_BITCOIN_INTERNAL)
         TransformD64_4way = sha256d64_sse41::Transform_4way;
-        ret = "sse4(1way+4way)";
-#if defined(ENABLE_AVX2) && !defined(BUILD_BITCOIN_INTERNAL)
-        if (((ecx >> 27) & 1) && ((ecx >> 28) & 1)) { // XSAVE and AVX
-            cpuid(7, 0, eax, ebx, ecx, edx);
-            if ((ebx >> 5) & 1) { // AVX2 flag
-                if (AVXEnabled()) { // OS has enabled AVX registers
-                    TransformD64_8way = sha256d64_avx2::Transform_8way;
-                    ret += ",avx2(8way)";
-                }
-            }
-        }
-#endif
-#else
-        ret = "sse4";
+        ret += ",sse41(4way)";
 #endif
     }
+
+#if defined(ENABLE_AVX2) && !defined(BUILD_BITCOIN_INTERNAL)
+    if (have_avx2 && have_avx && enabled_avx) {
+        TransformD64_8way = sha256d64_avx2::Transform_8way;
+        ret += ",avx2(8way)";
+    }
+#endif
 #endif
 
     assert(SelfTest());