fix #1984 & fix cmake (#2000)

examples/CMakeLists.txt

@@ -13,7 +13,7 @@ if(NOT OpenCV_FOUND)
 endif()
 
 if(OpenCV_FOUND)
-    if(${OpenCV_VERSION_MAJOR} GREATER_EQUAL 4)
+    if(${OpenCV_VERSION_MAJOR} GREATER 3)
         set(CMAKE_CXX_STANDARD 11)
     endif()
     include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../src)

src/cpu.cpp

@@ -214,8 +214,8 @@ int cpu_support_arm_asimdhp()
 
 int cpu_support_x86_avx2()
 {
-#if defined(__x86_64__)
-#ifdef _MSC_VER
+#if (_M_AMD64 || __x86_64__) || (_M_IX86 || __i386__)
+#if defined(_MSC_VER)
     // TODO move to init function
     int cpu_info[4];
     __cpuid(cpu_info, 0);
@@ -235,10 +235,18 @@ int cpu_support_x86_avx2()
 
     __cpuid(cpu_info, 7);
     return cpu_info[1] & 0x00000020;
-#else
-    // TODO gcc-specific
+#elif defined(__clang__)
+#if __clang_major__ >= 6
     __builtin_cpu_init();
+#endif
     return __builtin_cpu_supports("avx2");
+#elif defined(__GNUC__)
+    __builtin_cpu_init();
+    return __builtin_cpu_supports("avx2");
+#else
+    // TODO: other x86 compilers checking avx2 here
+    NCNN_LOGE("AVX2 detection method is unknown for current compiler");
+    return 0;
 #endif
 #else
     return 0;