reject xuantie gcc

CMakeLists.txt

@@ -419,6 +419,11 @@ elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "^(riscv)")
     set(NCNN_TARGET_ARCH riscv)
 
     if(CMAKE_SIZEOF_VOID_P EQUAL 8)
+        check_cxx_source_compiles("#if __riscv_v == 7000\nint main() { return 0; }\n#endif\n" NCNN_COMPILER_IS_XUANTIE_GCC)
+        if(NCNN_COMPILER_IS_XUANTIE_GCC)
+            message(FATAL_ERROR "Since XuanTie GCC Toolchain is not yet ready for RVV1.0, the risc-v optimized code of ncnn has been migrated to the RVV1.0 standard. Please use the old version of ncnn, which can be found at https://github.com/Tencent/ncnn/releases/download/20240820/ncnn-20240820-full-source.zip")
+        endif()
+
         set(CMAKE_REQUIRED_FLAGS "-march=rv64gcv")
         check_cxx_source_compiles("#include <riscv_vector.h>\nint main() { vfloat32m8_t _s, _w; float _v; size_t vl; _s = __riscv_vfmacc_vf_f32m8(_s, _v, _w, vl); return 0; }" NCNN_COMPILER_SUPPORT_RISCV_V)
 

docs/how-to-build/how-to-build.md

@@ -667,6 +667,10 @@ Pick `build-XYZ/install` folder for further usage.
 
 ### Build for AllWinner D1
 
+**Important Note**
+
+Since XuanTie GCC Toolchain is not yet ready for RVV1.0, the RISC-V optimized code of ncnn has been migrated to the RVV1.0 standard. The compilation process here uses the old version of ncnn, which can be found at https://github.com/Tencent/ncnn/releases/download/20240820/ncnn-20240820-full-source.zip
+
 Download c906 toolchain package from https://xuantie.t-head.cn/community/download?id=4224193099938729984
 
 ```shell