riscv: Improve the RVV availability check

In some cases, the vector extension is not supported, although the
compiler allows the "v" flag in `-march` and includes `<riscv_vector>`
without raising an error.

Signed-off-by: Cosmin Truta <ctruta@gmail.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3bd74c6..cfc4cc1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -331,9 +331,28 @@
     if(index EQUAL -1)
       message(FATAL_ERROR "PNG_RISCV_RVV must be one of [${PNG_RISCV_RVV_POSSIBLE_VALUES}]")
     elseif(NOT PNG_RISCV_RVV STREQUAL "off")
-      check_c_compiler_flag("-march=rv64gv1p0" COMPILER_SUPPORTS_RVV)
+
+      set(_SAVED_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+      set(CMAKE_REQUIRED_FLAGS "-march=rv64gv1p0")
+
+      check_c_source_compiles("
+        #include <riscv_vector.h>
+        #include <asm/hwcap.h>
+        #ifndef COMPAT_HWCAP_ISA_V /* added in linux-6.5 */
+        #error \"COMPAT_HWCAP_ISA_V is not available\"
+        #endif
+        int main() {
+          const float src[] = { 0.0f, 0.0f, 0.0f, 0.0f };
+          uint64_t ptr[2] = {0x0908060504020100, 0xFFFFFFFF0E0D0C0A};
+          vuint8m1_t a = __riscv_vreinterpret_v_u64m1_u8m1(__riscv_vle64_v_u64m1(ptr, 2));
+          vfloat32m1_t val = __riscv_vle32_v_f32m1((const float*)(src), 4);
+          return (int)__riscv_vfmv_f_s_f32m1_f32(val);
+      }" COMPILER_SUPPORTS_RVV)
+
+      set(CMAKE_REQUIRED_FLAGS ${_SAVED_CMAKE_REQUIRED_FLAGS})
+
       if(NOT COMPILER_SUPPORTS_RVV)
-        message(FATAL_ERROR "Compiler does not support -march=rv64gv1p0 option")
+        message(FATAL_ERROR "Compiler does not support RISC-V Vector extension or its unable to detect it")
       endif()
       set(libpng_riscv_sources
           riscv/filter_rvv_intrinsics.c

configure.ac

diff --git a/configure.ac b/configure.ac
index 2b86959..9f4ed5a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -700,7 +700,7 @@
          AC_DEFINE([PNG_RISCV_RVV_OPT], [2],
                    [Enable RISC-V Vector optimizations])
          AC_MSG_WARN([--enable-riscv-rvv: please specify 'check' or 'api', if]
-            [you want the optimizations unconditionally pass e.g. '-march=rv64gv']
+            [you want the optimizations unconditionally pass e.g. '-march=rv64gv1p0']
             [to the compiler.]);;
       *)
          AC_MSG_ERROR([--enable-riscv-rvv=${enable_riscv_rvv}: invalid value])
@@ -716,11 +716,19 @@
    AC_MSG_CHECKING(whether to use RISC-V RVV intrinsics)
 
    save_CFLAGS=$CFLAGS
-   CFLAGS="$CFLAGS -march=rv64gv"
+   CFLAGS="$CFLAGS -march=rv64gv1p0"
    AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 #include <riscv_vector.h>
-int main(){
-   return 0;
+#include <asm/hwcap.h>
+#ifndef COMPAT_HWCAP_ISA_V /* added in linux-6.5 */
+#error "COMPAT_HWCAP_ISA_V is not available"
+#endif
+int main() {
+   const float src[] = { 0.0f, 0.0f, 0.0f, 0.0f };
+   uint64_t ptr[2] = {0x0908060504020100, 0xFFFFFFFF0E0D0C0A};
+   vuint8m1_t a = __riscv_vreinterpret_v_u64m1_u8m1(__riscv_vle64_v_u64m1(ptr, 2));
+   vfloat32m1_t val = __riscv_vle32_v_f32m1((const float*)(src), 4);
+   return (int)__riscv_vfmv_f_s_f32m1_f32(val);
 }]])],compiler_support_riscv_rvv=yes)
    AC_MSG_RESULT($compiler_support_riscv_rvv)
    if test "$compiler_support_riscv_rvv" = "yes"; then