lavu/riscv: helper to read the vector length

libavutil/riscv/cpu.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright © 2022 Rémi Denis-Courmont.
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef AVUTIL_RISCV_CPU_H
+#define AVUTIL_RISCV_CPU_H
+
+#include "config.h"
+#include <stddef.h>
+#include "libavutil/cpu.h"
+
+#if HAVE_RVV
+/**
+ * Returns the vector size in bytes (always a power of two and at least 4).
+ * This is undefined behaviour if vectors are not implemented.
+ */
+static inline size_t ff_get_rv_vlenb(void)
+{
+    size_t vlenb;
+
+    __asm__ (
+        ".option push\n"
+        ".option arch, +v\n"
+        "    csrr %0, vlenb\n"
+        ".option pop\n" : "=r" (vlenb));
+    return vlenb;
+}
+#endif
+#endif