lavc/videodsp: RISC-V zicbop prefetch

There are currently no ways to run-time detect the CPU capability, so we
take it for granted (in the worst case, it will execute NOPs).

libavcodec/riscv/Makefile

@@ -67,6 +67,8 @@ RVV-OBJS-$(CONFIG_UTVIDEO_DECODER) += riscv/utvideodsp_rvv.o
 OBJS-$(CONFIG_VC1DSP) += riscv/vc1dsp_init.o
 RV-OBJS-$(CONFIG_VC1DSP) += riscv/vc1dsp_rvi.o
 RVV-OBJS-$(CONFIG_VC1DSP) += riscv/vc1dsp_rvv.o
+OBJS-$(CONFIG_VIDEODSP) += riscv/videodsp_init.o
+RV-OBJS-$(CONFIG_VIDEODSP) += riscv/videodsp.o
 OBJS-$(CONFIG_VP7_DECODER) += riscv/vp7dsp_init.o
 RVV-OBJS-$(CONFIG_VP7_DECODER) += riscv/vp7dsp_rvv.o
 OBJS-$(CONFIG_VP8DSP) += riscv/vp8dsp_init.o

libavcodec/riscv/videodsp.S

@@ -0,0 +1,34 @@
+/*
+ * 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
+ */
+
+#include "config.h"
+#include "libavutil/riscv/asm.S"
+
+#if HAVE_RV_ZICBOP
+func ff_prefetch_rv_zicbop, zicbop
+1:
+        addi    a2, a2, -1
+        prefetch.r  (a0)
+        add     a0, a0, a1
+        bnez    a2, 1b
+
+        ret
+endfunc
+#endif

libavcodec/riscv/videodsp_init.c

@@ -0,0 +1,36 @@
+/*
+ * 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
+ */
+
+#include "config.h"
+#include "libavutil/attributes.h"
+#include "libavutil/cpu.h"
+#include "libavcodec/videodsp.h"
+
+void ff_prefetch_rv_zicbop(const uint8_t *mem, ptrdiff_t stride, int h);
+
+av_cold void ff_videodsp_init_riscv(VideoDSPContext *ctx, int bpc)
+{
+#if HAVE_RV_ZICBOP
+    /* TODO: Since we pay for the indirect function call anyway, we should
+     * only set this if Cache-Block Operation Prefetch (Zicbop) is actually
+     * supported and otherwise save a few cycles of NOPs.
+     * But so far there are no means to detect Zicbop (in user mode).
+     */
+    ctx->prefetch = ff_prefetch_rv_zicbop;
+#endif
+}

libavcodec/videodsp.c

@@ -51,6 +51,8 @@ av_cold void ff_videodsp_init(VideoDSPContext *ctx, int bpc)
     ff_videodsp_init_arm(ctx, bpc);
 #elif ARCH_PPC
     ff_videodsp_init_ppc(ctx, bpc);
+#elif ARCH_RISCV
+    ff_videodsp_init_riscv(ctx, bpc);
 #elif ARCH_X86
     ff_videodsp_init_x86(ctx, bpc);
 #elif ARCH_MIPS

libavcodec/videodsp.h

@@ -81,6 +81,7 @@ void ff_videodsp_init(VideoDSPContext *ctx, int bpc);
 void ff_videodsp_init_aarch64(VideoDSPContext *ctx, int bpc);
 void ff_videodsp_init_arm(VideoDSPContext *ctx, int bpc);
 void ff_videodsp_init_ppc(VideoDSPContext *ctx, int bpc);
+void ff_videodsp_init_riscv(VideoDSPContext *ctx, int bpc);
 void ff_videodsp_init_x86(VideoDSPContext *ctx, int bpc);
 void ff_videodsp_init_mips(VideoDSPContext *ctx, int bpc);
 void ff_videodsp_init_loongarch(VideoDSPContext *ctx, int bpc);