lavc/startcode: add R-V Zbb startcode_find_candidate

The main loop processes 8 bytes in 5 instructions.
For comparison, the optimal plain strnlen() requires 4 instructions per
byte (6.4x worse): LBU; ADDI; BEQZ; BNE. The current libavcodec C code
involves 5 instructions per byte (8x worse). Actual benchmarks may be
slightly less favourable due to latency from ORC.B to BNE.

libavcodec/h264dsp.c

@@ -158,6 +158,8 @@ av_cold void ff_h264dsp_init(H264DSPContext *c, const int bit_depth,
     ff_h264dsp_init_arm(c, bit_depth, chroma_format_idc);
 #elif ARCH_PPC
     ff_h264dsp_init_ppc(c, bit_depth, chroma_format_idc);
+#elif ARCH_RISCV
+    ff_h264dsp_init_riscv(c, bit_depth, chroma_format_idc);
 #elif ARCH_X86
     ff_h264dsp_init_x86(c, bit_depth, chroma_format_idc);
 #elif ARCH_MIPS

libavcodec/h264dsp.h

@@ -125,6 +125,8 @@ void ff_h264dsp_init_arm(H264DSPContext *c, const int bit_depth,
                          const int chroma_format_idc);
 void ff_h264dsp_init_ppc(H264DSPContext *c, const int bit_depth,
                          const int chroma_format_idc);
+void ff_h264dsp_init_riscv(H264DSPContext *c, const int bit_depth,
+                           const int chroma_format_idc);
 void ff_h264dsp_init_x86(H264DSPContext *c, const int bit_depth,
                          const int chroma_format_idc);
 void ff_h264dsp_init_mips(H264DSPContext *c, const int bit_depth,

libavcodec/riscv/Makefile

@@ -28,6 +28,7 @@ OBJS-$(CONFIG_JPEG2000_DECODER) += riscv/jpeg2000dsp_init.o
 RVV-OBJS-$(CONFIG_JPEG2000_DECODER) += riscv/jpeg2000dsp_rvv.o
 OBJS-$(CONFIG_H264CHROMA) += riscv/h264_chroma_init_riscv.o
 RVV-OBJS-$(CONFIG_H264CHROMA) += riscv/h264_mc_chroma.o
+OBJS-$(CONFIG_H264DSP) += riscv/h264dsp_init.o
 OBJS-$(CONFIG_HUFFYUV_DECODER) += riscv/huffyuvdsp_init.o
 RVV-OBJS-$(CONFIG_HUFFYUV_DECODER) += riscv/huffyuvdsp_rvv.o
 OBJS-$(CONFIG_IDCTDSP) += riscv/idctdsp_init.o
@@ -51,6 +52,7 @@ OBJS-$(CONFIG_RV34DSP) += riscv/rv34dsp_init.o
 RVV-OBJS-$(CONFIG_RV34DSP) += riscv/rv34dsp_rvv.o
 OBJS-$(CONFIG_RV40_DECODER) += riscv/rv40dsp_init.o
 RVV-OBJS-$(CONFIG_RV40_DECODER) += riscv/rv40dsp_rvv.o
+RV-OBJS-$(CONFIG_STARTCODE) += riscv/startcode_rvb.o
 OBJS-$(CONFIG_SVQ1_ENCODER) += riscv/svqenc_init.o
 RVV-OBJS-$(CONFIG_SVQ1_ENCODER) += riscv/svqenc_rvv.o
 OBJS-$(CONFIG_TAK_DECODER) += riscv/takdsp_init.o

libavcodec/riscv/h264dsp_init.c

@@ -0,0 +1,40 @@
+/*
+ * Copyright © 2024 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 <stdint.h>
+
+#include "libavutil/attributes.h"
+#include "libavutil/cpu.h"
+#include "libavcodec/h264dsp.h"
+
+extern int ff_startcode_find_candidate_rvb(const uint8_t *, int);
+
+av_cold void ff_h264dsp_init_riscv(H264DSPContext *dsp, const int bit_depth,
+                                   const int chroma_format_idc)
+{
+#if HAVE_RV
+    int flags = av_get_cpu_flags();
+
+    if (flags & AV_CPU_FLAG_RVB_BASIC)
+        dsp->startcode_find_candidate = ff_startcode_find_candidate_rvb;
+#endif
+}

libavcodec/riscv/startcode_rvb.S

@@ -0,0 +1,83 @@
+/*
+ * Copyright © 2024 Rémi Denis-Courmont.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "libavutil/riscv/asm.S"
+
+        .macro  lx rd, addr
+#if (__riscv_xlen == 32)
+        lw      \rd, \addr
+#elif (__riscv_xlen == 64)
+        ld      \rd, \addr
+#else
+        lq      \rd, \addr
+#endif
+        .endm
+
+func ff_startcode_find_candidate_rvb, zbb
+        add     a1, a0, a1
+
+        // Potentially unaligned head
+        andi    t0, a0, -(__riscv_xlen / 8)
+        beq     a0, a1, 2f
+
+        andi    t1, a0, (__riscv_xlen / 8) - 1
+        lx      t2, (t0)
+        li      t3, __riscv_xlen
+        orc.b   t2, t2
+        slli    t1, t1, 3
+        not     t2, t2
+        sub     t3, t3, t1
+        srl     t2, t2, t1
+        addi    t0, t0, __riscv_xlen / 8
+        sll     t2, t2, t1
+        bnez    t2, 4f
+
+        // Main loop (including potentially short tail)
+        bge     t0, a1, 2f
+        li      t3, -1
+1:
+        lx      t2, (t0)
+        addi    t0, t0, __riscv_xlen / 8
+        orc.b   t2, t2
+        bne     t2, t3, 3f // t2 != -1 iff (at least one) zero byte
+        blt     t0, a1, 1b
+
+2:      // No zero byte found
+        sub     a0, a1, a0
+        ret
+
+3:      // Zero byte found in main loop
+        not     t2, t2
+4:      // Zero byte found in head
+        ctz     t2, t2
+        addi    t0, t0, -(__riscv_xlen / 8) // back-track
+        srl     t2, t2, 3
+        add     t0, t0, t2
+        // Uncomment the following line for exact POSIX C strnlen() semantics.
+        //minu    t0, t0, a1 // ignore zero byte in tail
+        sub     a0, t0, a0
+        ret
+endfunc

libavcodec/riscv/vc1dsp_init.c

@@ -33,6 +33,7 @@ void ff_put_pixels16x16_rvi(uint8_t *dst, const uint8_t *src, ptrdiff_t line_siz
 void ff_put_pixels8x8_rvi(uint8_t *dst, const uint8_t *src, ptrdiff_t line_size, int rnd);
 void ff_avg_pixels16x16_rvv(uint8_t *dst, const uint8_t *src, ptrdiff_t line_size, int rnd);
 void ff_avg_pixels8x8_rvv(uint8_t *dst, const uint8_t *src, ptrdiff_t line_size, int rnd);
+int ff_startcode_find_candidate_rvb(const uint8_t *, int);
 
 av_cold void ff_vc1dsp_init_riscv(VC1DSPContext *dsp)
 {
@@ -45,7 +46,9 @@ av_cold void ff_vc1dsp_init_riscv(VC1DSPContext *dsp)
         dsp->put_vc1_mspel_pixels_tab[0][0] = ff_put_pixels16x16_rvi;
     }
 # endif
-#if HAVE_RVV
+    if (flags & AV_CPU_FLAG_RVB_BASIC)
+        dsp->startcode_find_candidate = ff_startcode_find_candidate_rvb;
+# if HAVE_RVV
     if (flags & AV_CPU_FLAG_RVV_I32 && ff_rv_vlen_least(128)) {
         dsp->vc1_inv_trans_4x8_dc = ff_vc1_inv_trans_4x8_dc_rvv;
         dsp->vc1_inv_trans_4x4_dc = ff_vc1_inv_trans_4x4_dc_rvv;
@@ -56,6 +59,6 @@ av_cold void ff_vc1dsp_init_riscv(VC1DSPContext *dsp)
             dsp->avg_vc1_mspel_pixels_tab[1][0] = ff_avg_pixels8x8_rvv;
         }
     }
-#endif
+# endif
 #endif
 }