lavc/bswapdsp: RISC-V B bswap_buf

Simply taking the Zbb REV8 instruction into use in a simple loop gives
some significant savings:

bswap_buf_c: 1081.0
bswap_buf_rvb_b: 771.0

But we can also use the 64-bit REV8 as a pseudo-SIMD instruction with
just one additional shift, and one fewer load, effectively doubling the
bandwidth. Consequently, this patch is useful even if the compile-time
target has Zbb enabled for C code:

bswap_buf_c: 1081.0
bswap_buf_rvb_b: 341.0  (this patch)

On the other hand, this approach fails miserably for bswap16_buf as the
ratio of shifts and stores becomes unfavorable compared to naïve C:

bswap16_buf_c: 1542.0
bswap16_buf_rvb_b: 1803.7

Unrolling to process 128 bits (4 samples) at a time actually worsens
performance ever so slightly:

bswap_buf_c: 1081.0
bswap_buf_rvb_b: 408.5

libavcodec/bswapdsp.c

@@ -51,7 +51,9 @@ av_cold void ff_bswapdsp_init(BswapDSPContext *c)
     c->bswap_buf   = bswap_buf;
     c->bswap16_buf = bswap16_buf;
 
-#if ARCH_X86
+#if ARCH_RISCV
+    ff_bswapdsp_init_riscv(c);
+#elif ARCH_X86
     ff_bswapdsp_init_x86(c);
 #endif
 }

libavcodec/bswapdsp.h

@@ -27,6 +27,7 @@ typedef struct BswapDSPContext {
 } BswapDSPContext;
 
 void ff_bswapdsp_init(BswapDSPContext *c);
+void ff_bswapdsp_init_riscv(BswapDSPContext *c);
 void ff_bswapdsp_init_x86(BswapDSPContext *c);
 
 #endif /* AVCODEC_BSWAPDSP_H */

libavcodec/riscv/Makefile

@@ -5,6 +5,8 @@ RVV-OBJS-$(CONFIG_ALAC_DECODER) += riscv/alacdsp_rvv.o
 OBJS-$(CONFIG_AUDIODSP) += riscv/audiodsp_init.o \
                            riscv/audiodsp_rvf.o
 RVV-OBJS-$(CONFIG_AUDIODSP) += riscv/audiodsp_rvv.o
+OBJS-$(CONFIG_BSWAPDSP) += riscv/bswapdsp_init.o \
+                           riscv/bswapdsp_rvb.o
 OBJS-$(CONFIG_FMTCONVERT) += riscv/fmtconvert_init.o
 RVV-OBJS-$(CONFIG_FMTCONVERT) += riscv/fmtconvert_rvv.o
 OBJS-$(CONFIG_IDCTDSP) += riscv/idctdsp_init.o

libavcodec/riscv/bswapdsp_init.c

@@ -0,0 +1,38 @@
+/*
+ * 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 <stdint.h>
+
+#include "config.h"
+#include "libavutil/attributes.h"
+#include "libavutil/cpu.h"
+#include "libavcodec/bswapdsp.h"
+
+void ff_bswap32_buf_rvb(uint32_t *dst, const uint32_t *src, int len);
+
+av_cold void ff_bswapdsp_init_riscv(BswapDSPContext *c)
+{
+#if (__riscv_xlen >= 64)
+    int cpu_flags = av_get_cpu_flags();
+
+    if (cpu_flags & AV_CPU_FLAG_RVB_BASIC)
+        c->bswap_buf = ff_bswap32_buf_rvb;
+#endif
+}

libavcodec/riscv/bswapdsp_rvb.S

@@ -0,0 +1,68 @@
+/*
+ * 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 (__riscv_xlen >= 64)
+func ff_bswap32_buf_rvb, zbb
+        andi    t0, a1, 4
+        beqz    t0, 1f
+        /* Align a1 (input) to 64-bit */
+        lwu     t0, (a1)
+        addi    a0, a0, 4
+        rev8    t0, t0
+        addi    a2, a2, -1
+        srli    t0, t0, __riscv_xlen - 32
+        addi    a1, a1, 4
+        sw      t0, -4(a0)
+1:
+        andi    a3, a2, -2
+        sh2add  a2, a2, a0
+        beqz    a3, 3f
+        sh2add  a3, a3, a0
+2:      /* 2 elements (64 bits) at a time on a 64-bit boundary */
+        ld      t0,  (a1)
+        addi    a0, a0, 8
+        rev8    t0, t0
+#if (__riscv_xlen == 64)
+        srli    t2, t0, 32
+        sw      t0, -4(a0)
+#else
+        srli    t1, t0, __riscv_xlen - 64
+        srli    t2, t0, __riscv_xlen - 32
+        sw      t1, -4(a0)
+#endif
+        addi    a1, a1, 8
+        sw      t2, -8(a0)
+        bne     a0, a3, 2b
+3:
+        beq     a0, a2, 5f
+4:      /* Process last element */
+        lwu     t0, (a1)
+        addi    a0, a0, 4
+        rev8    t0, t0
+        addi    a1, a1, 4
+        srli    t0, t0, __riscv_xlen - 32
+        sw      t0, -4(a0)
+5:
+        ret
+endfunc
+#endif