lavc/audiodsp: RISC-V F vector_clipf

RV64G supports MIN & MAX instructions natively only on floating point
registers, not general purpose ones. The later would require the Zbb
extension. Due to that, it is actually faster to perform the clipping
"properly" in FPU.

Benchmarks on SiFive U74-MC (courtesy of Shanghai StarFive Tech):
audiodsp.vector_clipf_c: 29551.5
audiodsp.vector_clipf_rvf: 17871.0

Also tried unrolling with 2 or 8 elements but it gets worse either way.

libavcodec/audiodsp.c

@@ -113,6 +113,8 @@ av_cold void ff_audiodsp_init(AudioDSPContext *c)
     ff_audiodsp_init_arm(c);
 #elif ARCH_PPC
     ff_audiodsp_init_ppc(c);
+#elif ARCH_RISCV
+    ff_audiodsp_init_riscv(c);
 #elif ARCH_X86
     ff_audiodsp_init_x86(c);
 #endif

libavcodec/audiodsp.h

@@ -55,6 +55,7 @@ typedef struct AudioDSPContext {
 void ff_audiodsp_init(AudioDSPContext *c);
 void ff_audiodsp_init_arm(AudioDSPContext *c);
 void ff_audiodsp_init_ppc(AudioDSPContext *c);
+void ff_audiodsp_init_riscv(AudioDSPContext *c);
 void ff_audiodsp_init_x86(AudioDSPContext *c);
 
 #endif /* AVCODEC_AUDIODSP_H */

libavcodec/riscv/Makefile

@@ -0,0 +1,2 @@
+OBJS-$(CONFIG_AUDIODSP) += riscv/audiodsp_init.o \
+                           riscv/audiodsp_rvf.o

libavcodec/riscv/audiodsp_init.c

@@ -0,0 +1,33 @@
+/*
+ * 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 "libavutil/attributes.h"
+#include "libavutil/cpu.h"
+#include "libavcodec/audiodsp.h"
+
+void ff_vector_clipf_rvf(float *dst, const float *src, int len, float min, float max);
+
+av_cold void ff_audiodsp_init_riscv(AudioDSPContext *c)
+{
+    int flags = av_get_cpu_flags();
+
+    if (flags & AV_CPU_FLAG_RVF)
+        c->vector_clipf = ff_vector_clipf_rvf;
+}

libavcodec/riscv/audiodsp_rvf.S

@@ -0,0 +1,49 @@
+/*
+ * 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 "libavutil/riscv/asm.S"
+
+func ff_vector_clipf_rvf, f
+NOHWF   fmv.w.x fa0, a3
+NOHWF   fmv.w.x fa1, a4
+1:
+        flw     ft0,   (a1)
+        flw     ft1,  4(a1)
+        fmax.s  ft0, ft0, fa0
+        flw     ft2,  8(a1)
+        fmax.s  ft1, ft1, fa0
+        flw     ft3, 12(a1)
+        fmax.s  ft2, ft2, fa0
+        addi    a2, a2, -4
+        fmax.s  ft3, ft3, fa0
+        addi    a1, a1, 16
+        fmin.s  ft0, ft0, fa1
+        fmin.s  ft1, ft1, fa1
+        fsw     ft0,   (a0)
+        fmin.s  ft2, ft2, fa1
+        fsw     ft1,  4(a0)
+        fmin.s  ft3, ft3, fa1
+        fsw     ft2,  8(a0)
+        fsw     ft3, 12(a0)
+        addi    a0, a0, 16
+        bnez    a2, 1b
+
+        ret
+endfunc