lavc/alacdsp: RISC-V V decorrelate_stereo

To avoid data dependencies, this does the following unroll, which
requires one extra but probably free addition:

    coeff = (b * left_weight) >> decorr_shift;
    b += a;
    a -= coeff;
    b -= coeff;
    swap(a, b);

libavcodec/alacdsp.c

@@ -58,7 +58,9 @@ av_cold void ff_alacdsp_init(ALACDSPContext *c)
     c->append_extra_bits[0] =
     c->append_extra_bits[1] = append_extra_bits;
 
-#if ARCH_X86
+#if ARCH_RISCV
+    ff_alacdsp_init_riscv(c);
+#elif ARCH_X86
     ff_alacdsp_init_x86(c);
 #endif
 }

libavcodec/alacdsp.h

@@ -29,6 +29,7 @@ typedef struct ALACDSPContext {
 } ALACDSPContext;
 
 void ff_alacdsp_init(ALACDSPContext *c);
+void ff_alacdsp_init_riscv(ALACDSPContext *c);
 void ff_alacdsp_init_x86(ALACDSPContext *c);
 
 #endif /* AVCODEC_ALACDSP_H */

libavcodec/riscv/Makefile

@@ -1,5 +1,7 @@
 OBJS-$(CONFIG_AAC_DECODER) += riscv/aacpsdsp_init.o
 RVV-OBJS-$(CONFIG_AAC_DECODER) += riscv/aacpsdsp_rvv.o
+OBJS-$(CONFIG_ALAC_DECODER) += riscv/alacdsp_init.o
+RVV-OBJS-$(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

libavcodec/riscv/alacdsp_init.c

@@ -0,0 +1,39 @@
+/*
+ * 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 "libavutil/attributes.h"
+#include "libavutil/cpu.h"
+#include "libavcodec/avcodec.h"
+#include "libavcodec/alacdsp.h"
+
+void ff_alac_decorrelate_stereo_rvv(int32_t *buffer[2], int nb_samples,
+                                    int decorr_shift, int decorr_left_weight);
+
+av_cold void ff_alacdsp_init_riscv(ALACDSPContext *c)
+{
+#if HAVE_RVV && (__riscv_xlen == 64)
+    int flags = av_get_cpu_flags();
+
+    if (flags & AV_CPU_FLAG_RVV_I32)
+        c->decorrelate_stereo = ff_alac_decorrelate_stereo_rvv;
+#endif
+}

libavcodec/riscv/alacdsp_rvv.S

@@ -0,0 +1,46 @@
+/*
+ * 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_alac_decorrelate_stereo_rvv, zve32x
+        ld          a4, 8(a0)
+        ld          a0, 0(a0)
+1:
+        vsetvli     t0, a1, e32, m1, ta, ma
+        vle32.v     v24, (a4)
+        sub         a1, a1, t0
+        vle32.v     v16, (a0)
+        vmul.vx     v8, v24, a3
+        vadd.vv     v24, v24, v16
+        vsra.vx     v8, v8, a2
+        vsub.vv     v16, v16, v8
+        vsub.vv     v24, v24, v8
+        vse32.v     v16, (a4)
+        sh2add      a4, t0, a4
+        vse32.v     v24, (a0)
+        sh2add      a0, t0, a0
+        bnez        a1, 1b
+
+        ret
+endfunc
+#endif