lavc/lpc: R-V V compute_autocorr

The loop iterates over the length of the vector, not the order. This is
to avoid reloading the same data for each lag value. However this means
the loop only works if the maximum order is no larger than VLENB.

The loop is roughly equivalent to:

    for (size_t j = 0; j < lag; j++)
        autoc[j] = 1.;

    while (len > lag) {
        for (ptrdiff_t j = 0; j < lag; j++)
            autoc[j] += data[j] * *data;
        data++;
        len--;
    }

    while (len > 0) {
        for (ptrdiff_t j = 0; j < len; j++)
            autoc[j] += data[j] * *data;
        data++;
        len--;
    }

Since register pressure is only at 50%, it should be possible to implement
the same loop for order up to 2xVLENB. But this is left for future work.

Performance numbers are all over the place from ~1.25x to ~4x speedups,
but at least they are always noticeably better than nothing.

libavcodec/riscv/lpc_init.c

@@ -22,16 +22,22 @@
 
 #include "libavutil/attributes.h"
 #include "libavutil/cpu.h"
+#include "libavutil/riscv/cpu.h"
 #include "libavcodec/lpc.h"
 
 void ff_lpc_apply_welch_window_rvv(const int32_t *, ptrdiff_t, double *);
+void ff_lpc_compute_autocorr_rvv(const double *, ptrdiff_t, int, double *);
 
 av_cold void ff_lpc_init_riscv(LPCContext *c)
 {
 #if HAVE_RVV && (__riscv_xlen >= 64)
     int flags = av_get_cpu_flags();
 
-    if ((flags & AV_CPU_FLAG_RVV_F64) && (flags & AV_CPU_FLAG_RVB_ADDR))
+    if ((flags & AV_CPU_FLAG_RVV_F64) && (flags & AV_CPU_FLAG_RVB_ADDR)) {
         c->lpc_apply_welch_window = ff_lpc_apply_welch_window_rvv;
+
+        if (ff_get_rv_vlenb() >= c->max_order)
+            c->lpc_compute_autocorr = ff_lpc_compute_autocorr_rvv;
+    }
 #endif
 }

libavcodec/riscv/lpc_rvv.S

@@ -85,4 +85,33 @@ func ff_lpc_apply_welch_window_rvv, zve64d
 
         ret
 endfunc
+
+func ff_lpc_compute_autocorr_rvv, zve64d
+        li        t0, 1
+        vsetvli   zero, a2, e64, m8, ta, ma
+        fcvt.d.l  ft0, t0
+        vle64.v   v0, (a0)
+        sh3add    a0, a2, a0   # data += lag
+        vfmv.v.f  v16, ft0
+        bge       a2, a1, 2f
+1:
+        vfmv.f.s  ft0, v0
+        fld       ft1, (a0)    # ft1 = data[lag + i]
+        vfmacc.vf v16, ft0, v0 # v16[j] += data[i] * data[i + j]
+        addi      a1, a1, -1
+        vfslide1down.vf v0, v0, ft1
+        addi      a0, a0, 8
+        bgt       a1, a2, 1b   # while (len > lag);
+2:
+        vfmv.f.s  ft0, v0
+        vsetvli   zero, a1, e64, m8, tu, ma
+        vfmacc.vf v16, ft0, v0
+        addi      a1, a1, -1
+        vslide1down.vx v0, v0, zero
+        bnez      a1, 2b       # while (len > 0);
+
+        vsetvli   zero, a2, e64, m8, ta, ma
+        vse64.v   v16, (a3)
+        ret
+endfunc
 #endif