lavu/tx: invert permutation lookups

out[lut[i]] = in[i] lookups were 4.04 times(!) slower than
out[i] = in[lut[i]] lookups for an out-of-place FFT of length 4096.

The permutes remain unchanged for anything but out-of-place monolithic
FFT, as those benefit quite a lot from the current order (it means
there's only 1 lookup necessary to add to an offset, rather than
a full gather).

The code was based around non-power-of-two FFTs, so this wasn't
benchmarked early on.

libavutil/tx.c

@@ -91,7 +91,7 @@ int ff_tx_gen_compound_mapping(AVTXContext *s)
     return 0;
 }
 
-int ff_tx_gen_ptwo_revtab(AVTXContext *s)
+int ff_tx_gen_ptwo_revtab(AVTXContext *s, int invert_lookup)
 {
     const int m = s->m, inv = s->inv;
 
@@ -101,7 +101,10 @@ int ff_tx_gen_ptwo_revtab(AVTXContext *s)
     /* Default */
     for (int i = 0; i < m; i++) {
         int k = -split_radix_permutation(i, m, inv) & (m - 1);
-        s->revtab[k] = i;
+        if (invert_lookup)
+            s->revtab[i] = k;
+        else
+            s->revtab[k] = i;
     }
 
     return 0;

libavutil/tx_priv.h

@@ -123,7 +123,7 @@ struct AVTXContext {
 /* Shared functions */
 int ff_tx_type_is_mdct(enum AVTXType type);
 int ff_tx_gen_compound_mapping(AVTXContext *s);
-int ff_tx_gen_ptwo_revtab(AVTXContext *s);
+int ff_tx_gen_ptwo_revtab(AVTXContext *s, int invert_lookup);
 int ff_tx_gen_ptwo_inplace_revtab_idx(AVTXContext *s);
 
 /* Also used by SIMD init */

libavutil/tx_template.c

@@ -410,7 +410,7 @@ static void monolithic_fft(AVTXContext *s, void *_out, void *_in,
         } while ((src = *inplace_idx++));
     } else {
         for (int i = 0; i < m; i++)
-            out[s->revtab[i]] = in[i];
+            out[i] = in[s->revtab[i]];
     }
 
     fft_dispatch[mb](out);
@@ -738,7 +738,7 @@ int TX_NAME(ff_tx_init_mdct_fft)(AVTXContext *s, av_tx_fn *tx,
     if (n != 1)
         init_cos_tabs(0);
     if (m != 1) {
-        if ((err = ff_tx_gen_ptwo_revtab(s)))
+        if ((err = ff_tx_gen_ptwo_revtab(s, n == 1 && !(flags & AV_TX_INPLACE))))
             return err;
         if (flags & AV_TX_INPLACE) {
             if (is_mdct) /* In-place MDCTs are not supported yet */