lavu/tx: support in-place FFT transforms

This commit adds support for in-place FFT transforms. Since our
internal transforms were all in-place anyway, this only changes
the permutation on the input.

Unfortunately, research papers were of no help here. All focused
on dry hardware implementations, where permutes are free, or on
software implementations where binary bloat is of no concern so
storing dozen times the transforms for each permutation and version
is not considered bad practice.
Still, for a pure C implementation, it's only around 28% slower
than the multi-megabyte FFTW3 in unaligned mode.

Unlike a closed permutation like with PFA, split-radix FFT bit-reversals
contain multiple NOPs, multiple simple swaps, and a few chained swaps,
so regular single-loop single-state permute loops were not possible.
Instead, we filter out parts of the input indices which are redundant.
This allows for a single branch, and with some clever AVX512 asm,
could possibly be SIMD'd without refactoring.

The inplace_idx array is guaranteed to never be larger than the
revtab array, and in practice only requires around log2(len) entries.

The power-of-two MDCTs can be done in-place as well. And it's
possible to eliminate a copy in the compound MDCTs too, however
it'll be slower than doing them out of place, and we'd need to dirty
the input array.

diff --git a/doc/APIchanges b/doc/APIchanges
index c353d2d2812a17fe6228f22be64473252332d901..33be750af2ef14b19ccabdf116d57db8e74f551d 100644 (file)
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -15,6 +15,9 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2021-02-21 - xxxxxxxxxx - lavu 56.66.100 - tx.h
+  Add enum AVTXFlags and AVTXFlags.AV_TX_INPLACE
+
 2021-02-14 - xxxxxxxxxx - lavd 58.12.100 - avdevice.h
   Deprecated avdevice_capabilities_create() and
   avdevice_capabilities_free().

diff --git a/libavutil/tx.c b/libavutil/tx.c
index 3b0568a5e17b8be54c5cdb3d251d2761a3baa080..49d5e125ae914539ebea53c9a5f3f9d4e79a7c91 100644 (file)
--- a/libavutil/tx.c
+++ b/libavutil/tx.c
@@ -107,6 +107,42 @@ int ff_tx_gen_ptwo_revtab(AVTXContext *s)
     return 0;
 }
 
+int ff_tx_gen_ptwo_inplace_revtab_idx(AVTXContext *s)
+{
+    int nb_inplace_idx = 0;
+
+    if (!(s->inplace_idx = av_malloc(s->m*sizeof(*s->inplace_idx))))
+        return AVERROR(ENOMEM);
+
+    for (int d = 1; d < s->m; d++) {
+        int src = d;
+        int dst = s->revtab[src];
+
+        if (dst <= src)
+            continue;
+
+        int found = 0;
+        int start_src = src;
+        do {
+            src = dst;
+            for (int j = 0; j < nb_inplace_idx; j++) {
+                if (dst == s->inplace_idx[j]) {
+                    found = 1;
+                    break;
+                }
+            }
+            dst = s->revtab[dst];
+        } while (dst != start_src && !found);
+
+        if (!found)
+            s->inplace_idx[nb_inplace_idx++] = start_src;
+    }
+
+    s->inplace_idx[nb_inplace_idx++] = 0;
+
+    return 0;
+}
+
 av_cold void av_tx_uninit(AVTXContext **ctx)
 {
     if (!(*ctx))
@@ -115,6 +151,7 @@ av_cold void av_tx_uninit(AVTXContext **ctx)
     av_free((*ctx)->pfatab);
     av_free((*ctx)->exptab);
     av_free((*ctx)->revtab);
+    av_free((*ctx)->inplace_idx);
     av_free((*ctx)->tmp);
 
     av_freep(ctx);

diff --git a/libavutil/tx.h b/libavutil/tx.h
index 14d6311e020cacce2fc976ac7aef2660c95c209e..983b5e9307c2ea4cb03245cebbcdfa6109f31a0d 100644 (file)
--- a/libavutil/tx.h
+++ b/libavutil/tx.h
@@ -98,6 +98,18 @@ enum AVTXType {
  */
 typedef void (*av_tx_fn)(AVTXContext *s, void *out, void *in, ptrdiff_t stride);
 
+/**
+ * Flags for av_tx_init()
+ */
+enum AVTXFlags {
+    /**
+     * Performs an in-place transformation on the input. The output parameter
+     * to av_tn_fn() will be ignored. May be unsupported or slower for some
+     * transform types.
+     */
+    AV_TX_INPLACE = 1ULL << 0,
+};
+
 /**
  * Initialize a transform context with the given configuration
  * (i)MDCTs with an odd length are currently not supported.
@@ -108,7 +120,7 @@ typedef void (*av_tx_fn)(AVTXContext *s, void *out, void *in, ptrdiff_t stride);
  * @param inv whether to do an inverse or a forward transform
  * @param len the size of the transform in samples
  * @param scale pointer to the value to scale the output if supported by type
- * @param flags currently unused
+ * @param flags a bitmask of AVTXFlags or 0
  *
  * @return 0 on success, negative error code on failure
  */

diff --git a/libavutil/tx_priv.h b/libavutil/tx_priv.h
index 18a07c312c6d484198af9a3be8311fa59582fb8a..e9fba02a35ff31c85eddeb90cdbd6cff5e93bf3b 100644 (file)
--- a/libavutil/tx_priv.h
+++ b/libavutil/tx_priv.h
@@ -106,22 +106,25 @@ typedef void FFTComplex;
 
 /* Used by asm, reorder with care */
 struct AVTXContext {
-    int n;              /* Nptwo part */
-    int m;              /* Ptwo part */
-    int inv;            /* Is inverted */
+    int n;              /* Non-power-of-two part */
+    int m;              /* Power-of-two part */
+    int inv;            /* Is inverse */
     int type;           /* Type */
+    uint64_t flags;     /* Flags */
     double scale;       /* Scale */
 
     FFTComplex *exptab; /* MDCT exptab */
     FFTComplex *tmp;    /* Temporary buffer needed for all compound transforms */
     int        *pfatab; /* Input/Output mapping for compound transforms */
     int        *revtab; /* Input mapping for power of two transforms */
+    int   *inplace_idx; /* Required indices to revtab for in-place transforms */
 };
 
 /* 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_inplace_revtab_idx(AVTXContext *s);
 
 /* Also used by SIMD init */
 static inline int split_radix_permutation(int i, int n, int inverse)

diff --git a/libavutil/tx_template.c b/libavutil/tx_template.c
index 155e879f8e7df524cc88a978b1ba1bd2587ccb04..f43173e920ed479e6c4415858dfb01b07dac3328 100644 (file)
--- a/libavutil/tx_template.c
+++ b/libavutil/tx_template.c
@@ -392,8 +392,28 @@ static void monolithic_fft(AVTXContext *s, void *_out, void *_in,
     FFTComplex *in = _in;
     FFTComplex *out = _out;
     int m = s->m, mb = av_log2(m);
-    for (int i = 0; i < m; i++)
-        out[s->revtab[i]] = in[i];
+
+    if (s->flags & AV_TX_INPLACE) {
+        FFTComplex tmp;
+        int src, dst, *inplace_idx = s->inplace_idx;
+
+        out = in;
+        src = *inplace_idx++;
+
+        do {
+            tmp = out[src];
+            dst = s->revtab[src];
+            do {
+                FFSWAP(FFTComplex, tmp, out[dst]);
+                dst = s->revtab[dst];
+            } while (dst != src); /* Can be > as well, but is less predictable */
+            out[dst] = tmp;
+        } while ((src = *inplace_idx++));
+    } else {
+        for (int i = 0; i < m; i++)
+            out[s->revtab[i]] = in[i];
+    }
+
     fft_dispatch[mb](out);
 }
 
@@ -678,6 +698,7 @@ int TX_NAME(ff_tx_init_mdct_fft)(AVTXContext *s, av_tx_fn *tx,
     s->m = m;
     s->inv = inv;
     s->type = type;
+    s->flags = flags;
 
     /* If we weren't able to split the length into factors we can handle,
      * resort to using the naive and slow FT. This also filters out
@@ -685,6 +706,8 @@ int TX_NAME(ff_tx_init_mdct_fft)(AVTXContext *s, av_tx_fn *tx,
     if (len > 1 || m == 1) {
         if (is_mdct && (l & 1)) /* Odd (i)MDCTs are not supported yet */
             return AVERROR(ENOSYS);
+        if (flags & AV_TX_INPLACE) /* Neither are in-place naive transforms */
+            return AVERROR(ENOSYS);
         s->n = l;
         s->m = 1;
         *tx = naive_fft;
@@ -716,7 +739,14 @@ int TX_NAME(ff_tx_init_mdct_fft)(AVTXContext *s, av_tx_fn *tx,
     if (n != 1)
         init_cos_tabs(0);
     if (m != 1) {
-        ff_tx_gen_ptwo_revtab(s);
+        if ((err = ff_tx_gen_ptwo_revtab(s)))
+            return err;
+        if (flags & AV_TX_INPLACE) {
+            if (is_mdct) /* In-place MDCTs are not supported yet */
+                return AVERROR(ENOSYS);
+            if ((err = ff_tx_gen_ptwo_inplace_revtab_idx(s)))
+                return err;
+        }
         for (int i = 4; i <= av_log2(m); i++)
             init_cos_tabs(i);
     }

diff --git a/libavutil/version.h b/libavutil/version.h
index b2165754f96186b6c7adca3b9f76e7daaae8a205..b7c5892a37ed65c44becc12a10a35dc713197a38 100644 (file)
--- a/libavutil/version.h
+++ b/libavutil/version.h
@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  56
-#define LIBAVUTIL_VERSION_MINOR  65
+#define LIBAVUTIL_VERSION_MINOR  66
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \