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.
Patch attached.
>From 29e162de51aa71a5bd1d7e85406a053785c08e76 Mon Sep 17 00:00:00 2001
From: Lynne <d...@lynne.ee>
Date: Wed, 10 Feb 2021 17:58:22 +0100
Subject: [PATCH] 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.
---
libavutil/tx.c | 37 +++++++++++++++++++++++++++++++++++++
libavutil/tx.h | 14 +++++++++++++-
libavutil/tx_priv.h | 9 ++++++---
libavutil/tx_template.c | 33 ++++++++++++++++++++++++++++++---
4 files changed, 86 insertions(+), 7 deletions(-)
diff --git a/libavutil/tx.c b/libavutil/tx.c
index 3b0568a5e1..49d5e125ae 100644
--- 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 f49eb8c4c7..0cf2434847 100644
--- a/libavutil/tx.h
+++ b/libavutil/tx.h
@@ -93,6 +93,18 @@ 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.
@@ -103,7 +115,7 @@ 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 18a07c312c..e9fba02a35 100644
--- 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 155e879f8e..4cf6967cbe 100644
--- a/libavutil/tx_template.c
+++ b/libavutil/tx_template.c
@@ -392,8 +392,25 @@ 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) {
+ int src, *inplace_idx = s->inplace_idx;
+ out = in;
+ while (src = *inplace_idx++) {
+ int dst = s->revtab[src];
+
+ int start_src = src;
+ FFTComplex tmp = out[src];
+ do {
+ FFSWAP(FFTComplex, tmp, out[dst]);
+ src = dst;
+ dst = s->revtab[dst];
+ } while (dst != start_src);
+ out[dst] = tmp;
+ }
+ } else {
+ for (int i = 0; i < m; i++)
+ out[s->revtab[i]] = in[i];
+ }
fft_dispatch[mb](out);
}
@@ -678,6 +695,7 @@ 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 +703,8 @@ 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 +736,14 @@ 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);
}
--
2.30.0
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