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obu: Improve short-signaling reference frame index calculation

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Reduces code size a fair amount, and with some loop unrolling
by the compiler the code becomes nearly branchless.
This commit is contained in:
Henrik Gramner
2025-07-09 14:24:07 +02:00
parent fa30043ba0
commit 716164239a
1 changed files with 48 additions and 70 deletions
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src/obu.c
+48 -70
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@@ -516,95 +516,73 @@ static int parse_frame_hdr(Dav1dContext *const c, GetBits *const gb) {
dav1d_get_bits(gb, seqhdr->order_hint_n_bits);
if (seqhdr->order_hint) {
hdr->frame_ref_short_signaling = dav1d_get_bit(gb);
if (hdr->frame_ref_short_signaling) { // FIXME: Nearly verbatim copy from section 7.8
if (hdr->frame_ref_short_signaling) {
hdr->refidx[0] = dav1d_get_bits(gb, 3);
hdr->refidx[1] = hdr->refidx[2] = -1;
hdr->refidx[3] = dav1d_get_bits(gb, 3);
hdr->refidx[4] = hdr->refidx[5] = hdr->refidx[6] = -1;
int shifted_frame_offset[8];
const int current_frame_offset = 1 << (seqhdr->order_hint_n_bits - 1);
for (int i = 0; i < 8; i++) {
if (!c->refs[i].p.p.frame_hdr) goto error;
shifted_frame_offset[i] = current_frame_offset +
get_poc_diff(seqhdr->order_hint_n_bits,
c->refs[i].p.p.frame_hdr->frame_offset,
hdr->frame_offset);
}
int used_frame[8] = { 0 };
used_frame[hdr->refidx[0]] = 1;
used_frame[hdr->refidx[3]] = 1;
int latest_frame_offset = -1;
for (int i = 0; i < 8; i++) {
const int hint = shifted_frame_offset[i];
if (!used_frame[i] && hint >= current_frame_offset &&
hint >= latest_frame_offset)
{
hdr->refidx[6] = i;
latest_frame_offset = hint;
/* +1 allows for unconditional stores, as unused
* values can be dumped into frame_offset[-1]. */
int frame_offset_mem[8+1];
int *const frame_offset = &frame_offset_mem[1];
int earliest_ref = -1;
for (int i = 0, earliest_offset = INT_MAX; i < 8; i++) {
const Dav1dFrameHeader *const refhdr = c->refs[i].p.p.frame_hdr;
if (!refhdr) goto error;
const int diff = get_poc_diff(seqhdr->order_hint_n_bits,
refhdr->frame_offset,
hdr->frame_offset);
frame_offset[i] = diff;
if (diff < earliest_offset) {
earliest_offset = diff;
earliest_ref = i;
}
}
if (latest_frame_offset != -1)
used_frame[hdr->refidx[6]] = 1;
frame_offset[hdr->refidx[0]] = INT_MIN; // = reference frame is used
frame_offset[hdr->refidx[3]] = INT_MIN;
assert(earliest_ref >= 0);
int earliest_frame_offset = INT_MAX;
for (int i = 0; i < 8; i++) {
const int hint = shifted_frame_offset[i];
if (!used_frame[i] && hint >= current_frame_offset &&
hint < earliest_frame_offset)
{
hdr->refidx[4] = i;
earliest_frame_offset = hint;
int refidx = -1;
for (int i = 0, latest_offset = 0; i < 8; i++) {
const int hint = frame_offset[i];
if (hint >= latest_offset) {
latest_offset = hint;
refidx = i;
}
}
if (earliest_frame_offset != INT_MAX)
used_frame[hdr->refidx[4]] = 1;
frame_offset[refidx] = INT_MIN;
hdr->refidx[6] = refidx;
earliest_frame_offset = INT_MAX;
for (int i = 0; i < 8; i++) {
const int hint = shifted_frame_offset[i];
if (!used_frame[i] && hint >= current_frame_offset &&
(hint < earliest_frame_offset))
{
hdr->refidx[5] = i;
earliest_frame_offset = hint;
for (int i = 4; i < 6; i++) {
/* Unsigned compares to handle negative values. */
unsigned earliest_offset = UINT8_MAX;
refidx = -1;
for (int j = 0; j < 8; j++) {
const unsigned hint = frame_offset[j];
if (hint < earliest_offset) {
earliest_offset = hint;
refidx = j;
}
}
frame_offset[refidx] = INT_MIN;
hdr->refidx[i] = refidx;
}
if (earliest_frame_offset != INT_MAX)
used_frame[hdr->refidx[5]] = 1;
for (int i = 1; i < 7; i++) {
if (hdr->refidx[i] < 0) {
latest_frame_offset = -1;
refidx = hdr->refidx[i];
if (refidx < 0) {
unsigned latest_offset = ~UINT8_MAX;
for (int j = 0; j < 8; j++) {
const int hint = shifted_frame_offset[j];
if (!used_frame[j] && hint < current_frame_offset &&
hint >= latest_frame_offset)
{
hdr->refidx[i] = j;
latest_frame_offset = hint;
const unsigned hint = frame_offset[j];
if (hint >= latest_offset) {
latest_offset = hint;
refidx = j;
}
}
if (latest_frame_offset != -1)
used_frame[hdr->refidx[i]] = 1;
frame_offset[refidx] = INT_MIN;
hdr->refidx[i] = refidx >= 0 ? refidx : earliest_ref;
}
}
earliest_frame_offset = INT_MAX;
int ref = -1;
for (int i = 0; i < 8; i++) {
const int hint = shifted_frame_offset[i];
if (hint < earliest_frame_offset) {
ref = i;
earliest_frame_offset = hint;
}
}
for (int i = 0; i < 7; i++) {
if (hdr->refidx[i] < 0)
hdr->refidx[i] = ref;
}
}
}
for (int i = 0; i < 7; i++) {