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Bit magic for backdrop accumulation

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Use bit counting rather than iterating backdrop increments one by one.
A nice if not huge speedup.
This commit is contained in:
Raph Levien 2020-05-22 07:13:27 -07:00
parent a616b4d010
commit 7d040dff37
2 changed files with 20 additions and 18 deletions
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38
piet-gpu/shader/coarse.comp
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@ -80,6 +80,14 @@ void alloc_chunk(inout uint chunk_n_segs, inout SegChunkRef seg_chunk_ref,
}
}
// Accumulate delta to backdrop.
//
// Each bit for which bd_bitmap is 1 and bd_sign is 1 counts as +1, and each
// bit for which bd_bitmap is 1 and bd_sign is 0 counts as -1.
int count_backdrop(uint bd_bitmap, uint bd_sign) {
return bitCount(bd_bitmap & bd_sign) - bitCount(bd_bitmap & ~bd_sign);
}
void main() {
// Could use either linear or 2d layouts for both dispatch and
// invocations within the workgroup. We'll use variables to abstract.
@ -275,33 +283,31 @@ void main() {
uint slice_ix = 0;
uint bitmap = sh_bitmaps[0][th_ix];
uint bd_bitmap = sh_backdrop[0][th_ix];
uint combined = bitmap | bd_bitmap;
uint bd_sign = sh_bd_sign[0];
while (true) {
if (combined == 0) {
if (bitmap == 0) {
backdrop += count_backdrop(bd_bitmap, bd_sign);
slice_ix++;
if (slice_ix == N_SLICE) {
break;
}
bitmap = sh_bitmaps[slice_ix][th_ix];
bd_bitmap = sh_backdrop[slice_ix][th_ix];
combined = bitmap | bd_bitmap;
if (combined == 0) {
bd_sign = sh_bd_sign[slice_ix];
if (bitmap == 0) {
continue;
}
}
uint element_ref_ix = slice_ix * 32 + findLSB(combined);
uint element_ref_ix = slice_ix * 32 + findLSB(bitmap);
uint element_ix = sh_elements[(rd_ix + element_ref_ix) % N_RINGBUF];
// TODO: use bit magic to aggregate this calculation.
if ((bd_bitmap & (1 << (element_ref_ix & 31))) != 0) {
if ((sh_bd_sign[slice_ix] & (1 << (element_ref_ix & 31))) != 0) {
backdrop += 1;
} else {
backdrop -= 1;
}
}
// Bits up to and including the lsb
uint bd_mask = (bitmap - 1) ^ bitmap;
backdrop += count_backdrop(bd_bitmap & bd_mask, bd_sign);
// Clear bits that have been consumed.
bd_bitmap &= ~bd_mask;
bitmap &= ~bd_mask;
if ((bitmap & (1 << (element_ref_ix & 31))) != 0) {
// At this point, we read the element again from global memory.
// If that turns out to be expensive, maybe we can pack it into
// shared memory (or perhaps just the tag).
@ -378,10 +384,6 @@ void main() {
}
break;
}
}
// clear LSB
combined &= combined - 1;
}
barrier();

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piet-gpu/shader/coarse.spv
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