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Merge pull request #120 from linebender/element_barrier

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Add memory barrier to elements shader
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Raph Levien 2021-11-05 13:38:13 -07:00 committed by GitHub
parent c648038967 95aad3e6c7
commit b0b0f33c3c
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GPG key ID: 4AEE18F83AFDEB23
2 changed files with 51 additions and 25 deletions
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42
piet-gpu/shader/elements.comp
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@ -175,6 +175,7 @@ shared State sh_state[WG_SIZE];
shared uint sh_part_ix;
shared State sh_prefix;
shared uint sh_flag;
void main() {
State th_state[N_ROWS];
@ -219,17 +220,20 @@ void main() {
// Publish aggregate for this partition
if (gl_LocalInvocationID.x == WG_SIZE - 1) {
// Note: with memory model, we'd want to generate the atomic store version of this.
State_write(state_aggregate_ref(part_ix), agg);
if (part_ix == 0) {
State_write(state_prefix_ref(part_ix), agg);
}
}
// Write flag with release semantics; this is done portably with a barrier.
memoryBarrierBuffer();
if (gl_LocalInvocationID.x == WG_SIZE - 1) {
uint flag = FLAG_AGGREGATE_READY;
if (part_ix == 0) {
State_write(state_prefix_ref(part_ix), agg);
flag = FLAG_PREFIX_READY;
}
state[state_flag_index(part_ix)] = flag;
}
if (part_ix != 0) {
// step 4 of paper: decoupled lookback
uint look_back_ix = part_ix - 1;
@ -237,20 +241,35 @@ void main() {
State their_agg;
uint their_ix = 0;
while (true) {
flag = state[state_flag_index(look_back_ix)];
// Read flag with acquire semantics.
if (gl_LocalInvocationID.x == WG_SIZE - 1) {
sh_flag = state[state_flag_index(look_back_ix)];
}
// The flag load is done only in the last thread. However, because the
// translation of memoryBarrierBuffer to Metal requires uniform control
// flow, we broadcast it to all threads.
barrier();
memoryBarrierBuffer();
uint flag = sh_flag;
if (flag == FLAG_PREFIX_READY) {
if (gl_LocalInvocationID.x == WG_SIZE - 1) {
State their_prefix = State_read(state_prefix_ref(look_back_ix));
exclusive = combine_state(their_prefix, exclusive);
}
break;
} else if (flag == FLAG_AGGREGATE_READY) {
if (gl_LocalInvocationID.x == WG_SIZE - 1) {
their_agg = State_read(state_aggregate_ref(look_back_ix));
exclusive = combine_state(their_agg, exclusive);
}
look_back_ix--;
their_ix = 0;
continue;
}
// else spin
if (gl_LocalInvocationID.x == WG_SIZE - 1) {
// Unfortunately there's no guarantee of forward progress of other
// workgroups, so compute a bit of the aggregate before trying again.
// In the worst case, spinning stops when the aggregate is complete.
@ -265,23 +284,30 @@ void main() {
if (their_ix == PARTITION_SIZE) {
exclusive = combine_state(their_agg, exclusive);
if (look_back_ix == 0) {
break;
}
sh_flag = FLAG_PREFIX_READY;
} else {
look_back_ix--;
their_ix = 0;
}
}
}
barrier();
flag = sh_flag;
if (flag == FLAG_PREFIX_READY) {
break;
}
}
// step 5 of paper: compute inclusive prefix
if (gl_LocalInvocationID.x == WG_SIZE - 1) {
State inclusive_prefix = combine_state(exclusive, agg);
sh_prefix = exclusive;
State_write(state_prefix_ref(part_ix), inclusive_prefix);
}
}
memoryBarrierBuffer();
if (gl_LocalInvocationID.x == WG_SIZE - 1 && part_ix != 0) {
if (gl_LocalInvocationID.x == WG_SIZE - 1) {
state[state_flag_index(part_ix)] = FLAG_PREFIX_READY;
}
}
barrier();
if (part_ix != 0) {
exclusive = sh_prefix;

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piet-gpu/shader/elements.spv
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