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pan/va: Handle terminal barriers

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If a shader ends with a workgroup barrier, it must wait for slot #7 at the end
to finish the barrier. After inserting flow control, we get:

   BARRIER
   NOP.wait
   NOP.end

Currently, the flow control pass assumes that .end implies all other control
flow, and will merge this down to

   BARRIER.end

However, this is incorrect. Slot #7 is no longer waited on. In theory, this
cannot affect the correctness of the shader. In practice, the hardware checks
that all barriers are reached. Terminating without waiting on slot #7 first
raises an INSTR_BARRIER_FAULT. We need to weaken the flow control merging
slightly to avoid this incorrect merge, instead emitting:

   BARRIER.wait
   NOP.end

Of course, all of these cases are inefficient: terminal barriers shouldn't be
emitted in the first place. I wrote out an optimization for this. We can merge
it if we find a workload that it actually helps.

Fixes test_half.vstore_half.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17264>
This commit is contained in:
Alyssa Rosenzweig 2022-06-27 15:37:49 -04:00 • committed by Marge Bot
parent 4ee6345d2e
commit 154929d731
2 changed files with 15 additions and 2 deletions
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11
src/panfrost/bifrost/valhall/test/test-merge-flow.cpp
View file

@ -292,3 +292,14 @@ TEST_F(MergeFlow, DeletePointlessDiscard) {
});
}
TEST_F(MergeFlow, PreserveTerminalBarriers) {
CASE({
bi_barrier(b);
flow(WAIT);
flow(END);
},
{
bi_barrier(b)->flow = VA_FLOW_WAIT;
flow(END);
});
}

6
src/panfrost/bifrost/valhall/va_merge_flow.c
View file

@ -83,9 +83,11 @@ merge_end_reconverge(bi_block *block)
if (last->op != BI_OPCODE_NOP) return;
if (last->flow != VA_FLOW_RECONVERGE && last->flow != VA_FLOW_END) return;
/* End implies all other flow control, so remove blocking flow control */
/* End implies all other flow control except for waiting on barriers (slot
* #7, with VA_FLOW_WAIT), so remove blocking flow control.
*/
if (last->flow == VA_FLOW_END) {
while (penult->op == BI_OPCODE_NOP) {
while (penult->op == BI_OPCODE_NOP && penult->flow != VA_FLOW_WAIT) {
bi_remove_instruction(penult);
/* There may be nothing left */