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agx: Update explanation of sample_mask behaviour

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We discovered today that these (probably) trigger depth/stencil testing, which
has significant implications for the correct/performant use.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23832>
This commit is contained in:
Alyssa Rosenzweig 2023-06-14 11:46:01 -04:00 • committed by Marge Bot
parent 942c206cd1
commit baf67144bd
1 changed files with 16 additions and 9 deletions
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25
src/asahi/compiler/agx_nir_lower_sample_mask.c
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@ -12,7 +12,7 @@
*
* foreach sample in TARGET {
* if sample in LIVE {
* mark sample live
* run depth/stencil test and update
* } else {
* kill sample
* }
@ -28,23 +28,30 @@
* sample_mask must follow these rules:
*
* 1. All sample_mask instructions affecting a sample must execute before a
* local_store_pixel instruction targeting that sample.
* local_store_pixel instruction targeting that sample. This ensures that
* nothing is written for discarded samples (whether discarded in shader or
* due to a failed depth/stencil test).
*
* 2. If sample_mask is used anywhere in a shader, the state of every sample
* must be set on all execution paths. That is, the union of the TARGET sets
* of executed sample_masks must contain all samples.
* 2. If sample_mask is used anywhere in a shader, then on every execution path,
* every sample must be killed or else run depth/stencil tests exactly ONCE.
*
* 3. If a sample is killed, future sample_mask instructions will have
* no effect. The following code sequence correctly implements a conditional
* discard (if there are no other sample_mask instructions in the shader):
* 3. If a sample is killed, future sample_mask instructions have
* no effect on that sample. The following code sequence correctly implements
* a conditional discard (if there are no other sample_mask instructions in
* the shader):
*
* sample_mask discarded, 0
* sample_mask ~0, ~0
*
* but this sequence is incorrect:
*
* sample_mask ~0, ~discarded
* sample_mask ~0, ~0 <-- incorrect: depth/stencil tests run twice
*
* 4. If zs_emit is used anywhere in the shader, sample_mask must not be used.
* Instead, zs_emit with depth = NaN can be emitted.
*
* This pass legalizes sample_mask instructions to satisfy these rules.
* This pass legalizes some sample_mask instructions to satisfy these rules.
*/
#define ALL_SAMPLES (0xFF)