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[naga] Remove non-essential override references via compaction #7703

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[naga] Remove non-essential override references via compaction #7703

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@andyleiserson andyleiserson commented May 19, 2025 •
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Adds a mode to compaction that removes unused functions, global variables, and named types and overrides. This mode is used everywhere except the compaction at the end of lowering, where it is important to preserve unused items for type checking and other validation of the module.

Pruning all but the active entry point and then compacting makes process_overrides tolerant of missing values for overrides that are not used by the active entry point.

Fixes #5885

This change also removes the compact feature from naga (enables the associated code unconditionally) due to the new use of compaction for overrides and lack of clear justification for the feature flag. See the first comment for more discussion.

Testing
Adds several cases in the validation test to verify that compaction removes references to non-essential overrides. Also enables the relevant CTS tests, which are now passing.

Squash or Rebase? Squash

Important TODOs

  • The call to compact within the benchmark added by Update naga benchmarks and add a compaction benchmark #7715 needs to updated with the 2nd argument in this PR.
  • This change should have two changelog entries: one for the overrides, and one for removal of the compact feature (assuming that's what we do).

Checklist

  • Run cargo fmt.
  • Run taplo format.
  • Run cargo clippy --tests. If applicable, add:
    • --target wasm32-unknown-unknown
  • Run cargo xtask test to run tests.

@andyleiserson andyleiserson requested a review from a team as a code owner May 19, 2025 19:21
@andyleiserson andyleiserson mentioned this pull request May 19, 2025
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andyleiserson commented May 19, 2025 •
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There is a question what to do about the compact feature. Currently wgsl-in requires compact, so in many environments it is already required. But when wgsl-in is not set, we need a strategy for controlling the use of compaction in process_overrides:

  1. Remove the compact feature and build the associated functionality unconditionally, as this PR currently does.
  2. Make process_overrides conditional upon the compact feature, and arrange for the feature to be set when necessary. (A change would be needed to the no-features clippy runs, because wgpu-hal uses process_overrides, but does not enable compact.)
  3. Make process_overrides available unconditionally, but when built without the compact feature, specifying all overrides is required.
  4. Make process_overrides conditional upon a new feature (overrides?) which would depend on compact.

I have listed in my order of preference, but I don't have all the context about use cases and build size concerns that might justify one of my non-preferred options.

@andyleiserson andyleiserson mentioned this pull request May 20, 2025
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andyleiserson commented May 20, 2025
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naga-cli/src/bin/naga.rs
let (module, info) = naga::back::pipeline_constants::process_overrides(
module,
info,
None,
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It already came in useful to be able to pass the actual entry point here, and I have the changes to do that, but unless there's a desire to have them in this PR, I'll hold them for a follow-up so that this one doesn't get any bigger.

This was referenced May 21, 2025
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Some performance data: compacting all of the wgsl snapshot tests, with the reachability changes (#7674), before this change:

compact/shader: compact time:   [298.90 µs 299.67 µs 300.65 µs]
                        thrpt:  [472.03 MiB/s 473.57 MiB/s 474.78 MiB/s]

After this change, with !preserve_unused:

compact/shader: compact time:   [336.43 µs 337.32 µs 338.28 µs]
                        thrpt:  [419.06 MiB/s 420.25 MiB/s 421.37 MiB/s]
                 change:
                        time:   [+12.296% +12.679% +13.089%] (p = 0.00 < 0.05)
                        thrpt:  [-11.574% -11.253% -10.950%]
                        Performance has regressed.

Without the reachability changes, after this change, with !preserve_unused:

compact/shader: compact time:   [194.58 µs 195.04 µs 195.50 µs]
                        thrpt:  [691.40 MiB/s 693.02 MiB/s 694.64 MiB/s]

Performance of other stages (with this change; wgsl-in includes a compaction and is ~1% slower, validation and hlsl-out are unchanged):

front/shader: wgsl-in   time:   [8.8573 ms 8.8659 ms 8.8749 ms]
                        thrpt:  [21.790 MiB/s 21.812 MiB/s 21.833 MiB/s]
validate/shader: validation
                        time:   [719.47 µs 720.31 µs 721.13 µs]
                        thrpt:  [268.17 MiB/s 268.47 MiB/s 268.79 MiB/s]
back/shader: hlsl-out   time:   [1.4870 ms 1.5040 ms 1.5299 ms]
                        thrpt:  [92.661 MiB/s 94.255 MiB/s 95.334 MiB/s]

@andyleiserson andyleiserson mentioned this pull request May 22, 2025
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@andyleiserson andyleiserson requested a review from crowlKats as a code owner May 22, 2025 21:06
@andyleiserson andyleiserson added the PR: run CTS PRs that should trigger the GitHub actions workflow for the WebGPU conformance test suite (CTS) label May 22, 2025
@andyleiserson
[naga] Remove non-essential override references via compaction
983d616 
Adds a mode to compaction that removes unused functions, global
variables, and named types and overrides. This mode is used
everywhere except the compaction at the end of lowering, where
it is important to preserve unused items for type checking and
other validation of the module.

Pruning all but the active entry point and then compacting makes
`process_overrides` tolerant of missing values for overrides that are
not used by the active entry point.

Fixes gfx-rs#5885
@andyleiserson
Update glsl snapshot test outputs
c107477
@andyleiserson
Remove the compact feature from naga
01e29b9
@andyleiserson
Add override tests to CTS test list
ad6c39a
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Naga shouldn't evaluate overrides unless they're used by the entry point
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