Make DCE mandatory and run it much earlier (even per-crate, shrinking `.rlib`s). #350

eddyb · 2025-07-25T20:00:07Z

The DCE ("dead code elimination", i.e. removing definitions not reachable from e.g. shader entry-points) pass was, for one reason or another, toggleable, and supposedly shaders should've still compiled without it.

However, the way this was achieved was by ignoring anything that DCE would've removed, in the "zombie" (deferred error) reporting pass, and removing the subset that also had "zombie" decorations.

Additionally, I believe that the concept of "optional DCE" has been outdated for a while, because of:

non-enumerable SPIR-T interned (Type, Const) / module-scoped (GlobalVar, Func) handles
- that is, you can't even observe a FooDef without finding a Foo handle elsewhere
- everything has to be reached from spirt::Module exports (linker ones + shader entry-points)
  (the DCE pass has a similar definition of "roots", even if rspirv makes it more ad-hoc)
- even if lowering a SPIR-V module into SPIR-T may allocate all original definitions,
  all unreachable ones are lost (tho returning some kind of Map<Id, _> might be nice)
moving away from the old "zombie" (deferred error) system, and favoring more legalization
- see also: Begin validating some SPIR-V extensions/capabilities (esp. IntN/FloatN) on SPIR-T. #332
- RUSTGPU_CODEGEN_ARGS=--no-early-report-zombies (reporting errors at the very end, on SPIR-T)
  has been around for a while, and it's only not the default due to the risk of random bugs

So this PR removes the --no-dce flag, applies DCE in a few more places, and gets to simplify the early "zombie" reporting pass (as it no longer has any reason to mutate the SPIR-V module, itself).

A niche (but welcome) benefit from running DCE really early, is that --dump-post-merge is now actually able to dump .spirt/.spirt.html, as well (instead of only .spv), since the pre-DCE SPIR-V module is really malformed (instructions referring to IDs never defined anywhere) and DCE is needed to clean that up.

However, the most exciting new use of DCE being added (in the last commit), is in the per-crate "optimize" stage (just before saving SPIR-V modules on disk for e.g. .rlibs and the final linking step).

We've been neglecting it because we're effectively stuck doing something like LTO, but there's no reason to not optimize CGUs (or, well, crates, because we run with 1 CGU for now - another thing I want to fix), and DCE is a pretty safe way to start (but we could also consider e.g. the mem2reg->inline->mem2reg sequence).

And it turns out we can save over 1 MiB of SPIR-V (that shader linking will never see again after this PR), from the minimal dependency set (core, compiler-builtins, glam, libm, num-traits, spirv-std):

(click for exact numbers and methodology)

Before

$ cargo compiletest --target-env spv1.3
$ echo target/compiletest-deps/spirv-unknown-spv1.3/debug/deps/*.rlib | xargs -n1 llvm-ar tv | rg '\.o$' | sort -n -k 3
--------- 0/0     48 Jan  1 02:00 1970 bitflags-3dcf3d9be677438b.bitflags.a47078eccd6f1016-cgu.0.rcgu.o
--------- 0/0     48 Jan  1 02:00 1970 compiletests_deps_helper-32dfa6c895360273.cetel80wcnyywktmqqi32z0t9.1c6l5wu.rcgu.o
--------- 0/0  10900 Jan  1 02:00 1970 spirv_std_types-688f22e3cf3995e3.0h0l2egaag7apdniux376o36z.09692e3.rcgu.o
--------- 0/0 1020660 Jan  1 02:00 1970 spirv_std-3fa5eb13241d7c30.9o2xt2nbmavkkxywdyb4v65wl.13t9lsz.rcgu.o
--------- 0/0 2416856 Jan  1 02:00 1970 num_traits-aa5c62cc8821fd9c.num_traits.92968d3e04044d3b-cgu.0.rcgu.o
--------- 0/0 2921312 Jan  1 02:00 1970 libm-4ae827bf8b5c3151.libm.30b39fea0ef0ed48-cgu.0.rcgu.o
--------- 0/0 3732808 Jan  1 02:00 1970 glam-ce3b52c9fe00e7e0.glam.158081abeaed99-cgu.0.rcgu.o
--------- 0/0 5711704 Jan  1 02:00 1970 compiler_builtins-b469d9463e1e9364.compiler_builtins.93b624f8cc19b5ca-cgu.0.rcgu.o
--------- 0/0 9499536 Jan  1 02:00 1970 core-0d15d1819711cec1.core.d3a3ec86f32056-cgu.0.rcgu.o
$ cargo run -p example-runner-wgpu --release
$ echo target/spirv-builder/spirv-unknown-vulkan1.1/release/deps/*.rlib | xargs -n1 llvm-ar tv | rg '\.o$' | sort -n -k 3 | rg -v shared-
--------- 0/0     88 Jan  1 02:00 1970 bitflags-0de303e2ee1c80c4.bitflags.ca8277fe98621545-cgu.0.rcgu.o
--------- 0/0     88 Jan  1 02:00 1970 bytemuck-e70aaa40b159da49.bytemuck.433330b546080123-cgu.0.rcgu.o
--------- 0/0  10812 Jan  1 02:00 1970 spirv_std_types-531bcff05bb52ca4.2kp5062ggpggjb8a9bw5p13g7.1i1tktr.rcgu.o
--------- 0/0 1017296 Jan  1 02:00 1970 spirv_std-acd900a964871511.5fh9vgyaa0jioh67zv8mt54km.14k8tk4.rcgu.o
--------- 0/0 2405552 Jan  1 02:00 1970 num_traits-cb12f45b10b98d97.num_traits.e2bc6a17b294da77-cgu.0.rcgu.o
--------- 0/0 2943520 Jan  1 02:00 1970 libm-f828430d7c4ee7de.libm.646417dd18176353-cgu.0.rcgu.o
--------- 0/0 3659432 Jan  1 02:00 1970 glam-63f13bd94f2750aa.glam.852b52b1c1856f37-cgu.0.rcgu.o
--------- 0/0 5396468 Jan  1 02:00 1970 compiler_builtins-2138275e1f227a29.compiler_builtins.b8f71e5756201651-cgu.0.rcgu.o
--------- 0/0 9258412 Jan  1 02:00 1970 core-55cff8dca7a6fe24.core.6da4bd542a2a13d-cgu.0.rcgu.o

After

$ cargo compiletest --target-env spv1.3
$ echo target/compiletest-deps/spirv-unknown-spv1.3/debug/deps/*.rlib | xargs -n1 llvm-ar tv | rg '\.o$' | sort -n -k 3
--------- 0/0     48 Jan  1 02:00 1970 bitflags-3dcf3d9be677438b.bitflags.a47078eccd6f1016-cgu.0.rcgu.o
--------- 0/0     48 Jan  1 02:00 1970 compiletests_deps_helper-32dfa6c895360273.cetel80wcnyywktmqqi32z0t9.1kwcmt2.rcgu.o
--------- 0/0  10900 Jan  1 02:00 1970 spirv_std_types-688f22e3cf3995e3.0h0l2egaag7apdniux376o36z.1ldvf4p.rcgu.o
--------- 0/0 850820 Jan  1 02:00 1970 spirv_std-3fa5eb13241d7c30.9o2xt2nbmavkkxywdyb4v65wl.0wbp380.rcgu.o
--------- 0/0 2239204 Jan  1 02:00 1970 num_traits-aa5c62cc8821fd9c.num_traits.92968d3e04044d3b-cgu.0.rcgu.o
--------- 0/0 2854988 Jan  1 02:00 1970 libm-4ae827bf8b5c3151.libm.30b39fea0ef0ed48-cgu.0.rcgu.o
--------- 0/0 3534688 Jan  1 02:00 1970 glam-ce3b52c9fe00e7e0.glam.158081abeaed99-cgu.0.rcgu.o
--------- 0/0 5507400 Jan  1 02:00 1970 compiler_builtins-b469d9463e1e9364.compiler_builtins.93b624f8cc19b5ca-cgu.0.rcgu.o
--------- 0/0 8925012 Jan  1 02:00 1970 core-0d15d1819711cec1.core.d3a3ec86f32056-cgu.0.rcgu.o
$ cargo run -p example-runner-wgpu --release
$ echo target/spirv-builder/spirv-unknown-vulkan1.1/release/deps/*.rlib | xargs -n1 llvm-ar tv | rg '\.o$' | sort -n -k 3 | rg -v shared-
--------- 0/0     88 Jan  1 02:00 1970 bitflags-0de303e2ee1c80c4.bitflags.ca8277fe98621545-cgu.0.rcgu.o
--------- 0/0     88 Jan  1 02:00 1970 bytemuck-e70aaa40b159da49.bytemuck.433330b546080123-cgu.0.rcgu.o
--------- 0/0  10812 Jan  1 02:00 1970 spirv_std_types-531bcff05bb52ca4.2kp5062ggpggjb8a9bw5p13g7.0jr4t2s.rcgu.o
--------- 0/0 847580 Jan  1 02:00 1970 spirv_std-acd900a964871511.5fh9vgyaa0jioh67zv8mt54km.1b0oz1f.rcgu.o
--------- 0/0 2228172 Jan  1 02:00 1970 num_traits-cb12f45b10b98d97.num_traits.e2bc6a17b294da77-cgu.0.rcgu.o
--------- 0/0 2876984 Jan  1 02:00 1970 libm-f828430d7c4ee7de.libm.646417dd18176353-cgu.0.rcgu.o
--------- 0/0 3459556 Jan  1 02:00 1970 glam-63f13bd94f2750aa.glam.852b52b1c1856f37-cgu.0.rcgu.o
--------- 0/0 5195832 Jan  1 02:00 1970 compiler_builtins-2138275e1f227a29.compiler_builtins.b8f71e5756201651-cgu.0.rcgu.o
--------- 0/0 8695756 Jan  1 02:00 1970 core-55cff8dca7a6fe24.core.6da4bd542a2a13d-cgu.0.rcgu.o

Debug (via `cargo compiletest`)

	Before	After	Change
Total	24.14 MiB	22.81 MiB	-1.33 MiB (-5.49%)
^{Largest (core)}	9.06 MiB	8.51 MiB	-0.55 MiB (-6.05%)
^{Most impacted (spirv-std)}	0.97 MiB	0.81 MiB	-0.16 MiB (-16.64%)

Release (via `example-runner-wgpu`)

	Before	After	Change
Total	23.55 MiB	22.23 MiB	-1.31 MiB (-5.58%)
^{Largest (core)}	8.83 MiB	8.29 MiB	-0.54 MiB (-6.08%)
^{Most impacted (spirv-std)}	0.97 MiB	0.81 MiB	-0.16 MiB (-16.68%)

_{(the overall impact of such a change might look even better, if/once we can find a way to remove e.g. the OpSource embedded source code,
which amounts to e.g. almost 4 MiB of core's 8-9 MiB - but that's more future work, alongside with increasing CGU count etc.)}

…ore (thanks to DCE).

…vs `optimize*` (centralized, even if noop).

…`s/linking/etc.).

LegNeato

This looks awesome and is surprisingly straightforward. Nice! 🍻

eddyb added 3 commits July 25, 2025 21:23

linker: make DCE mandatory and run it very early on, as well.

d429da3

linker: early zombie reporting doesn't need to mutate the module anym…

84929d9

…ore (thanks to DCE).

Triage WriteBackendMethods into "unimplemented" (w/ better errors) …

447cfa2

…vs `optimize*` (centralized, even if noop).

eddyb requested review from LegNeato, Firestar99 and schell as code owners July 25, 2025 20:00

Run DCE in per-crate optimize, too (before saving SPIR-V for `.rlib…

7ac082b

…`s/linking/etc.).

eddyb force-pushed the max-dce branch from 28df2e9 to 7ac082b Compare July 25, 2025 20:00

eddyb enabled auto-merge July 25, 2025 20:49

LegNeato approved these changes Jul 26, 2025

View reviewed changes

eddyb added this pull request to the merge queue Jul 26, 2025

Merged via the queue into Rust-GPU:main with commit 87ea628 Jul 26, 2025
13 checks passed

eddyb deleted the max-dce branch July 26, 2025 03:28

Firestar99 mentioned this pull request Jul 28, 2025

Make SpirvValue(Kind) representation and operations more orthogonal and robust (lossless). #348

Draft

eddyb mentioned this pull request Jul 29, 2025

Soundness fix: respect read_scalar errors in read_from_const_alloc. #353

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Make DCE mandatory and run it much earlier (even per-crate, shrinking `.rlib`s). #350

Make DCE mandatory and run it much earlier (even per-crate, shrinking `.rlib`s). #350

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eddyb commented Jul 25, 2025

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Make DCE mandatory and run it much earlier (even per-crate, shrinking .rlibs). #350

Make DCE mandatory and run it much earlier (even per-crate, shrinking .rlibs). #350

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eddyb commented Jul 25, 2025

Before

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Debug (via cargo compiletest)

Release (via example-runner-wgpu)

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Make DCE mandatory and run it much earlier (even per-crate, shrinking `.rlib`s). #350

Make DCE mandatory and run it much earlier (even per-crate, shrinking `.rlib`s). #350

Debug (via `cargo compiletest`)

Release (via `example-runner-wgpu`)