improve(desktop-gpui): write BGRA unpad through a raw pointer

[kyzer1023]

Summary

unpad_rgba_to_bgra (Linux/Windows editor playback via frame_image, and the portable camera-blur readback) allocated with vec![0; len] and then overwrote every byte. This writes through Vec::with_capacity + *mut u8 + set_len instead: same skip-zero allocation, tight nested loop, no iterator zip.

An earlier spare_capacity_mut / MaybeUninit::write version was faster on rustc 1.99 but ~0.78x slower on rustc 1.88 (the pinned toolchain), because 1.88 doesn't inline Iterator::zip over the MaybeUninit chunks. The raw-pointer loop is a win on both.

Change

• apps/desktop-gpui/kernels/src/frame.rs: raw-pointer unpad, no zero fill. Only unpad_rgba_to_bgra changes; main's reorder_32bit_row / convert_32bit_rows work is untouched.
• apps/desktop-gpui/kernels/benches/frame_unpad.rs: before/after harness with the previous implementation inlined (results consumed with let _ = black_box(...) per AGENTS.md)
• apps/desktop-gpui/kernels/Cargo.toml: keeps all of main's benches (including overlay_image_inspection, frame_export, screenshot_alpha) and appends frame_unpad
• Regression test for wgpu 256-byte aligned stride

Rebased onto main (0bc36e0d). Single commit.

Relationship to #2513

#2513 edits the same file (kernels/src/frame.rs) but different functions (swap_red_blue_into, convert_32bit_rows). The changes are complementary; landing both is a same-file merge, not overlapping edits.

Benchmarks

On the rebased commit. Intel Xeon, 4 cores, KVM; Ubuntu 24.04.5 LTS, Linux 6.12.94+. GPUI-like release profile (opt-level=3, lto=thin, codegen-units=1), 5 warmup, median of 30. Baseline is main's vec![0] path.

Toolchain	1248×702	1080p	4K
rustc 1.88.0	0.440 → 0.341 ms (1.29×)	1.094 → 0.841 ms (1.30×)	4.268 → 3.289 ms (1.30×)
rustc 1.99.1	0.437 → 0.339 ms (1.29×)	1.080 → 0.837 ms (1.29×)	4.373 → 3.386 ms (1.29×)

Earlier runs on previous bases matched (1.29–1.32× on both toolchains), with byte-identical output to the previous implementation and Miri clean on 11 tests.

Tests

Full cap-gpui-kernels suite: 40 passed, 0 failed, 1 ignored (benchmark_waveform_peaks) on both rustc 1.88.0 and 1.99.1.

The full GPUI app was not built: the [patch."https://github.com/wingleeio/zed"] entry points at /zed-cap, which isn't available in the test VM, so the kernels crate was extracted into a standalone workspace.

Repro

With /zed-cap checked out:

cd apps/desktop-gpui
cargo test -p cap-gpui-kernels --lib
cargo bench -p cap-gpui-kernels --bench frame_unpad -- --nocapture

Without it, extract apps/desktop-gpui/kernels into a one-crate workspace and run:

export FFMPEG_DIR=$PWD/target/native-deps
export LD_LIBRARY_PATH=$FFMPEG_DIR/lib
cargo test -p cap-gpui-kernels --lib
cargo bench -p cap-gpui-kernels --bench frame_unpad -- --nocapture

Confidence Score: 5/5

The PR appears safe to merge; no blocking issues remain.

Summary

Rewrites unpad_rgba_to_bgra to fill allocated space through a raw pointer instead of zeroing it first.

• Frame conversion now skips a zero-fill pass.

_{Reviews (3) · Last reviewed commit: "improve(desktop-gpui): write BGRA unpad ..." · Reviewed by Greptile}

[vercel]

@kyzer1023 is attempting to deploy a commit to the Cap Software Inc Team on Vercel.

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[greptile-apps]

Benchmark silently discards its result

black_box(f(...)) returns Option<Vec<u8>>, but the benchmark silently discards it. AGENTS.md requires every Option result to be explicitly handled. Use let _ = black_box(f(...)); to mark the discard as intentional and avoid the unused_must_use warning. This repository requirement must be satisfied before merging.

Suggested change

	black_box(f(
	black_box(&src),
	black_box(width),
	black_box(stride),
	black_box(height),
	));
	let _ = black_box(f(
	black_box(&src),
	black_box(width),
	black_box(stride),
	black_box(height),
	));

Context Used: AGENTS.md (source)

Prompt To Fix With AI

This is a comment left during a code review.
Path: apps/desktop-gpui/kernels/benches/frame_unpad.rs
Line: 21-26

Comment:
**Benchmark silently discards its result**

`black_box(f(...))` returns `Option<Vec<u8>>`, but the benchmark silently discards it. `AGENTS.md` requires every `Option` result to be explicitly handled. Use `let _ = black_box(f(...));` to mark the discard as intentional and avoid the `unused_must_use` warning. This repository requirement must be satisfied before merging.

```suggestion
                let _ = black_box(f(
                    black_box(&src),
                    black_box(width),
                    black_box(stride),
                    black_box(height),
                ));
```

**Context Used:** AGENTS.md ([source](https://github.com/capsoftware/cap/blob/main/AGENTS.md))

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

[kyzer1023]

Fixed in cb5d9d7: the benchmark now uses let _ = black_box(f(...)); to make the discard explicit, per AGENTS.md.

[kyzer1023]

@greptileai review

[kyzer1023]

@greptileai review