Padding of array<matCx3, 4> is clobbered in storage-bound writes from shader

[ErichDonGubler]

Description

When running some webgpu:shader,execution,padding:* tests in Firefox, I noticed that the following were failing:

• webgpu:shader,execution,padding:matCx3:*
• webgpu:shader,execution,padding:array_of_matCx3:*

Repro steps

I've reduced this to an MRE repo (erichdongubler-mre/wgpu-matCx3-padding-clobber) that runs natively on the wgpu crate, converting …:array_of_matCx3:columns=2;use_struct=false to Rust code. I recommend using the following

RUST_LOG=debug cargo run

Expected vs observed behavior

Examining the MRE's output, it seems we're failing these tests because we clobber the padding in the first matCx3 element, but not the others. Weird!

Extra materials

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Platform

This is confirmed to be an issue on MacOS. Firefox's CI reports issues on DX12, too, but I haven't investigated that platform thoroughly or reproduced locally (despite non-exhaustive attempts with the MRE above).

[ErichDonGubler]

CC @sagudev, who may be interested in a CTS-relevant issue.

[ErichDonGubler]

I wonder if this is relevant to #5739?