Add alignas(T) to HeapVectorBacking to preserve element alignment [chromium/src : main]

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Keishi Hattori (Gerrit)

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Aug 4, 2026, 2:45:11 AM (6 days ago) Aug 4

to Michael Lippautz, Chromium LUCI CQ, android-bu...@system.gserviceaccount.com, chromium...@chromium.org, Kentaro Hara, blink-...@chromium.org, kinuko...@chromium.org, kouhe...@chromium.org, oilpan-rev...@chromium.org

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File-level comment, Patchset 6 (Latest):

Keishi Hattori . resolved

PTAL. This CL serves as the immediate fix for libc++ hardening aborts triggered by nested vectors (e.g., HeapVector<HeapVector<...>>) on 32-bit DCHECK builds.

Root Cause:
In DCHECK builds, VectorBufferBase previously used an int64_t for modifications_, artificially boosting alignof(HeapVector) to 8 bytes on 32-bit systems. Oilpan's backing store wrapper (HeapVectorBacking<T>) did not inherit element alignment, resulting in 4-byte aligned backing heap allocations. Casting a 4-byte aligned backing buffer to an 8-byte aligned inner HeapVector pointer triggered fatal memory alignment aborts.

I landed an immediate fix by changing the counter type(https://crrev.com/c/8176251) but I thought we should fix this too.

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