[release/10.0] Fix alignment of 8-byte thread statics on direct TLS path#129785
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[release/10.0] Fix alignment of 8-byte thread statics on direct TLS path#129785github-actions[bot] wants to merge 1 commit into
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GetTLSIndexForThreadStatic computes the alignment for a thread static placed on the direct-on-thread-local-data bump allocator. The chain of size checks was missing an `else` before the `>= 4` case, so an 8-byte field (long/double) first set alignment = 8 and then immediately overwrote it with alignment = 4. The resulting offset could land on a 4-mod-8 boundary, producing a misaligned long/double. On arm64 this causes Interlocked.Increment(ref threadStaticLong) to throw DataMisalignedException, since the atomic lowers to ldaxr/stlxr which fault on a misaligned address. Fixes #129733 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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jkotas
approved these changes
Jun 24, 2026
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Backport of #129749 to release/10.0
/cc @EgorBo
Customer Impact
Some TLS (ThreadStatic) fields could be allocated with 4b alignment while demanding 8b e.g.
long. This may lead to DataMisalignedException in certain cases (e.g. using Interlocked on arm64, or double fields on arm32) or/and reiceve a perf penalty from misaligned access.Regression
It was introduced in .NET 9.0
Testing
Validated locally that 64-bit TLS variables never get any alignment less than 64 bytes.
Risk
Low to none.