Landing: db3b32439f9c
Project / Subsystem
gcc / middle-end
Date
2026-08-23
Author
Dominic P
Commit
db3b32439f9c8e98cea5625b9026cc42fd066917
Source
github
Perf win
Yes
Breaking
No
All attributes
- project
- gcc
- subsystem
- middle-end
- patch_id
- —
- commit_hash
- db3b32439f9c8e98cea5625b9026cc42fd066917
- source_type
- github
- headline
- Avoid spurious volatile load before bit-field store
- tldr
- Fixes a bug where volatile bit-field stores incorrectly emitted a read-modify-write cycle.
- author
- Dominic P
- outcome
- committed
- performance_win
- true
- breaking_change
- false
- series_id
- gcc:middle-end: don't read a memory destination fully overwritten by a bit-field store [pr71048]
- series_parts
- []
- tags
-
- • optimization
- • volatile
- • embedded
- discussion_id_link
- —
- bugzilla_pr
- —
- date
- 2026-08-23T00:00:00.000Z
When storing to a bit-field that occupies an entire memory unit, the compiler previously emitted a read of the memory destination before the write, even if no surrounding bits needed preserving. For non-volatile destinations, this read was optimized away, but for volatile destinations, it remained as a spurious memory access. This change modifies the store operation to directly write the value when the bit-field fills the entire unit, eliminating the unnecessary volatile read and preventing incorrect behavior with memory-mapped I/O registers that have side effects on reads.