ExprAttrs::eval: Don't write to the shared empty Bindings#537
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Evaluating an empty attrset literal wrote its position into
Bindings::emptyBindings, the shared static object that
EvalMemory::allocBindings() returns for zero-capacity bindings. Under
parallel evaluation this is a data race (the position of every '{ }'
was whichever one was evaluated last), and 'perf c2c' on an Intel
i7-1260P showed it also causes false sharing: emptyBindings happens
to share a cache line with Counter::enabled, which is read by every
thread in allocValue()/callFunction()/maybeThunk(), so each write
invalidated that line across all cores.
Skip the write for the shared empty bindings; '{ }' now has a
deterministic (undefined) position instead of a racy one, and the
cache line stays clean. Verified with perf c2c that the line no
longer appears among contended cache lines.
Assisted-by: Claude Fable 5 <noreply@anthropic.com>
cole-h
approved these changes
Jul 6, 2026
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Motivation
This is a (probably harmless) data race, but more importantly, it causes a lot of false sharing on the cache line holding
emptyBindings, which is bad for parallel eval.Taken from #534.
Context