libexpr: Optimise Value::type(), ValueStorage::getInternalType()#15271
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Ericson2314 merged 2 commits intomasterfrom Feb 17, 2026
Merged
libexpr: Optimise Value::type(), ValueStorage::getInternalType()#15271Ericson2314 merged 2 commits intomasterfrom
Value::type(), ValueStorage::getInternalType()#15271Ericson2314 merged 2 commits intomasterfrom
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This reduces the churn when changing up the order of values in a follow-up commit. This should have been done from the start ideally to improve readability.
Using nix::unreachable() in getInternalType() and type() turns out to be quite expensive and prevents inlining. Also Value::type got compiled to a jump table which has a high overhead from indirect jumps. Using an explicit lookup table turns out to be more efficient. This does mean that we lose out on nice diagnostics from nix::unreachable calls, but this code is probably one of the hottests functions in the whole evaluator, so I think the tradeoff is worth it. The nixUnreachableWhenHardened boils down to nix::unreachable when UBSan is enabled so we still have good coverage there.
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This has a noticeable ~3% wall time reduction and ~5% less instructions executed for common eval scenarios. |
Ericson2314
approved these changes
Feb 17, 2026
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Motivation
Using
nix::unreachable()ingetInternalType()andtype()turnsout to be quite expensive and prevents inlining. Also
Value::typegot compiled to a jump table which has a high overhead from indirect
jumps. Using an explicit lookup table turns out to be more efficient.
This does mean that we lose out on nice diagnostics from nix::unreachable
calls, but this code is probably one of the hottests functions in the whole
evaluator, so I think the tradeoff is worth it. The nixUnreachableWhenHardened
boils down to nix::unreachable when UBSan is enabled so we still have good
coverage there.
Context
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