[move][tracing] Update tracing to support not generating unneeded values internally for sparse traces#25900
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[move][tracing] Update tracing to support not generating unneeded values internally for sparse traces#25900
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| // Macro: resolve stack values as Push effects, or skip if !wants_effects. | ||
| // - push_effect!() — resolve the top-of-stack (index 0) as a single Push effect. | ||
| // - push_effect!(n) — resolve n stack values as Push effects (in reverse order). | ||
| macro_rules! push_effect { |
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Please write a macro emit_effects! that takes a body and reuse it everywhere.
Also, this macro is weird to see littered in the code below. I think preserving the intent of that code by using some emit_effects! { ... } will help a lot.
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Updated. Much nicer!
cgswords
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Mar 19, 2026
| } | ||
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| /// Skip building effects: clear any captured pre-effects and return empty. | ||
| fn skip_effects(&mut self) -> Vec<EF> { |
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Why do we clear them here? I appreciate we are trying to save some work, but generating the pre-effects is work too. We should never run afoul of this in the limit.
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Was able to remove without much issue so good call.
…ues internally for sparse traces
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Description
This adds
wants_effects()to theTracertrait so the VM tracer can skip expensive value resolution when the tracer will discard effect events and those values are not needed.When
InstructionOnlyTracerorFunctionOnlyTraceris used, the VM tracer was eagerly converting every runtime value into aSerializableMoveValueviainto_annotated_move_valuewhich incurred a recursive tree traversal with StructTag/Identifier cloning) for Push/Pop/Read/Write effects — only for those effects to be immediately filtered out bypush_event. This is the dominant per-instruction cost in the tracer.Changes:
Tracer::wants_effects() -> boolto the trait, and this is set to false byInstructionOnlyTracerandFunctionOnlyTracerwants_effects: boolin VMTracer at construction.start_instruction_impl: early-return path that skips all value resolution exceptStLocside effects (store_global/insert_local) and index capture forVecImmBorrow/VecMutBorrow.end_instruction_impl: usepush_effect!/emit!macros to gate value resolution behindwants_effects, while always performing type-stack management. Write-effect arms (StLoc,WriteRef,VecPushBack,VecSwap) use inline conditionals since their resolution logic is treated in a more special manner.The instruction-only trace baselines shrink (no effect events emitted), and full trace baselines see minor changes from the
FunctionOnlyTracer's notify fix (which was incorrectly keepingInstructionevents). The changes in the instruction only traces is because the trace IDs used for the frame IDs change (because the trace is smaller).Test plan
Existing tracing tests. This reduces generation of sparse traces
Release notes
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