perf(interpreter): add unlikely() hint to gas-block charge in run() hot loop#810
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zhoutianxia1 wants to merge 3 commits into
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perf(interpreter): add unlikely() hint to gas-block charge in run() hot loop#810zhoutianxia1 wants to merge 3 commits into
zhoutianxia1 wants to merge 3 commits into
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Two compile errors prevented grey from building on Windows: 1. build.rs: PathBuf::display() emits Windows backslash separators which Rust's lexer then interprets as escape sequences inside include_bytes! string literals (e.g. \j, \g, \o). Fix by replacing '\\' with '/' before embedding the path into the generated service_blobs.rs. 2. node.rs: tokio::signal::unix is not available on Windows. Wrap the SIGTERM, SIGUSR1, and SIGHUP signal registrations in #[cfg(unix)] and provide std::future::pending::<()>() placeholders on non-Unix so the select! arms compile and are simply never selected.
…ot loop Gas-block boundaries are sparse (~5-15% of instructions). The �b_gas_cost > 0 guard is almost always false, but without an explicit hint LLVM may not lay out the fall-through path optimally. Changes: - Add #![feature(core_intrinsics)] + #![allow(internal_features)] to crates/javm/src/lib.rs (project is already nightly-only per rust-toolchain.toml) - Import core::intrinsics::unlikely in interpreter/mod.rs - Wrap �b_gas_cost > 0 check with unlikely() so LLVM moves the charge block to a cold section, keeping the sequential hot path as fall-through Expected effect: 1-3% I-cache improvement on instruction-dense workloads (blake2b, keccak, ed25519). No change in semantics or gas values. Also add docs/interpreter-gas-branch-misprediction-analysis.md with full frequency analysis, cache impact study, and micro-architectural cost estimate. Refs: jarchain#400 (Optimize javm interpreter performance)
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Summary
Gas-block boundaries are sparse (~5-15% of instructions for typical workloads like blake2b, keccak, ed25519). The
bb_gas_cost > 0guard in therun()hot loop is almost always false, but without an explicit hint LLVM cannot prove this and may not lay out the fall-through path as the primary code stream.This PR adds
core::intrinsics::unlikely()around the guard so LLVM moves the gas-charge block to a cold section, keeping the sequential hot path as fall-through code - reducing I-cache footprint per iteration.Changes
grey/crates/javm/src/lib.rs: add#![feature(core_intrinsics)]+#![allow(internal_features)]. The project already requires nightly (rust-toolchain.tomlpins nightly edition 2024).grey/crates/javm/src/interpreter/mod.rs: importcore::intrinsics::unlikely; wrapbb_gas_cost > 0check.docs/interpreter-gas-branch-misprediction-analysis.md: full analysis including frequency study, cache impact, micro-architectural cost estimate, recommendations.Analysis
Key findings:
unlikelymoves the cold charge block out-of-line, shrinking the hot path cache footprintCorrectness
Zero change in semantics or gas values.
unlikelyis a pure code-generation hint.cargo check -p javmpasses with no errors and no warnings.Refs: #400