std/sys/unix/time: make it easier for LLVM to optimize Instant subtraction.
#75545
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r? @sfackler (rust_highfive has picked a reviewer for you, use r? to override) |
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LGTM other than the 1 nit |
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Rollup of 4 pull requests Successful merges: - rust-lang#75376 (Set CMAKE_SYSTEM_NAME when cross-compiling) - rust-lang#75448 (merge `as_local_hir_id` with `local_def_id_to_hir_id`) - rust-lang#75513 (Recover gracefully from `struct` parse errors) - rust-lang#75545 (std/sys/unix/time: make it easier for LLVM to optimize `Instant` subtraction.) Failed merges: - rust-lang#75514 (Replaced `log` with `tracing`) r? @ghost
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This PR is the minimal change necessary to get LLVM to optimize
if self.t.tv_nsec >= other.t.tv_nsecto branchless instructions (at least on x86_64), inspired by @m-ou-se's own attempts at optimizingInstantsubtraction.I stumbled over this by looking at the total number of instructions executed by
rustc -Z self-profile, and found that after disabling ASLR, the largest source of non-determinism remaining was from thisiftaking one branch or the other, depending on the values involved.The reason this code is even called so many times to make a difference, is that
measureme(the-Z self-profileimplementation) currently usesInstant::elapsedfor its event timestamps (of which there can be millions).I doubt it's critical to land this, although perhaps it could slightly improve some forms of benchmarking.