boa/core/profiler/src/lib.rs

//! The **`boa_profiler`** crate is a code profiler for Boa.
//!
//! # Crate Overview
//!
//! This crate provides a code profiler for Boa. For more information, please
//! see Boa's page on [profiling][profiler-md].
//!
//! [profiler-md]: https://github.com/boa-dev/boa/blob/main/docs/profiling.md
#![doc = include_str!("../ABOUT.md")]
#![doc(
    html_logo_url = "https://raw.githubusercontent.com/boa-dev/boa/main/assets/logo.svg",
    html_favicon_url = "https://raw.githubusercontent.com/boa-dev/boa/main/assets/logo.svg"
)]
#![cfg_attr(not(test), forbid(clippy::unwrap_used))]
#![cfg_attr(not(feature = "profiler"), no_std)]

use core::fmt::{self, Debug};

#[cfg(feature = "profiler")]
use measureme::{EventId, Profiler as MeasuremeProfiler, StringId, TimingGuard};
#[cfg(feature = "profiler")]
use once_cell::sync::OnceCell;
#[cfg(feature = "profiler")]
use rustc_hash::FxHashMap;
#[cfg(feature = "profiler")]
use std::collections::hash_map::Entry;
#[cfg(feature = "profiler")]
use std::sync::RwLock;
#[cfg(feature = "profiler")]
use std::{
    path::Path,
    thread::{current, ThreadId},
};

/// Profiler for the Boa JavaScript engine.
#[cfg(feature = "profiler")]
pub struct Profiler {
    profiler: MeasuremeProfiler,
    string_cache: RwLock<FxHashMap<String, StringId>>,
}

/// This static instance must never be public, and its only access must be done through the
/// `global()` and `drop()` methods. This is because `get_or_init` manages synchronization and the
/// case of an empty value.
#[cfg(feature = "profiler")]
static mut INSTANCE: OnceCell<Profiler> = OnceCell::new();

#[cfg(feature = "profiler")]
impl Profiler {
    /// Start a new profiled event.
    pub fn start_event(&self, label: &str, category: &str) -> TimingGuard<'_> {
        let kind = self.get_or_alloc_string(category);
        let id = EventId::from_label(self.get_or_alloc_string(label));
        let thread_id = Self::thread_id_to_u32(current().id());
        self.profiler
            .start_recording_interval_event(kind, id, thread_id)
    }

    #[allow(clippy::significant_drop_tightening)]
    fn get_or_alloc_string(&self, s: &str) -> StringId {
        {
            // Check the cache only with the read lock first.
            let cache = self
                .string_cache
                .read()
                .expect("Some writer panicked while holding an exclusive lock.");
            if let Some(id) = cache.get(s) {
                return *id;
            }
        }
        let mut cache = self
            .string_cache
            .write()
            .expect("Some writer panicked while holding an exclusive lock.");
        let entry = cache.entry(s.into());
        match entry {
            Entry::Occupied(entry) => *entry.get(),
            Entry::Vacant(entry) => {
                let id = self.profiler.alloc_string(s);
                *entry.insert(id)
            }
        }
    }

    fn default() -> Self {
        let profiler =
            MeasuremeProfiler::new(Path::new("./my_trace")).expect("must be able to create file");
        Self {
            profiler,
            string_cache: RwLock::new(FxHashMap::default()),
        }
    }

    /// Return the global instance of the profiler.
    #[must_use]
    pub fn global() -> &'static Self {
        unsafe { INSTANCE.get_or_init(Self::default) }
    }

    /// Drop the global instance of the profiler.
    ///
    /// # Panics
    ///
    /// Calling `drop` will panic if `INSTANCE` cannot be taken back.
    pub fn drop(&self) {
        // In order to drop the INSTANCE we need to get ownership of it, which isn't possible on a static unless you make it a mutable static
        // mutating statics is unsafe, so we need to wrap it as so.
        // This is actually safe though because init and drop are only called at the beginning and end of the application
        unsafe {
            INSTANCE
                .take()
                .expect("Could not take back profiler instance");
        }
    }

    // Sadly we need to use the unsafe method until this is resolved:
    // https://github.com/rust-lang/rust/issues/67939
    // Once `as_64()` is in stable we can do this:
    // https://github.com/rust-lang/rust/pull/68531/commits/ea42b1c5b85f649728e3a3b334489bac6dce890a
    // Until then our options are: use rust-nightly or use unsafe {}
    #[allow(clippy::cast_possible_truncation)]
    fn thread_id_to_u32(tid: ThreadId) -> u32 {
        unsafe { std::mem::transmute::<ThreadId, u64>(tid) as u32 }
    }
}

impl Debug for Profiler {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        Debug::fmt("no debug implemented", f)
    }
}

/// An empty profiler that does nothing.
#[cfg(not(feature = "profiler"))]
#[derive(Copy, Clone)]
pub struct Profiler;

#[cfg(not(feature = "profiler"))]
impl Profiler {
    /// Does nothing.
    #[allow(clippy::unused_unit)]
    pub const fn start_event(&self, _label: &str, _category: &str) -> () {}

    /// Does nothing.
    pub const fn drop(&self) {}

    /// Does nothing.
    #[must_use]
    pub const fn global() -> Self {
        Self
    }
}