max / quasi

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1	//! Allocation counting, compiled in only under `--features count`.
2	//!
3	//! The count is the number that matters. It is stable across machines and
4	//! across load, which a wall-clock number on a shared dev box is not, and it is
5	//! what an emitter change actually moves. The timing run answers "is this
6	//! faster here today"; this one answers "does this do less work".
7	//!
8	//! # Why `dhat` rather than a hand-written `GlobalAlloc`
9	//!
10	//! The workspace sets `unsafe_code = "forbid"`, and a `GlobalAlloc`
11	//! implementation cannot be written without `unsafe`. `forbid` is not
12	//! overridable by an `#[allow]`, so a hand-rolled counter would mean this crate
13	//! opting out of the workspace lint block, and that block is meant to be
14	//! identical everywhere. A dependency that is off by default is the cheaper
15	//! trade: `dhat` holds the `unsafe`, and the bench holds none.
16	//!
17	//! It also answers the reason the two measurements are two builds. `dhat`'s
18	//! allocator is real instrumentation and a timing number taken through it would
19	//! be a number about the instrumentation, so `count` is never on in a timing
20	//! run.
21
22	/// The counting allocator, in place only in a `--features count` build.
23	#[global_allocator]
24	static ALLOC: dhat::Alloc = dhat::Alloc;
25
26	/// A running profiler, held for as long as measurements are being taken.
27	pub struct Session(dhat::Profiler);
28
29	impl Session {
30	/// Start counting.
31	///
32	/// `testing()` keeps `dhat` from writing its JSON output file: the bench
33	/// wants two integers, not a profile to open in a viewer.
34	#[must_use]
35	pub fn start() -> Self {
36	Self(dhat::Profiler::builder().testing().build())
37	}
38	}
39
40	/// Allocations and bytes so far in this session.
41	///
42	/// `total_blocks` is every allocation since the profiler started, which is what
43	/// "how many allocations does rendering this screen take" means. The live and
44	/// peak figures `dhat` also carries are about retention, and a renderer that
45	/// returns a `String` and drops everything else has nothing interesting there.
46	#[must_use]
47	pub fn read() -> (u64, u64) {
48	let stats = dhat::HeapStats::get();
49	(stats.total_blocks, stats.total_bytes)
50	}
51