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Start improving monomorphization items stats by lqd · Pull Request #105481 · rus...

 1 year ago
source link: https://github.com/rust-lang/rust/pull/105481
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@lqd lqd commented Dec 9, 2022 •

edited

As described in this zulip discussion, some stats about monomorphization collection would be interesting to have, in a different form than -Zprint-mono-items: to have some visibility into the cost of the mono items, we'd like to know how many are instantiated and what is their estimated size.

That can be a proxy to analyze sources of slow compile times, although in the future, we'd also like to add more realistic stats from the actual backend's lowering.

This PR adds a new -Z dump-mono-stats flag which will output some stats in a {crate_name}.mono-items.md file (the flag optionally takes an output directory parameter, for easier use within a workspace than printing to stdout).

For example,

fn compute<T>(collection: Vec<T>) -> usize {
    collection.len() + 19 - 0 * 9 - 18 - 1 * 1 // random code to increase the function's size
}

fn main() {
    dbg!(compute(vec![0u8, 1, 2]));
    dbg!(compute(vec![0u64, 1, 2]));
    dbg!(compute(vec!["0", "1", "2", "3"]));
}

will output a file with this markdown table (abridged for readability), for a debug build:

Item Instantiation count Estimated Cost Per Instantiation Total Estimated Cost
alloc::alloc::box_free 3 122 366
std::alloc::Global::alloc_impl 1 284 284
alloc::raw_vec::RawVec::<T, A>::current_memory 3 82 246
std::ptr::align_offset 1 222 222
std::slice::hack::into_vec 3 67 201
<std::vec::Vec<T, A> as std::ops::Drop>::drop 3 66 198
std::ptr::mut_ptr::<impl *mut T>::is_null 4 47 188
main 1 163 163
std::ptr::NonNull::::new_unchecked 4 37 148
...
Click for full output

Since we discussed it together, r? @wesleywiser.


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