rust/compiler/rustc_mir/src/monomorphize/partitioning/mod.rs

//! Partitioning Codegen Units for Incremental Compilation
//! ======================================================
//!
//! The task of this module is to take the complete set of monomorphizations of
//! a crate and produce a set of codegen units from it, where a codegen unit
//! is a named set of (mono-item, linkage) pairs. That is, this module
//! decides which monomorphization appears in which codegen units with which
//! linkage. The following paragraphs describe some of the background on the
//! partitioning scheme.
//!
//! The most important opportunity for saving on compilation time with
//! incremental compilation is to avoid re-codegenning and re-optimizing code.
//! Since the unit of codegen and optimization for LLVM is "modules" or, how
//! we call them "codegen units", the particulars of how much time can be saved
//! by incremental compilation are tightly linked to how the output program is
//! partitioned into these codegen units prior to passing it to LLVM --
//! especially because we have to treat codegen units as opaque entities once
//! they are created: There is no way for us to incrementally update an existing
//! LLVM module and so we have to build any such module from scratch if it was
//! affected by some change in the source code.
//!
//! From that point of view it would make sense to maximize the number of
//! codegen units by, for example, putting each function into its own module.
//! That way only those modules would have to be re-compiled that were actually
//! affected by some change, minimizing the number of functions that could have
//! been re-used but just happened to be located in a module that is
//! re-compiled.
//!
//! However, since LLVM optimization does not work across module boundaries,
//! using such a highly granular partitioning would lead to very slow runtime
//! code since it would effectively prohibit inlining and other inter-procedure
//! optimizations. We want to avoid that as much as possible.
//!
//! Thus we end up with a trade-off: The bigger the codegen units, the better
//! LLVM's optimizer can do its work, but also the smaller the compilation time
//! reduction we get from incremental compilation.
//!
//! Ideally, we would create a partitioning such that there are few big codegen
//! units with few interdependencies between them. For now though, we use the
//! following heuristic to determine the partitioning:
//!
//! - There are two codegen units for every source-level module:
//! - One for "stable", that is non-generic, code
//! - One for more "volatile" code, i.e., monomorphized instances of functions
//!   defined in that module
//!
//! In order to see why this heuristic makes sense, let's take a look at when a
//! codegen unit can get invalidated:
//!
//! 1. The most straightforward case is when the BODY of a function or global
//! changes. Then any codegen unit containing the code for that item has to be
//! re-compiled. Note that this includes all codegen units where the function
//! has been inlined.
//!
//! 2. The next case is when the SIGNATURE of a function or global changes. In
//! this case, all codegen units containing a REFERENCE to that item have to be
//! re-compiled. This is a superset of case 1.
//!
//! 3. The final and most subtle case is when a REFERENCE to a generic function
//! is added or removed somewhere. Even though the definition of the function
//! might be unchanged, a new REFERENCE might introduce a new monomorphized
//! instance of this function which has to be placed and compiled somewhere.
//! Conversely, when removing a REFERENCE, it might have been the last one with
//! that particular set of generic arguments and thus we have to remove it.
//!
//! From the above we see that just using one codegen unit per source-level
//! module is not such a good idea, since just adding a REFERENCE to some
//! generic item somewhere else would invalidate everything within the module
//! containing the generic item. The heuristic above reduces this detrimental
//! side-effect of references a little by at least not touching the non-generic
//! code of the module.
//!
//! A Note on Inlining
//! ------------------
//! As briefly mentioned above, in order for LLVM to be able to inline a
//! function call, the body of the function has to be available in the LLVM
//! module where the call is made. This has a few consequences for partitioning:
//!
//! - The partitioning algorithm has to take care of placing functions into all
//!   codegen units where they should be available for inlining. It also has to
//!   decide on the correct linkage for these functions.
//!
//! - The partitioning algorithm has to know which functions are likely to get
//!   inlined, so it can distribute function instantiations accordingly. Since
//!   there is no way of knowing for sure which functions LLVM will decide to
//!   inline in the end, we apply a heuristic here: Only functions marked with
//!   `#[inline]` are considered for inlining by the partitioner. The current
//!   implementation will not try to determine if a function is likely to be
//!   inlined by looking at the functions definition.
//!
//! Note though that as a side-effect of creating a codegen units per
//! source-level module, functions from the same module will be available for
//! inlining, even when they are not marked `#[inline]`.

mod default;
mod merging;

use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_data_structures::sync;
use rustc_hir::def_id::{CrateNum, DefIdSet, LOCAL_CRATE};
use rustc_middle::mir::mono::MonoItem;
use rustc_middle::mir::mono::{CodegenUnit, Linkage};
use rustc_middle::ty::query::Providers;
use rustc_middle::ty::TyCtxt;
use rustc_span::symbol::Symbol;

use crate::monomorphize::collector::InliningMap;
use crate::monomorphize::collector::{self, MonoItemCollectionMode};

trait Partitioner<'tcx> {
    fn place_root_mono_items(
        &mut self,
        tcx: TyCtxt<'tcx>,
        mono_items: &mut dyn Iterator<Item = MonoItem<'tcx>>,
    ) -> PreInliningPartitioning<'tcx>;

    fn merge_codegen_units(
        &mut self,
        tcx: TyCtxt<'tcx>,
        initial_partitioning: &mut PreInliningPartitioning<'tcx>,
        target_cgu_count: usize,
    );

    fn place_inlined_mono_items(
        &mut self,
        initial_partitioning: PreInliningPartitioning<'tcx>,
        inlining_map: &InliningMap<'tcx>,
    ) -> PostInliningPartitioning<'tcx>;

    fn internalize_symbols(
        &mut self,
        tcx: TyCtxt<'tcx>,
        partitioning: &mut PostInliningPartitioning<'tcx>,
        inlining_map: &InliningMap<'tcx>,
    );
}

fn get_partitioner<'tcx>(tcx: TyCtxt<'tcx>) -> Box<dyn Partitioner<'tcx>> {
    let strategy = match &tcx.sess.opts.debugging_opts.cgu_partitioning_strategy {
        None => "default",
        Some(s) => &s[..],
    };

    match strategy {
        "default" => Box::new(default::DefaultPartitioning),
        _ => tcx.sess.fatal("unknown partitioning strategy"),
    }
}

pub fn partition<'tcx>(
    tcx: TyCtxt<'tcx>,
    mono_items: &mut dyn Iterator<Item = MonoItem<'tcx>>,
    max_cgu_count: usize,
    inlining_map: &InliningMap<'tcx>,
) -> Vec<CodegenUnit<'tcx>> {
    let _prof_timer = tcx.prof.generic_activity("cgu_partitioning");

    let mut partitioner = get_partitioner(tcx);
    // In the first step, we place all regular monomorphizations into their
    // respective 'home' codegen unit. Regular monomorphizations are all
    // functions and statics defined in the local crate.
    let mut initial_partitioning = {
        let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_place_roots");
        partitioner.place_root_mono_items(tcx, mono_items)
    };

    initial_partitioning.codegen_units.iter_mut().for_each(|cgu| cgu.estimate_size(tcx));

    debug_dump(tcx, "INITIAL PARTITIONING:", initial_partitioning.codegen_units.iter());

    // Merge until we have at most `max_cgu_count` codegen units.
    {
        let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_merge_cgus");
        partitioner.merge_codegen_units(tcx, &mut initial_partitioning, max_cgu_count);
        debug_dump(tcx, "POST MERGING:", initial_partitioning.codegen_units.iter());
    }

    // In the next step, we use the inlining map to determine which additional
    // monomorphizations have to go into each codegen unit. These additional
    // monomorphizations can be drop-glue, functions from external crates, and
    // local functions the definition of which is marked with `#[inline]`.
    let mut post_inlining = {
        let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_place_inline_items");
        partitioner.place_inlined_mono_items(initial_partitioning, inlining_map)
    };

    post_inlining.codegen_units.iter_mut().for_each(|cgu| cgu.estimate_size(tcx));

    debug_dump(tcx, "POST INLINING:", post_inlining.codegen_units.iter());

    // Next we try to make as many symbols "internal" as possible, so LLVM has
    // more freedom to optimize.
    if !tcx.sess.link_dead_code() {
        let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_internalize_symbols");
        partitioner.internalize_symbols(tcx, &mut post_inlining, inlining_map);
    }

    // Finally, sort by codegen unit name, so that we get deterministic results.
    let PostInliningPartitioning {
        codegen_units: mut result,
        mono_item_placements: _,
        internalization_candidates: _,
    } = post_inlining;

    result.sort_by_cached_key(|cgu| cgu.name().as_str());

    result
}

pub struct PreInliningPartitioning<'tcx> {
    codegen_units: Vec<CodegenUnit<'tcx>>,
    roots: FxHashSet<MonoItem<'tcx>>,
    internalization_candidates: FxHashSet<MonoItem<'tcx>>,
}

/// For symbol internalization, we need to know whether a symbol/mono-item is
/// accessed from outside the codegen unit it is defined in. This type is used
/// to keep track of that.
#[derive(Clone, PartialEq, Eq, Debug)]
enum MonoItemPlacement {
    SingleCgu { cgu_name: Symbol },
    MultipleCgus,
}

struct PostInliningPartitioning<'tcx> {
    codegen_units: Vec<CodegenUnit<'tcx>>,
    mono_item_placements: FxHashMap<MonoItem<'tcx>, MonoItemPlacement>,
    internalization_candidates: FxHashSet<MonoItem<'tcx>>,
}

fn debug_dump<'a, 'tcx, I>(tcx: TyCtxt<'tcx>, label: &str, cgus: I)
where
    I: Iterator<Item = &'a CodegenUnit<'tcx>>,
    'tcx: 'a,
{
    if cfg!(debug_assertions) {
        debug!("{}", label);
        for cgu in cgus {
            debug!("CodegenUnit {} estimated size {} :", cgu.name(), cgu.size_estimate());

            for (mono_item, linkage) in cgu.items() {
                let symbol_name = mono_item.symbol_name(tcx).name;
                let symbol_hash_start = symbol_name.rfind('h');
                let symbol_hash =
                    symbol_hash_start.map(|i| &symbol_name[i..]).unwrap_or("<no hash>");

                debug!(
                    " - {} [{:?}] [{}] estimated size {}",
                    mono_item,
                    linkage,
                    symbol_hash,
                    mono_item.size_estimate(tcx)
                );
            }

            debug!("");
        }
    }
}

#[inline(never)] // give this a place in the profiler
fn assert_symbols_are_distinct<'a, 'tcx, I>(tcx: TyCtxt<'tcx>, mono_items: I)
where
    I: Iterator<Item = &'a MonoItem<'tcx>>,
    'tcx: 'a,
{
    let _prof_timer = tcx.prof.generic_activity("assert_symbols_are_distinct");

    let mut symbols: Vec<_> =
        mono_items.map(|mono_item| (mono_item, mono_item.symbol_name(tcx))).collect();

    symbols.sort_by_key(|sym| sym.1);

    for pair in symbols.windows(2) {
        let sym1 = &pair[0].1;
        let sym2 = &pair[1].1;

        if sym1 == sym2 {
            let mono_item1 = pair[0].0;
            let mono_item2 = pair[1].0;

            let span1 = mono_item1.local_span(tcx);
            let span2 = mono_item2.local_span(tcx);

            // Deterministically select one of the spans for error reporting
            let span = match (span1, span2) {
                (Some(span1), Some(span2)) => {
                    Some(if span1.lo().0 > span2.lo().0 { span1 } else { span2 })
                }
                (span1, span2) => span1.or(span2),
            };

            let error_message = format!("symbol `{}` is already defined", sym1);

            if let Some(span) = span {
                tcx.sess.span_fatal(span, &error_message)
            } else {
                tcx.sess.fatal(&error_message)
            }
        }
    }
}

fn collect_and_partition_mono_items<'tcx>(
    tcx: TyCtxt<'tcx>,
    cnum: CrateNum,
) -> (&'tcx DefIdSet, &'tcx [CodegenUnit<'tcx>]) {
    assert_eq!(cnum, LOCAL_CRATE);

    let collection_mode = match tcx.sess.opts.debugging_opts.print_mono_items {
        Some(ref s) => {
            let mode_string = s.to_lowercase();
            let mode_string = mode_string.trim();
            if mode_string == "eager" {
                MonoItemCollectionMode::Eager
            } else {
                if mode_string != "lazy" {
                    let message = format!(
                        "Unknown codegen-item collection mode '{}'. \
                                           Falling back to 'lazy' mode.",
                        mode_string
                    );
                    tcx.sess.warn(&message);
                }

                MonoItemCollectionMode::Lazy
            }
        }
        None => {
            if tcx.sess.link_dead_code() {
                MonoItemCollectionMode::Eager
            } else {
                MonoItemCollectionMode::Lazy
            }
        }
    };

    let (items, inlining_map) = collector::collect_crate_mono_items(tcx, collection_mode);

    tcx.sess.abort_if_errors();

    let (codegen_units, _) = tcx.sess.time("partition_and_assert_distinct_symbols", || {
        sync::join(
            || {
                &*tcx.arena.alloc_from_iter(partition(
                    tcx,
                    &mut items.iter().cloned(),
                    tcx.sess.codegen_units(),
                    &inlining_map,
                ))
            },
            || assert_symbols_are_distinct(tcx, items.iter()),
        )
    });

    let mono_items: DefIdSet = items
        .iter()
        .filter_map(|mono_item| match *mono_item {
            MonoItem::Fn(ref instance) => Some(instance.def_id()),
            MonoItem::Static(def_id) => Some(def_id),
            _ => None,
        })
        .collect();

    if tcx.sess.opts.debugging_opts.print_mono_items.is_some() {
        let mut item_to_cgus: FxHashMap<_, Vec<_>> = Default::default();

        for cgu in codegen_units {
            for (&mono_item, &linkage) in cgu.items() {
                item_to_cgus.entry(mono_item).or_default().push((cgu.name(), linkage));
            }
        }

        let mut item_keys: Vec<_> = items
            .iter()
            .map(|i| {
                let mut output = i.to_string();
                output.push_str(" @@");
                let mut empty = Vec::new();
                let cgus = item_to_cgus.get_mut(i).unwrap_or(&mut empty);
                cgus.sort_by_key(|(name, _)| *name);
                cgus.dedup();
                for &(ref cgu_name, (linkage, _)) in cgus.iter() {
                    output.push_str(" ");
                    output.push_str(&cgu_name.as_str());

                    let linkage_abbrev = match linkage {
                        Linkage::External => "External",
                        Linkage::AvailableExternally => "Available",
                        Linkage::LinkOnceAny => "OnceAny",
                        Linkage::LinkOnceODR => "OnceODR",
                        Linkage::WeakAny => "WeakAny",
                        Linkage::WeakODR => "WeakODR",
                        Linkage::Appending => "Appending",
                        Linkage::Internal => "Internal",
                        Linkage::Private => "Private",
                        Linkage::ExternalWeak => "ExternalWeak",
                        Linkage::Common => "Common",
                    };

                    output.push_str("[");
                    output.push_str(linkage_abbrev);
                    output.push_str("]");
                }
                output
            })
            .collect();

        item_keys.sort();

        for item in item_keys {
            println!("MONO_ITEM {}", item);
        }
    }

    (tcx.arena.alloc(mono_items), codegen_units)
}

pub fn provide(providers: &mut Providers) {
    providers.collect_and_partition_mono_items = collect_and_partition_mono_items;

    providers.is_codegened_item = |tcx, def_id| {
        let (all_mono_items, _) = tcx.collect_and_partition_mono_items(LOCAL_CRATE);
        all_mono_items.contains(&def_id)
    };

    providers.codegen_unit = |tcx, name| {
        let (_, all) = tcx.collect_and_partition_mono_items(LOCAL_CRATE);
        all.iter()
            .find(|cgu| cgu.name() == name)
            .unwrap_or_else(|| panic!("failed to find cgu with name {:?}", name))
    };
}