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rust/compiler/rustc_monomorphize/src/partitioning.rs

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//! 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]`.
use std::cmp;
use std::collections::hash_map::Entry;
use std::fs::{self, File};
use std::io::{BufWriter, Write};
use std::path::{Path, PathBuf};
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_data_structures::sync;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{DefId, DefIdSet, LOCAL_CRATE};
use rustc_hir::definitions::DefPathDataName;
use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
use rustc_middle::middle::exported_symbols::{SymbolExportInfo, SymbolExportLevel};
use rustc_middle::mir::mono::{
CodegenUnit, CodegenUnitNameBuilder, InstantiationMode, Linkage, MonoItem, MonoItemData,
Visibility,
};
use rustc_middle::query::Providers;
use rustc_middle::ty::print::{characteristic_def_id_of_type, with_no_trimmed_paths};
use rustc_middle::ty::{self, visit::TypeVisitableExt, InstanceDef, TyCtxt};
use rustc_session::config::{DumpMonoStatsFormat, SwitchWithOptPath};
use rustc_session::CodegenUnits;
use rustc_span::symbol::Symbol;
use crate::collector::UsageMap;
use crate::collector::{self, MonoItemCollectionMode};
use crate::errors::{CouldntDumpMonoStats, SymbolAlreadyDefined, UnknownCguCollectionMode};
struct PartitioningCx<'a, 'tcx> {
tcx: TyCtxt<'tcx>,
usage_map: &'a UsageMap<'tcx>,
}
struct PlacedMonoItems<'tcx> {
/// The codegen units, sorted by name to make things deterministic.
codegen_units: Vec<CodegenUnit<'tcx>>,
internalization_candidates: FxHashSet<MonoItem<'tcx>>,
}
// The output CGUs are sorted by name.
fn partition<'tcx, I>(
tcx: TyCtxt<'tcx>,
mono_items: I,
usage_map: &UsageMap<'tcx>,
) -> Vec<CodegenUnit<'tcx>>
where
I: Iterator<Item = MonoItem<'tcx>>,
{
let _prof_timer = tcx.prof.generic_activity("cgu_partitioning");
let cx = &PartitioningCx { tcx, usage_map };
// Place all mono items into a codegen unit. `place_mono_items` is
// responsible for initializing the CGU size estimates.
let PlacedMonoItems { mut codegen_units, internalization_candidates } = {
let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_place_items");
let placed = place_mono_items(cx, mono_items);
debug_dump(tcx, "PLACE", &placed.codegen_units);
placed
};
// Merge until we don't exceed the max CGU count.
// `merge_codegen_units` is responsible for updating the CGU size
// estimates.
{
let _prof_timer = tcx.prof.generic_activity("cgu_partitioning_merge_cgus");
merge_codegen_units(cx, &mut codegen_units);
debug_dump(tcx, "MERGE", &codegen_units);
}
// 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");
internalize_symbols(cx, &mut codegen_units, internalization_candidates);
debug_dump(tcx, "INTERNALIZE", &codegen_units);
}
// Mark one CGU for dead code, if necessary.
if tcx.sess.instrument_coverage() {
mark_code_coverage_dead_code_cgu(&mut codegen_units);
}
// Ensure CGUs are sorted by name, so that we get deterministic results.
if !codegen_units.is_sorted_by(|a, b| a.name().as_str() <= b.name().as_str()) {
let mut names = String::new();
for cgu in codegen_units.iter() {
names += &format!("- {}\n", cgu.name());
}
bug!("unsorted CGUs:\n{names}");
}
codegen_units
}
fn place_mono_items<'tcx, I>(cx: &PartitioningCx<'_, 'tcx>, mono_items: I) -> PlacedMonoItems<'tcx>
where
I: Iterator<Item = MonoItem<'tcx>>,
{
let mut codegen_units = FxHashMap::default();
let is_incremental_build = cx.tcx.sess.opts.incremental.is_some();
let mut internalization_candidates = FxHashSet::default();
// Determine if monomorphizations instantiated in this crate will be made
// available to downstream crates. This depends on whether we are in
// share-generics mode and whether the current crate can even have
// downstream crates.
let export_generics =
cx.tcx.sess.opts.share_generics() && cx.tcx.local_crate_exports_generics();
let cgu_name_builder = &mut CodegenUnitNameBuilder::new(cx.tcx);
let cgu_name_cache = &mut FxHashMap::default();
for mono_item in mono_items {
// Handle only root (GloballyShared) items directly here. Inlined (LocalCopy) items
// are handled at the bottom of the loop based on reachability, with one exception.
// The #[lang = "start"] item is the program entrypoint, so there are no calls to it in MIR.
// So even if its mode is LocalCopy, we need to treat it like a root.
match mono_item.instantiation_mode(cx.tcx) {
InstantiationMode::GloballyShared { .. } => {}
InstantiationMode::LocalCopy => {
if Some(mono_item.def_id()) != cx.tcx.lang_items().start_fn() {
continue;
}
}
}
let characteristic_def_id = characteristic_def_id_of_mono_item(cx.tcx, mono_item);
let is_volatile = is_incremental_build && mono_item.is_generic_fn(cx.tcx);
let cgu_name = match characteristic_def_id {
Some(def_id) => compute_codegen_unit_name(
cx.tcx,
cgu_name_builder,
def_id,
is_volatile,
cgu_name_cache,
),
None => fallback_cgu_name(cgu_name_builder),
};
let cgu = codegen_units.entry(cgu_name).or_insert_with(|| CodegenUnit::new(cgu_name));
let mut can_be_internalized = true;
let (linkage, visibility) = mono_item_linkage_and_visibility(
cx.tcx,
&mono_item,
&mut can_be_internalized,
export_generics,
);
if visibility == Visibility::Hidden && can_be_internalized {
internalization_candidates.insert(mono_item);
}
let size_estimate = mono_item.size_estimate(cx.tcx);
cgu.items_mut()
.insert(mono_item, MonoItemData { inlined: false, linkage, visibility, size_estimate });
// Get all inlined items that are reachable from `mono_item` without
// going via another root item. This includes drop-glue, functions from
// external crates, and local functions the definition of which is
// marked with `#[inline]`.
let mut reachable_inlined_items = FxHashSet::default();
get_reachable_inlined_items(cx.tcx, mono_item, cx.usage_map, &mut reachable_inlined_items);
// Add those inlined items. It's possible an inlined item is reachable
// from multiple root items within a CGU, which is fine, it just means
// the `insert` will be a no-op.
for inlined_item in reachable_inlined_items {
// This is a CGU-private copy.
cgu.items_mut().entry(inlined_item).or_insert_with(|| MonoItemData {
inlined: true,
linkage: Linkage::Internal,
visibility: Visibility::Default,
size_estimate: inlined_item.size_estimate(cx.tcx),
});
}
}
// Always ensure we have at least one CGU; otherwise, if we have a
// crate with just types (for example), we could wind up with no CGU.
if codegen_units.is_empty() {
let cgu_name = fallback_cgu_name(cgu_name_builder);
codegen_units.insert(cgu_name, CodegenUnit::new(cgu_name));
}
let mut codegen_units: Vec<_> = codegen_units.into_values().collect();
codegen_units.sort_by(|a, b| a.name().as_str().cmp(b.name().as_str()));
for cgu in codegen_units.iter_mut() {
cgu.compute_size_estimate();
}
return PlacedMonoItems { codegen_units, internalization_candidates };
fn get_reachable_inlined_items<'tcx>(
tcx: TyCtxt<'tcx>,
item: MonoItem<'tcx>,
usage_map: &UsageMap<'tcx>,
visited: &mut FxHashSet<MonoItem<'tcx>>,
) {
usage_map.for_each_inlined_used_item(tcx, item, |inlined_item| {
let is_new = visited.insert(inlined_item);
if is_new {
get_reachable_inlined_items(tcx, inlined_item, usage_map, visited);
}
});
}
}
// This function requires the CGUs to be sorted by name on input, and ensures
// they are sorted by name on return, for deterministic behaviour.
fn merge_codegen_units<'tcx>(
cx: &PartitioningCx<'_, 'tcx>,
codegen_units: &mut Vec<CodegenUnit<'tcx>>,
) {
assert!(cx.tcx.sess.codegen_units().as_usize() >= 1);
// A sorted order here ensures merging is deterministic.
assert!(codegen_units.is_sorted_by(|a, b| a.name().as_str() <= b.name().as_str()));
// This map keeps track of what got merged into what.
let mut cgu_contents: FxHashMap<Symbol, Vec<Symbol>> =
codegen_units.iter().map(|cgu| (cgu.name(), vec![cgu.name()])).collect();
// If N is the maximum number of CGUs, and the CGUs are sorted from largest
// to smallest, we repeatedly find which CGU in codegen_units[N..] has the
// greatest overlap of inlined items with codegen_units[N-1], merge that
// CGU into codegen_units[N-1], then re-sort by size and repeat.
//
// We use inlined item overlap to guide this merging because it minimizes
// duplication of inlined items, which makes LLVM be faster and generate
// better and smaller machine code.
//
// Why merge into codegen_units[N-1]? We want CGUs to have similar sizes,
// which means we don't want codegen_units[0..N] (the already big ones)
// getting any bigger, if we can avoid it. When we have more than N CGUs
// then at least one of the biggest N will have to grow. codegen_units[N-1]
// is the smallest of those, and so has the most room to grow.
let max_codegen_units = cx.tcx.sess.codegen_units().as_usize();
while codegen_units.len() > max_codegen_units {
// Sort small CGUs to the back.
codegen_units.sort_by_key(|cgu| cmp::Reverse(cgu.size_estimate()));
let cgu_dst = &codegen_units[max_codegen_units - 1];
// Find the CGU that overlaps the most with `cgu_dst`. In the case of a
// tie, favour the earlier (bigger) CGU.
let mut max_overlap = 0;
let mut max_overlap_i = max_codegen_units;
for (i, cgu_src) in codegen_units.iter().enumerate().skip(max_codegen_units) {
if cgu_src.size_estimate() <= max_overlap {
// None of the remaining overlaps can exceed `max_overlap`, so
// stop looking.
break;
}
let overlap = compute_inlined_overlap(cgu_dst, cgu_src);
if overlap > max_overlap {
max_overlap = overlap;
max_overlap_i = i;
}
}
let mut cgu_src = codegen_units.swap_remove(max_overlap_i);
let cgu_dst = &mut codegen_units[max_codegen_units - 1];
// Move the items from `cgu_src` to `cgu_dst`. Some of them may be
// duplicate inlined items, in which case the destination CGU is
// unaffected. Recalculate size estimates afterwards.
cgu_dst.items_mut().extend(cgu_src.items_mut().drain(..));
cgu_dst.compute_size_estimate();
// Record that `cgu_dst` now contains all the stuff that was in
// `cgu_src` before.
let mut consumed_cgu_names = cgu_contents.remove(&cgu_src.name()).unwrap();
cgu_contents.get_mut(&cgu_dst.name()).unwrap().append(&mut consumed_cgu_names);
}
// Having multiple CGUs can drastically speed up compilation. But for
// non-incremental builds, tiny CGUs slow down compilation *and* result in
// worse generated code. So we don't allow CGUs smaller than this (unless
// there is just one CGU, of course). Note that CGU sizes of 100,000+ are
// common in larger programs, so this isn't all that large.
const NON_INCR_MIN_CGU_SIZE: usize = 1800;
// Repeatedly merge the two smallest codegen units as long as: it's a
// non-incremental build, and the user didn't specify a CGU count, and
// there are multiple CGUs, and some are below the minimum size.
//
// The "didn't specify a CGU count" condition is because when an explicit
// count is requested we observe it as closely as possible. For example,
// the `compiler_builtins` crate sets `codegen-units = 10000` and it's
// critical they aren't merged. Also, some tests use explicit small values
// and likewise won't work if small CGUs are merged.
while cx.tcx.sess.opts.incremental.is_none()
&& matches!(cx.tcx.sess.codegen_units(), CodegenUnits::Default(_))
&& codegen_units.len() > 1
&& codegen_units.iter().any(|cgu| cgu.size_estimate() < NON_INCR_MIN_CGU_SIZE)
{
// Sort small cgus to the back.
codegen_units.sort_by_key(|cgu| cmp::Reverse(cgu.size_estimate()));
let mut smallest = codegen_units.pop().unwrap();
let second_smallest = codegen_units.last_mut().unwrap();
// Move the items from `smallest` to `second_smallest`. Some of them
// may be duplicate inlined items, in which case the destination CGU is
// unaffected. Recalculate size estimates afterwards.
second_smallest.items_mut().extend(smallest.items_mut().drain(..));
second_smallest.compute_size_estimate();
// Don't update `cgu_contents`, that's only for incremental builds.
}
let cgu_name_builder = &mut CodegenUnitNameBuilder::new(cx.tcx);
// Rename the newly merged CGUs.
if cx.tcx.sess.opts.incremental.is_some() {
// If we are doing incremental compilation, we want CGU names to
// reflect the path of the source level module they correspond to.
// For CGUs that contain the code of multiple modules because of the
// merging done above, we use a concatenation of the names of all
// contained CGUs.
let new_cgu_names: FxHashMap<Symbol, String> = cgu_contents
.into_iter()
// This `filter` makes sure we only update the name of CGUs that
// were actually modified by merging.
.filter(|(_, cgu_contents)| cgu_contents.len() > 1)
.map(|(current_cgu_name, cgu_contents)| {
let mut cgu_contents: Vec<&str> = cgu_contents.iter().map(|s| s.as_str()).collect();
// Sort the names, so things are deterministic and easy to
// predict. We are sorting primitive `&str`s here so we can
// use unstable sort.
cgu_contents.sort_unstable();
(current_cgu_name, cgu_contents.join("--"))
})
.collect();
for cgu in codegen_units.iter_mut() {
if let Some(new_cgu_name) = new_cgu_names.get(&cgu.name()) {
if cx.tcx.sess.opts.unstable_opts.human_readable_cgu_names {
cgu.set_name(Symbol::intern(new_cgu_name));
} else {
// If we don't require CGU names to be human-readable,
// we use a fixed length hash of the composite CGU name
// instead.
let new_cgu_name = CodegenUnit::mangle_name(new_cgu_name);
cgu.set_name(Symbol::intern(&new_cgu_name));
}
}
}
// A sorted order here ensures what follows can be deterministic.
codegen_units.sort_by(|a, b| a.name().as_str().cmp(b.name().as_str()));
} else {
// When compiling non-incrementally, we rename the CGUS so they have
// identical names except for the numeric suffix, something like
// `regex.f10ba03eb5ec7975-cgu.N`, where `N` varies.
//
// It is useful for debugging and profiling purposes if the resulting
// CGUs are sorted by name *and* reverse sorted by size. (CGU 0 is the
// biggest, CGU 1 is the second biggest, etc.)
//
// So first we reverse sort by size. Then we generate the names with
// zero-padded suffixes, which means they are automatically sorted by
// names. The numeric suffix width depends on the number of CGUs, which
// is always greater than zero:
// - [1,9] CGUs: `0`, `1`, `2`, ...
// - [10,99] CGUs: `00`, `01`, `02`, ...
// - [100,999] CGUs: `000`, `001`, `002`, ...
// - etc.
//
// If we didn't zero-pad the sorted-by-name order would be `XYZ-cgu.0`,
// `XYZ-cgu.1`, `XYZ-cgu.10`, `XYZ-cgu.11`, ..., `XYZ-cgu.2`, etc.
codegen_units.sort_by_key(|cgu| cmp::Reverse(cgu.size_estimate()));
let num_digits = codegen_units.len().ilog10() as usize + 1;
for (index, cgu) in codegen_units.iter_mut().enumerate() {
// Note: `WorkItem::short_description` depends on this name ending
// with `-cgu.` followed by a numeric suffix. Please keep it in
// sync with this code.
let suffix = format!("{index:0num_digits$}");
let numbered_codegen_unit_name =
cgu_name_builder.build_cgu_name_no_mangle(LOCAL_CRATE, &["cgu"], Some(suffix));
cgu.set_name(numbered_codegen_unit_name);
}
}
}
/// Compute the combined size of all inlined items that appear in both `cgu1`
/// and `cgu2`.
fn compute_inlined_overlap<'tcx>(cgu1: &CodegenUnit<'tcx>, cgu2: &CodegenUnit<'tcx>) -> usize {
// Either order works. We pick the one that involves iterating over fewer
// items.
let (src_cgu, dst_cgu) =
if cgu1.items().len() <= cgu2.items().len() { (cgu1, cgu2) } else { (cgu2, cgu1) };
let mut overlap = 0;
for (item, data) in src_cgu.items().iter() {
if data.inlined {
if dst_cgu.items().contains_key(item) {
overlap += data.size_estimate;
}
}
}
overlap
}
fn internalize_symbols<'tcx>(
cx: &PartitioningCx<'_, 'tcx>,
codegen_units: &mut [CodegenUnit<'tcx>],
internalization_candidates: FxHashSet<MonoItem<'tcx>>,
) {
/// For symbol internalization, we need to know whether a symbol/mono-item
/// is used 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(Symbol),
MultipleCgus,
}
let mut mono_item_placements = FxHashMap::default();
let single_codegen_unit = codegen_units.len() == 1;
if !single_codegen_unit {
for cgu in codegen_units.iter() {
for item in cgu.items().keys() {
// If there is more than one codegen unit, we need to keep track
// in which codegen units each monomorphization is placed.
match mono_item_placements.entry(*item) {
Entry::Occupied(e) => {
let placement = e.into_mut();
debug_assert!(match *placement {
MonoItemPlacement::SingleCgu(cgu_name) => cgu_name != cgu.name(),
MonoItemPlacement::MultipleCgus => true,
});
*placement = MonoItemPlacement::MultipleCgus;
}
Entry::Vacant(e) => {
e.insert(MonoItemPlacement::SingleCgu(cgu.name()));
}
}
}
}
}
// For each internalization candidates in each codegen unit, check if it is
// used from outside its defining codegen unit.
for cgu in codegen_units {
let home_cgu = MonoItemPlacement::SingleCgu(cgu.name());
for (item, data) in cgu.items_mut() {
if !internalization_candidates.contains(item) {
// This item is no candidate for internalizing, so skip it.
continue;
}
if !single_codegen_unit {
debug_assert_eq!(mono_item_placements[item], home_cgu);
if cx
.usage_map
.get_user_items(*item)
.iter()
.filter_map(|user_item| {
// Some user mono items might not have been
// instantiated. We can safely ignore those.
mono_item_placements.get(user_item)
})
.any(|placement| *placement != home_cgu)
{
// Found a user from another CGU, so skip to the next item
// without marking this one as internal.
continue;
}
}
// If we got here, we did not find any uses from other CGUs, so
// it's fine to make this monomorphization internal.
data.linkage = Linkage::Internal;
data.visibility = Visibility::Default;
}
}
}
fn mark_code_coverage_dead_code_cgu<'tcx>(codegen_units: &mut [CodegenUnit<'tcx>]) {
assert!(!codegen_units.is_empty());
// Find the smallest CGU that has exported symbols and put the dead
// function stubs in that CGU. We look for exported symbols to increase
// the likelihood the linker won't throw away the dead functions.
// FIXME(#92165): In order to truly resolve this, we need to make sure
// the object file (CGU) containing the dead function stubs is included
// in the final binary. This will probably require forcing these
// function symbols to be included via `-u` or `/include` linker args.
let dead_code_cgu = codegen_units
.iter_mut()
.filter(|cgu| cgu.items().iter().any(|(_, data)| data.linkage == Linkage::External))
.min_by_key(|cgu| cgu.size_estimate());
// If there are no CGUs that have externally linked items, then we just
// pick the first CGU as a fallback.
let dead_code_cgu = if let Some(cgu) = dead_code_cgu { cgu } else { &mut codegen_units[0] };
dead_code_cgu.make_code_coverage_dead_code_cgu();
}
fn characteristic_def_id_of_mono_item<'tcx>(
tcx: TyCtxt<'tcx>,
mono_item: MonoItem<'tcx>,
) -> Option<DefId> {
match mono_item {
MonoItem::Fn(instance) => {
let def_id = match instance.def {
ty::InstanceDef::Item(def) => def,
ty::InstanceDef::VTableShim(..)
| ty::InstanceDef::ReifyShim(..)
| ty::InstanceDef::FnPtrShim(..)
| ty::InstanceDef::ClosureOnceShim { .. }
| ty::InstanceDef::ConstructCoroutineInClosureShim { .. }
| ty::InstanceDef::CoroutineKindShim { .. }
| ty::InstanceDef::Intrinsic(..)
| ty::InstanceDef::DropGlue(..)
| ty::InstanceDef::Virtual(..)
| ty::InstanceDef::CloneShim(..)
| ty::InstanceDef::ThreadLocalShim(..)
| ty::InstanceDef::FnPtrAddrShim(..) => return None,
};
// If this is a method, we want to put it into the same module as
// its self-type. If the self-type does not provide a characteristic
// DefId, we use the location of the impl after all.
if tcx.trait_of_item(def_id).is_some() {
let self_ty = instance.args.type_at(0);
// This is a default implementation of a trait method.
return characteristic_def_id_of_type(self_ty).or(Some(def_id));
}
if let Some(impl_def_id) = tcx.impl_of_method(def_id) {
if tcx.sess.opts.incremental.is_some()
&& tcx.trait_id_of_impl(impl_def_id) == tcx.lang_items().drop_trait()
{
// Put `Drop::drop` into the same cgu as `drop_in_place`
// since `drop_in_place` is the only thing that can
// call it.
return None;
}
// When polymorphization is enabled, methods which do not depend on their generic
// parameters, but the self-type of their impl block do will fail to normalize.
if !tcx.sess.opts.unstable_opts.polymorphize || !instance.has_param() {
// This is a method within an impl, find out what the self-type is:
let impl_self_ty = tcx.instantiate_and_normalize_erasing_regions(
instance.args,
ty::ParamEnv::reveal_all(),
tcx.type_of(impl_def_id),
);
if let Some(def_id) = characteristic_def_id_of_type(impl_self_ty) {
return Some(def_id);
}
}
}
Some(def_id)
}
MonoItem::Static(def_id) => Some(def_id),
MonoItem::GlobalAsm(item_id) => Some(item_id.owner_id.to_def_id()),
}
}
fn compute_codegen_unit_name(
tcx: TyCtxt<'_>,
name_builder: &mut CodegenUnitNameBuilder<'_>,
def_id: DefId,
volatile: bool,
cache: &mut CguNameCache,
) -> Symbol {
// Find the innermost module that is not nested within a function.
let mut current_def_id = def_id;
let mut cgu_def_id = None;
// Walk backwards from the item we want to find the module for.
loop {
if current_def_id.is_crate_root() {
if cgu_def_id.is_none() {
// If we have not found a module yet, take the crate root.
cgu_def_id = Some(def_id.krate.as_def_id());
}
break;
} else if tcx.def_kind(current_def_id) == DefKind::Mod {
if cgu_def_id.is_none() {
cgu_def_id = Some(current_def_id);
}
} else {
// If we encounter something that is not a module, throw away
// any module that we've found so far because we now know that
// it is nested within something else.
cgu_def_id = None;
}
current_def_id = tcx.parent(current_def_id);
}
let cgu_def_id = cgu_def_id.unwrap();
*cache.entry((cgu_def_id, volatile)).or_insert_with(|| {
let def_path = tcx.def_path(cgu_def_id);
let components = def_path.data.iter().map(|part| match part.data.name() {
DefPathDataName::Named(name) => name,
DefPathDataName::Anon { .. } => unreachable!(),
});
let volatile_suffix = volatile.then_some("volatile");
name_builder.build_cgu_name(def_path.krate, components, volatile_suffix)
})
}
// Anything we can't find a proper codegen unit for goes into this.
fn fallback_cgu_name(name_builder: &mut CodegenUnitNameBuilder<'_>) -> Symbol {
name_builder.build_cgu_name(LOCAL_CRATE, &["fallback"], Some("cgu"))
}
fn mono_item_linkage_and_visibility<'tcx>(
tcx: TyCtxt<'tcx>,
mono_item: &MonoItem<'tcx>,
can_be_internalized: &mut bool,
export_generics: bool,
) -> (Linkage, Visibility) {
if let Some(explicit_linkage) = mono_item.explicit_linkage(tcx) {
return (explicit_linkage, Visibility::Default);
}
let vis = mono_item_visibility(tcx, mono_item, can_be_internalized, export_generics);
(Linkage::External, vis)
}
type CguNameCache = FxHashMap<(DefId, bool), Symbol>;
fn static_visibility<'tcx>(
tcx: TyCtxt<'tcx>,
can_be_internalized: &mut bool,
def_id: DefId,
) -> Visibility {
if tcx.is_reachable_non_generic(def_id) {
*can_be_internalized = false;
default_visibility(tcx, def_id, false)
} else {
Visibility::Hidden
}
}
fn mono_item_visibility<'tcx>(
tcx: TyCtxt<'tcx>,
mono_item: &MonoItem<'tcx>,
can_be_internalized: &mut bool,
export_generics: bool,
) -> Visibility {
let instance = match mono_item {
// This is pretty complicated; see below.
MonoItem::Fn(instance) => instance,
// Misc handling for generics and such, but otherwise:
MonoItem::Static(def_id) => return static_visibility(tcx, can_be_internalized, *def_id),
MonoItem::GlobalAsm(item_id) => {
return static_visibility(tcx, can_be_internalized, item_id.owner_id.to_def_id());
}
};
let def_id = match instance.def {
InstanceDef::Item(def_id) | InstanceDef::DropGlue(def_id, Some(_)) => def_id,
// We match the visibility of statics here
InstanceDef::ThreadLocalShim(def_id) => {
return static_visibility(tcx, can_be_internalized, def_id);
}
// These are all compiler glue and such, never exported, always hidden.
InstanceDef::VTableShim(..)
| InstanceDef::ReifyShim(..)
| InstanceDef::FnPtrShim(..)
| InstanceDef::Virtual(..)
| InstanceDef::Intrinsic(..)
| InstanceDef::ClosureOnceShim { .. }
| InstanceDef::ConstructCoroutineInClosureShim { .. }
| InstanceDef::CoroutineKindShim { .. }
| InstanceDef::DropGlue(..)
| InstanceDef::CloneShim(..)
| InstanceDef::FnPtrAddrShim(..) => return Visibility::Hidden,
};
// The `start_fn` lang item is actually a monomorphized instance of a
// function in the standard library, used for the `main` function. We don't
// want to export it so we tag it with `Hidden` visibility but this symbol
// is only referenced from the actual `main` symbol which we unfortunately
// don't know anything about during partitioning/collection. As a result we
// forcibly keep this symbol out of the `internalization_candidates` set.
//
// FIXME: eventually we don't want to always force this symbol to have
// hidden visibility, it should indeed be a candidate for
// internalization, but we have to understand that it's referenced
// from the `main` symbol we'll generate later.
//
// This may be fixable with a new `InstanceDef` perhaps? Unsure!
if tcx.lang_items().start_fn() == Some(def_id) {
*can_be_internalized = false;
return Visibility::Hidden;
}
let is_generic = instance.args.non_erasable_generics(tcx, def_id).next().is_some();
// Upstream `DefId` instances get different handling than local ones.
let Some(def_id) = def_id.as_local() else {
return if export_generics && is_generic {
// If it is an upstream monomorphization and we export generics, we must make
// it available to downstream crates.
*can_be_internalized = false;
default_visibility(tcx, def_id, true)
} else {
Visibility::Hidden
};
};
if is_generic {
if export_generics {
if tcx.is_unreachable_local_definition(def_id) {
// This instance cannot be used from another crate.
Visibility::Hidden
} else {
// This instance might be useful in a downstream crate.
*can_be_internalized = false;
default_visibility(tcx, def_id.to_def_id(), true)
}
} else {
// We are not exporting generics or the definition is not reachable
// for downstream crates, we can internalize its instantiations.
Visibility::Hidden
}
} else {
// If this isn't a generic function then we mark this a `Default` if
// this is a reachable item, meaning that it's a symbol other crates may
// use when they link to us.
if tcx.is_reachable_non_generic(def_id.to_def_id()) {
*can_be_internalized = false;
debug_assert!(!is_generic);
return default_visibility(tcx, def_id.to_def_id(), false);
}
// If this isn't reachable then we're gonna tag this with `Hidden`
// visibility. In some situations though we'll want to prevent this
// symbol from being internalized.
//
// There's two categories of items here:
//
// * First is weak lang items. These are basically mechanisms for
// libcore to forward-reference symbols defined later in crates like
// the standard library or `#[panic_handler]` definitions. The
// definition of these weak lang items needs to be referencable by
// libcore, so we're no longer a candidate for internalization.
// Removal of these functions can't be done by LLVM but rather must be
// done by the linker as it's a non-local decision.
//
// * Second is "std internal symbols". Currently this is primarily used
// for allocator symbols. Allocators are a little weird in their
// implementation, but the idea is that the compiler, at the last
// minute, defines an allocator with an injected object file. The
// `alloc` crate references these symbols (`__rust_alloc`) and the
// definition doesn't get hooked up until a linked crate artifact is
// generated.
//
// The symbols synthesized by the compiler (`__rust_alloc`) are thin
// veneers around the actual implementation, some other symbol which
// implements the same ABI. These symbols (things like `__rg_alloc`,
// `__rdl_alloc`, `__rde_alloc`, etc), are all tagged with "std
// internal symbols".
//
// The std-internal symbols here **should not show up in a dll as an
// exported interface**, so they return `false` from
// `is_reachable_non_generic` above and we'll give them `Hidden`
// visibility below. Like the weak lang items, though, we can't let
// LLVM internalize them as this decision is left up to the linker to
// omit them, so prevent them from being internalized.
let attrs = tcx.codegen_fn_attrs(def_id);
if attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL) {
*can_be_internalized = false;
}
Visibility::Hidden
}
}
fn default_visibility(tcx: TyCtxt<'_>, id: DefId, is_generic: bool) -> Visibility {
if !tcx.sess.default_hidden_visibility() {
return Visibility::Default;
}
// Generic functions never have export-level C.
if is_generic {
return Visibility::Hidden;
}
// Things with export level C don't get instantiated in
// downstream crates.
if !id.is_local() {
return Visibility::Hidden;
}
// C-export level items remain at `Default`, all other internal
// items become `Hidden`.
match tcx.reachable_non_generics(id.krate).get(&id) {
Some(SymbolExportInfo { level: SymbolExportLevel::C, .. }) => Visibility::Default,
_ => Visibility::Hidden,
}
}
fn debug_dump<'a, 'tcx: 'a>(tcx: TyCtxt<'tcx>, label: &str, cgus: &[CodegenUnit<'tcx>]) {
let dump = move || {
use std::fmt::Write;
let mut num_cgus = 0;
let mut all_cgu_sizes = Vec::new();
// Note: every unique root item is placed exactly once, so the number
// of unique root items always equals the number of placed root items.
//
// Also, unreached inlined items won't be counted here. This is fine.
let mut inlined_items = FxHashSet::default();
let mut root_items = 0;
let mut unique_inlined_items = 0;
let mut placed_inlined_items = 0;
let mut root_size = 0;
let mut unique_inlined_size = 0;
let mut placed_inlined_size = 0;
for cgu in cgus.iter() {
num_cgus += 1;
all_cgu_sizes.push(cgu.size_estimate());
for (item, data) in cgu.items() {
if !data.inlined {
root_items += 1;
root_size += data.size_estimate;
} else {
if inlined_items.insert(item) {
unique_inlined_items += 1;
unique_inlined_size += data.size_estimate;
}
placed_inlined_items += 1;
placed_inlined_size += data.size_estimate;
}
}
}
all_cgu_sizes.sort_unstable_by_key(|&n| cmp::Reverse(n));
let unique_items = root_items + unique_inlined_items;
let placed_items = root_items + placed_inlined_items;
let items_ratio = placed_items as f64 / unique_items as f64;
let unique_size = root_size + unique_inlined_size;
let placed_size = root_size + placed_inlined_size;
let size_ratio = placed_size as f64 / unique_size as f64;
let mean_cgu_size = placed_size as f64 / num_cgus as f64;
assert_eq!(placed_size, all_cgu_sizes.iter().sum::<usize>());
let s = &mut String::new();
let _ = writeln!(s, "{label}");
let _ = writeln!(
s,
"- unique items: {unique_items} ({root_items} root + {unique_inlined_items} inlined), \
unique size: {unique_size} ({root_size} root + {unique_inlined_size} inlined)\n\
- placed items: {placed_items} ({root_items} root + {placed_inlined_items} inlined), \
placed size: {placed_size} ({root_size} root + {placed_inlined_size} inlined)\n\
- placed/unique items ratio: {items_ratio:.2}, \
placed/unique size ratio: {size_ratio:.2}\n\
- CGUs: {num_cgus}, mean size: {mean_cgu_size:.1}, sizes: {}",
list(&all_cgu_sizes),
);
let _ = writeln!(s);
for (i, cgu) in cgus.iter().enumerate() {
let name = cgu.name();
let size = cgu.size_estimate();
let num_items = cgu.items().len();
let mean_size = size as f64 / num_items as f64;
let mut placed_item_sizes: Vec<_> =
cgu.items().values().map(|data| data.size_estimate).collect();
placed_item_sizes.sort_unstable_by_key(|&n| cmp::Reverse(n));
let sizes = list(&placed_item_sizes);
let _ = writeln!(s, "- CGU[{i}]");
let _ = writeln!(s, " - {name}, size: {size}");
let _ =
writeln!(s, " - items: {num_items}, mean size: {mean_size:.1}, sizes: {sizes}",);
for (item, data) in cgu.items_in_deterministic_order(tcx) {
let linkage = data.linkage;
let symbol_name = item.symbol_name(tcx).name;
let symbol_hash_start = symbol_name.rfind('h');
let symbol_hash = symbol_hash_start.map_or("<no hash>", |i| &symbol_name[i..]);
let kind = if !data.inlined { "root" } else { "inlined" };
let size = data.size_estimate;
let _ = with_no_trimmed_paths!(writeln!(
s,
" - {item} [{linkage:?}] [{symbol_hash}] ({kind}, size: {size})"
));
}
let _ = writeln!(s);
}
return std::mem::take(s);
// Converts a slice to a string, capturing repetitions to save space.
// E.g. `[4, 4, 4, 3, 2, 1, 1, 1, 1, 1]` -> "[4 (x3), 3, 2, 1 (x5)]".
fn list(ns: &[usize]) -> String {
let mut v = Vec::new();
if ns.is_empty() {
return "[]".to_string();
}
let mut elem = |curr, curr_count| {
if curr_count == 1 {
v.push(format!("{curr}"));
} else {
v.push(format!("{curr} (x{curr_count})"));
}
};
let mut curr = ns[0];
let mut curr_count = 1;
for &n in &ns[1..] {
if n != curr {
elem(curr, curr_count);
curr = n;
curr_count = 1;
} else {
curr_count += 1;
}
}
elem(curr, curr_count);
format!("[{}]", v.join(", "))
}
};
debug!("{}", dump());
}
#[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 &[(mono_item1, ref sym1), (mono_item2, ref sym2)] in symbols.array_windows() {
if sym1 == sym2 {
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),
};
tcx.dcx().emit_fatal(SymbolAlreadyDefined { span, symbol: sym1.to_string() });
}
}
}
fn collect_and_partition_mono_items(tcx: TyCtxt<'_>, (): ()) -> (&DefIdSet, &[CodegenUnit<'_>]) {
let collection_mode = match tcx.sess.opts.unstable_opts.print_mono_items {
Some(ref s) => {
let mode = s.to_lowercase();
let mode = mode.trim();
if mode == "eager" {
MonoItemCollectionMode::Eager
} else {
if mode != "lazy" {
tcx.dcx().emit_warn(UnknownCguCollectionMode { mode });
}
MonoItemCollectionMode::Lazy
}
}
None => {
if tcx.sess.link_dead_code() {
MonoItemCollectionMode::Eager
} else {
MonoItemCollectionMode::Lazy
}
}
};
let (items, usage_map) = collector::collect_crate_mono_items(tcx, collection_mode);
tcx.dcx().abort_if_errors();
let (codegen_units, _) = tcx.sess.time("partition_and_assert_distinct_symbols", || {
sync::join(
|| {
let mut codegen_units = partition(tcx, items.iter().copied(), &usage_map);
codegen_units[0].make_primary();
&*tcx.arena.alloc_from_iter(codegen_units)
},
|| assert_symbols_are_distinct(tcx, items.iter()),
)
});
if tcx.prof.enabled() {
// Record CGU size estimates for self-profiling.
for cgu in codegen_units {
tcx.prof.artifact_size(
"codegen_unit_size_estimate",
cgu.name().as_str(),
cgu.size_estimate() as u64,
);
}
}
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();
// Output monomorphization stats per def_id
if let SwitchWithOptPath::Enabled(ref path) = tcx.sess.opts.unstable_opts.dump_mono_stats {
if let Err(err) =
dump_mono_items_stats(tcx, codegen_units, path, tcx.crate_name(LOCAL_CRATE))
{
tcx.dcx().emit_fatal(CouldntDumpMonoStats { error: err.to_string() });
}
}
if tcx.sess.opts.unstable_opts.print_mono_items.is_some() {
let mut item_to_cgus: FxHashMap<_, Vec<_>> = Default::default();
for cgu in codegen_units {
for (&mono_item, &data) in cgu.items() {
item_to_cgus.entry(mono_item).or_default().push((cgu.name(), data.linkage));
}
}
let mut item_keys: Vec<_> = items
.iter()
.map(|i| {
let mut output = with_no_trimmed_paths!(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(' ');
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('[');
output.push_str(linkage_abbrev);
output.push(']');
}
output
})
.collect();
item_keys.sort();
for item in item_keys {
println!("MONO_ITEM {item}");
}
}
(tcx.arena.alloc(mono_items), codegen_units)
}
/// Outputs stats about instantiation counts and estimated size, per `MonoItem`'s
/// def, to a file in the given output directory.
fn dump_mono_items_stats<'tcx>(
tcx: TyCtxt<'tcx>,
codegen_units: &[CodegenUnit<'tcx>],
output_directory: &Option<PathBuf>,
crate_name: Symbol,
) -> Result<(), Box<dyn std::error::Error>> {
let output_directory = if let Some(ref directory) = output_directory {
fs::create_dir_all(directory)?;
directory
} else {
Path::new(".")
};
let format = tcx.sess.opts.unstable_opts.dump_mono_stats_format;
let ext = format.extension();
let filename = format!("{crate_name}.mono_items.{ext}");
let output_path = output_directory.join(&filename);
let file = File::create(&output_path)?;
let mut file = BufWriter::new(file);
// Gather instantiated mono items grouped by def_id
let mut items_per_def_id: FxHashMap<_, Vec<_>> = Default::default();
for cgu in codegen_units {
cgu.items()
.keys()
// Avoid variable-sized compiler-generated shims
.filter(|mono_item| mono_item.is_user_defined())
.for_each(|mono_item| {
items_per_def_id.entry(mono_item.def_id()).or_default().push(mono_item);
});
}
#[derive(serde::Serialize)]
struct MonoItem {
name: String,
instantiation_count: usize,
size_estimate: usize,
total_estimate: usize,
}
// Output stats sorted by total instantiated size, from heaviest to lightest
let mut stats: Vec<_> = items_per_def_id
.into_iter()
.map(|(def_id, items)| {
let name = with_no_trimmed_paths!(tcx.def_path_str(def_id));
let instantiation_count = items.len();
let size_estimate = items[0].size_estimate(tcx);
let total_estimate = instantiation_count * size_estimate;
MonoItem { name, instantiation_count, size_estimate, total_estimate }
})
.collect();
stats.sort_unstable_by_key(|item| cmp::Reverse(item.total_estimate));
if !stats.is_empty() {
match format {
DumpMonoStatsFormat::Json => serde_json::to_writer(file, &stats)?,
DumpMonoStatsFormat::Markdown => {
writeln!(
file,
"| Item | Instantiation count | Estimated Cost Per Instantiation | Total Estimated Cost |"
)?;
writeln!(file, "| --- | ---: | ---: | ---: |")?;
for MonoItem { name, instantiation_count, size_estimate, total_estimate } in stats {
writeln!(
file,
"| `{name}` | {instantiation_count} | {size_estimate} | {total_estimate} |"
)?;
}
}
}
}
Ok(())
}
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(());
all_mono_items.contains(&def_id)
};
providers.codegen_unit = |tcx, name| {
let (_, all) = tcx.collect_and_partition_mono_items(());
all.iter()
.find(|cgu| cgu.name() == name)
.unwrap_or_else(|| panic!("failed to find cgu with name {name:?}"))
};
}
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