1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
use std::collections::hash_map::Entry;

use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_hir::def::DefKind;
use rustc_hir::def_id::{DefId, 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, Linkage, Visibility};
use rustc_middle::mir::mono::{InstantiationMode, MonoItem};
use rustc_middle::ty::print::characteristic_def_id_of_type;
use rustc_middle::ty::{self, visit::TypeVisitable, DefIdTree, InstanceDef, TyCtxt};
use rustc_span::symbol::Symbol;

use super::PartitioningCx;
use crate::collector::InliningMap;
use crate::partitioning::merging;
use crate::partitioning::{
    MonoItemPlacement, Partitioner, PostInliningPartitioning, PreInliningPartitioning,
};

pub struct DefaultPartitioning;

impl<'tcx> Partitioner<'tcx> for DefaultPartitioning {
    fn place_root_mono_items(
        &mut self,
        cx: &PartitioningCx<'_, 'tcx>,
        mono_items: &mut dyn Iterator<Item = MonoItem<'tcx>>,
    ) -> PreInliningPartitioning<'tcx> {
        let mut roots = FxHashSet::default();
        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 {
            match mono_item.instantiation_mode(cx.tcx) {
                InstantiationMode::GloballyShared { .. } => {}
                InstantiationMode::LocalCopy => 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();

            let codegen_unit_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 codegen_unit = codegen_units
                .entry(codegen_unit_name)
                .or_insert_with(|| CodegenUnit::new(codegen_unit_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);
            }

            codegen_unit.items_mut().insert(mono_item, (linkage, visibility));
            roots.insert(mono_item);
        }

        // 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 codegen_unit_name = fallback_cgu_name(cgu_name_builder);
            codegen_units.insert(codegen_unit_name, CodegenUnit::new(codegen_unit_name));
        }

        PreInliningPartitioning {
            codegen_units: codegen_units
                .into_iter()
                .map(|(_, codegen_unit)| codegen_unit)
                .collect(),
            roots,
            internalization_candidates,
        }
    }

    fn merge_codegen_units(
        &mut self,
        cx: &PartitioningCx<'_, 'tcx>,
        initial_partitioning: &mut PreInliningPartitioning<'tcx>,
    ) {
        merging::merge_codegen_units(cx, initial_partitioning);
    }

    fn place_inlined_mono_items(
        &mut self,
        cx: &PartitioningCx<'_, 'tcx>,
        initial_partitioning: PreInliningPartitioning<'tcx>,
    ) -> PostInliningPartitioning<'tcx> {
        let mut new_partitioning = Vec::new();
        let mut mono_item_placements = FxHashMap::default();

        let PreInliningPartitioning {
            codegen_units: initial_cgus,
            roots,
            internalization_candidates,
        } = initial_partitioning;

        let single_codegen_unit = initial_cgus.len() == 1;

        for old_codegen_unit in initial_cgus {
            // Collect all items that need to be available in this codegen unit.
            let mut reachable = FxHashSet::default();
            for root in old_codegen_unit.items().keys() {
                follow_inlining(*root, cx.inlining_map, &mut reachable);
            }

            let mut new_codegen_unit = CodegenUnit::new(old_codegen_unit.name());

            // Add all monomorphizations that are not already there.
            for mono_item in reachable {
                if let Some(linkage) = old_codegen_unit.items().get(&mono_item) {
                    // This is a root, just copy it over.
                    new_codegen_unit.items_mut().insert(mono_item, *linkage);
                } else {
                    if roots.contains(&mono_item) {
                        bug!(
                            "GloballyShared mono-item inlined into other CGU: \
                              {:?}",
                            mono_item
                        );
                    }

                    // This is a CGU-private copy.
                    new_codegen_unit
                        .items_mut()
                        .insert(mono_item, (Linkage::Internal, Visibility::Default));
                }

                if !single_codegen_unit {
                    // 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(mono_item) {
                        Entry::Occupied(e) => {
                            let placement = e.into_mut();
                            debug_assert!(match *placement {
                                MonoItemPlacement::SingleCgu { cgu_name } => {
                                    cgu_name != new_codegen_unit.name()
                                }
                                MonoItemPlacement::MultipleCgus => true,
                            });
                            *placement = MonoItemPlacement::MultipleCgus;
                        }
                        Entry::Vacant(e) => {
                            e.insert(MonoItemPlacement::SingleCgu {
                                cgu_name: new_codegen_unit.name(),
                            });
                        }
                    }
                }
            }

            new_partitioning.push(new_codegen_unit);
        }

        return PostInliningPartitioning {
            codegen_units: new_partitioning,
            mono_item_placements,
            internalization_candidates,
        };

        fn follow_inlining<'tcx>(
            mono_item: MonoItem<'tcx>,
            inlining_map: &InliningMap<'tcx>,
            visited: &mut FxHashSet<MonoItem<'tcx>>,
        ) {
            if !visited.insert(mono_item) {
                return;
            }

            inlining_map.with_inlining_candidates(mono_item, |target| {
                follow_inlining(target, inlining_map, visited);
            });
        }
    }

    fn internalize_symbols(
        &mut self,
        cx: &PartitioningCx<'_, 'tcx>,
        partitioning: &mut PostInliningPartitioning<'tcx>,
    ) {
        if partitioning.codegen_units.len() == 1 {
            // Fast path for when there is only one codegen unit. In this case we
            // can internalize all candidates, since there is nowhere else they
            // could be accessed from.
            for cgu in &mut partitioning.codegen_units {
                for candidate in &partitioning.internalization_candidates {
                    cgu.items_mut().insert(*candidate, (Linkage::Internal, Visibility::Default));
                }
            }

            return;
        }

        // Build a map from every monomorphization to all the monomorphizations that
        // reference it.
        let mut accessor_map: FxHashMap<MonoItem<'tcx>, Vec<MonoItem<'tcx>>> = Default::default();
        cx.inlining_map.iter_accesses(|accessor, accessees| {
            for accessee in accessees {
                accessor_map.entry(*accessee).or_default().push(accessor);
            }
        });

        let mono_item_placements = &partitioning.mono_item_placements;

        // For each internalization candidates in each codegen unit, check if it is
        // accessed from outside its defining codegen unit.
        for cgu in &mut partitioning.codegen_units {
            let home_cgu = MonoItemPlacement::SingleCgu { cgu_name: cgu.name() };

            for (accessee, linkage_and_visibility) in cgu.items_mut() {
                if !partitioning.internalization_candidates.contains(accessee) {
                    // This item is no candidate for internalizing, so skip it.
                    continue;
                }
                debug_assert_eq!(mono_item_placements[accessee], home_cgu);

                if let Some(accessors) = accessor_map.get(accessee) {
                    if accessors
                        .iter()
                        .filter_map(|accessor| {
                            // Some accessors might not have been
                            // instantiated. We can safely ignore those.
                            mono_item_placements.get(accessor)
                        })
                        .any(|placement| *placement != home_cgu)
                    {
                        // Found an accessor 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 accesses from other CGUs,
                // so it's fine to make this monomorphization internal.
                *linkage_and_visibility = (Linkage::Internal, Visibility::Default);
            }
        }
    }
}

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.did,
                ty::InstanceDef::VTableShim(..)
                | ty::InstanceDef::ReifyShim(..)
                | ty::InstanceDef::FnPtrShim(..)
                | ty::InstanceDef::ClosureOnceShim { .. }
                | ty::InstanceDef::Intrinsic(..)
                | ty::InstanceDef::DropGlue(..)
                | ty::InstanceDef::Virtual(..)
                | ty::InstanceDef::CloneShim(..) => 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.substs.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.needs_subst() {
                    // This is a method within an impl, find out what the self-type is:
                    let impl_self_ty = tcx.subst_and_normalize_erasing_regions(
                        instance.substs,
                        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.def_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 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 if tcx.is_reachable_non_generic(*def_id) {
                *can_be_internalized = false;
                default_visibility(tcx, *def_id, false)
            } else {
                Visibility::Hidden
            };
        }
        MonoItem::GlobalAsm(item_id) => {
            return if tcx.is_reachable_non_generic(item_id.def_id) {
                *can_be_internalized = false;
                default_visibility(tcx, item_id.def_id.to_def_id(), false)
            } else {
                Visibility::Hidden
            };
        }
    };

    let def_id = match instance.def {
        InstanceDef::Item(def) => def.did,
        InstanceDef::DropGlue(def_id, Some(_)) => 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::DropGlue(..)
        | InstanceDef::CloneShim(..) => 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.substs.non_erasable_generics().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
        // access 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.target.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,
    }
}