rustc_codegen_ssa/mir/
mod.rs

1use std::iter;
2
3use rustc_index::IndexVec;
4use rustc_index::bit_set::DenseBitSet;
5use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
6use rustc_middle::mir::{Body, Local, UnwindTerminateReason, traversal};
7use rustc_middle::ty::layout::{FnAbiOf, HasTyCtxt, HasTypingEnv, TyAndLayout};
8use rustc_middle::ty::{self, Instance, Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
9use rustc_middle::{bug, mir, span_bug};
10use rustc_target::callconv::{FnAbi, PassMode};
11use tracing::{debug, instrument};
12
13use crate::base;
14use crate::traits::*;
15
16mod analyze;
17mod block;
18mod constant;
19mod coverageinfo;
20pub mod debuginfo;
21mod intrinsic;
22mod locals;
23pub mod naked_asm;
24pub mod operand;
25pub mod place;
26mod rvalue;
27mod statement;
28
29pub use self::block::store_cast;
30use self::debuginfo::{FunctionDebugContext, PerLocalVarDebugInfo};
31use self::operand::{OperandRef, OperandValue};
32use self::place::PlaceRef;
33
34// Used for tracking the state of generated basic blocks.
35enum CachedLlbb<T> {
36    /// Nothing created yet.
37    None,
38
39    /// Has been created.
40    Some(T),
41
42    /// Nothing created yet, and nothing should be.
43    Skip,
44}
45
46type PerLocalVarDebugInfoIndexVec<'tcx, V> =
47    IndexVec<mir::Local, Vec<PerLocalVarDebugInfo<'tcx, V>>>;
48
49/// Master context for codegenning from MIR.
50pub struct FunctionCx<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
51    instance: Instance<'tcx>,
52
53    mir: &'tcx mir::Body<'tcx>,
54
55    debug_context: Option<FunctionDebugContext<'tcx, Bx::DIScope, Bx::DILocation>>,
56
57    llfn: Bx::Function,
58
59    cx: &'a Bx::CodegenCx,
60
61    fn_abi: &'tcx FnAbi<'tcx, Ty<'tcx>>,
62
63    /// When unwinding is initiated, we have to store this personality
64    /// value somewhere so that we can load it and re-use it in the
65    /// resume instruction. The personality is (afaik) some kind of
66    /// value used for C++ unwinding, which must filter by type: we
67    /// don't really care about it very much. Anyway, this value
68    /// contains an alloca into which the personality is stored and
69    /// then later loaded when generating the DIVERGE_BLOCK.
70    personality_slot: Option<PlaceRef<'tcx, Bx::Value>>,
71
72    /// A backend `BasicBlock` for each MIR `BasicBlock`, created lazily
73    /// as-needed (e.g. RPO reaching it or another block branching to it).
74    // FIXME(eddyb) rename `llbbs` and other `ll`-prefixed things to use a
75    // more backend-agnostic prefix such as `cg` (i.e. this would be `cgbbs`).
76    cached_llbbs: IndexVec<mir::BasicBlock, CachedLlbb<Bx::BasicBlock>>,
77
78    /// The funclet status of each basic block
79    cleanup_kinds: Option<IndexVec<mir::BasicBlock, analyze::CleanupKind>>,
80
81    /// When targeting MSVC, this stores the cleanup info for each funclet BB.
82    /// This is initialized at the same time as the `landing_pads` entry for the
83    /// funclets' head block, i.e. when needed by an unwind / `cleanup_ret` edge.
84    funclets: IndexVec<mir::BasicBlock, Option<Bx::Funclet>>,
85
86    /// This stores the cached landing/cleanup pad block for a given BB.
87    // FIXME(eddyb) rename this to `eh_pads`.
88    landing_pads: IndexVec<mir::BasicBlock, Option<Bx::BasicBlock>>,
89
90    /// Cached unreachable block
91    unreachable_block: Option<Bx::BasicBlock>,
92
93    /// Cached terminate upon unwinding block and its reason
94    terminate_block: Option<(Bx::BasicBlock, UnwindTerminateReason)>,
95
96    /// A bool flag for each basic block indicating whether it is a cold block.
97    /// A cold block is a block that is unlikely to be executed at runtime.
98    cold_blocks: IndexVec<mir::BasicBlock, bool>,
99
100    /// The location where each MIR arg/var/tmp/ret is stored. This is
101    /// usually an `PlaceRef` representing an alloca, but not always:
102    /// sometimes we can skip the alloca and just store the value
103    /// directly using an `OperandRef`, which makes for tighter LLVM
104    /// IR. The conditions for using an `OperandRef` are as follows:
105    ///
106    /// - the type of the local must be judged "immediate" by `is_llvm_immediate`
107    /// - the operand must never be referenced indirectly
108    ///     - we should not take its address using the `&` operator
109    ///     - nor should it appear in a place path like `tmp.a`
110    /// - the operand must be defined by an rvalue that can generate immediate
111    ///   values
112    ///
113    /// Avoiding allocs can also be important for certain intrinsics,
114    /// notably `expect`.
115    locals: locals::Locals<'tcx, Bx::Value>,
116
117    /// All `VarDebugInfo` from the MIR body, partitioned by `Local`.
118    /// This is `None` if no variable debuginfo/names are needed.
119    per_local_var_debug_info: Option<PerLocalVarDebugInfoIndexVec<'tcx, Bx::DIVariable>>,
120
121    /// Caller location propagated if this function has `#[track_caller]`.
122    caller_location: Option<OperandRef<'tcx, Bx::Value>>,
123}
124
125impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
126    pub fn monomorphize<T>(&self, value: T) -> T
127    where
128        T: Copy + TypeFoldable<TyCtxt<'tcx>>,
129    {
130        debug!("monomorphize: self.instance={:?}", self.instance);
131        self.instance.instantiate_mir_and_normalize_erasing_regions(
132            self.cx.tcx(),
133            self.cx.typing_env(),
134            ty::EarlyBinder::bind(value),
135        )
136    }
137}
138
139enum LocalRef<'tcx, V> {
140    Place(PlaceRef<'tcx, V>),
141    /// `UnsizedPlace(p)`: `p` itself is a thin pointer (indirect place).
142    /// `*p` is the wide pointer that references the actual unsized place.
143    ///
144    /// MIR only supports unsized args, not dynamically-sized locals, so
145    /// new unsized temps don't exist and we must reuse the referred-to place.
146    ///
147    /// FIXME: Since the removal of unsized locals in <https://github.com/rust-lang/rust/pull/142911>,
148    /// can we maybe use `Place` here? Or refactor it in another way? There are quite a few
149    /// `UnsizedPlace => bug` branches now.
150    UnsizedPlace(PlaceRef<'tcx, V>),
151    /// The backend [`OperandValue`] has already been generated.
152    Operand(OperandRef<'tcx, V>),
153    /// Will be a `Self::Operand` once we get to its definition.
154    PendingOperand,
155}
156
157impl<'tcx, V: CodegenObject> LocalRef<'tcx, V> {
158    fn new_operand(layout: TyAndLayout<'tcx>) -> LocalRef<'tcx, V> {
159        if layout.is_zst() {
160            // Zero-size temporaries aren't always initialized, which
161            // doesn't matter because they don't contain data, but
162            // we need something sufficiently aligned in the operand.
163            LocalRef::Operand(OperandRef::zero_sized(layout))
164        } else {
165            LocalRef::PendingOperand
166        }
167    }
168}
169
170///////////////////////////////////////////////////////////////////////////
171
172#[instrument(level = "debug", skip(cx))]
173pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
174    cx: &'a Bx::CodegenCx,
175    instance: Instance<'tcx>,
176) {
177    assert!(!instance.args.has_infer());
178
179    let tcx = cx.tcx();
180    let llfn = cx.get_fn(instance);
181
182    let mut mir = tcx.instance_mir(instance.def);
183
184    let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
185    debug!("fn_abi: {:?}", fn_abi);
186
187    if tcx.features().ergonomic_clones() {
188        let monomorphized_mir = instance.instantiate_mir_and_normalize_erasing_regions(
189            tcx,
190            ty::TypingEnv::fully_monomorphized(),
191            ty::EarlyBinder::bind(mir.clone()),
192        );
193        mir = tcx.arena.alloc(optimize_use_clone::<Bx>(cx, monomorphized_mir));
194    }
195
196    let debug_context = cx.create_function_debug_context(instance, fn_abi, llfn, &mir);
197
198    let start_llbb = Bx::append_block(cx, llfn, "start");
199    let mut start_bx = Bx::build(cx, start_llbb);
200
201    if mir.basic_blocks.iter().any(|bb| {
202        bb.is_cleanup || matches!(bb.terminator().unwind(), Some(mir::UnwindAction::Terminate(_)))
203    }) {
204        start_bx.set_personality_fn(cx.eh_personality());
205    }
206
207    let cleanup_kinds =
208        base::wants_new_eh_instructions(tcx.sess).then(|| analyze::cleanup_kinds(&mir));
209
210    let cached_llbbs: IndexVec<mir::BasicBlock, CachedLlbb<Bx::BasicBlock>> =
211        mir.basic_blocks
212            .indices()
213            .map(|bb| {
214                if bb == mir::START_BLOCK { CachedLlbb::Some(start_llbb) } else { CachedLlbb::None }
215            })
216            .collect();
217
218    let mut fx = FunctionCx {
219        instance,
220        mir,
221        llfn,
222        fn_abi,
223        cx,
224        personality_slot: None,
225        cached_llbbs,
226        unreachable_block: None,
227        terminate_block: None,
228        cleanup_kinds,
229        landing_pads: IndexVec::from_elem(None, &mir.basic_blocks),
230        funclets: IndexVec::from_fn_n(|_| None, mir.basic_blocks.len()),
231        cold_blocks: find_cold_blocks(tcx, mir),
232        locals: locals::Locals::empty(),
233        debug_context,
234        per_local_var_debug_info: None,
235        caller_location: None,
236    };
237
238    // It may seem like we should iterate over `required_consts` to ensure they all successfully
239    // evaluate; however, the `MirUsedCollector` already did that during the collection phase of
240    // monomorphization, and if there is an error during collection then codegen never starts -- so
241    // we don't have to do it again.
242
243    let (per_local_var_debug_info, consts_debug_info) =
244        fx.compute_per_local_var_debug_info(&mut start_bx).unzip();
245    fx.per_local_var_debug_info = per_local_var_debug_info;
246
247    let traversal_order = traversal::mono_reachable_reverse_postorder(mir, tcx, instance);
248    let memory_locals = analyze::non_ssa_locals(&fx, &traversal_order);
249
250    // Allocate variable and temp allocas
251    let local_values = {
252        let args = arg_local_refs(&mut start_bx, &mut fx, &memory_locals);
253
254        let mut allocate_local = |local: Local| {
255            let decl = &mir.local_decls[local];
256            let layout = start_bx.layout_of(fx.monomorphize(decl.ty));
257            assert!(!layout.ty.has_erasable_regions());
258
259            if local == mir::RETURN_PLACE {
260                match fx.fn_abi.ret.mode {
261                    PassMode::Indirect { .. } => {
262                        debug!("alloc: {:?} (return place) -> place", local);
263                        let llretptr = start_bx.get_param(0);
264                        return LocalRef::Place(PlaceRef::new_sized(llretptr, layout));
265                    }
266                    PassMode::Cast { ref cast, .. } => {
267                        debug!("alloc: {:?} (return place) -> place", local);
268                        let size = cast.size(&start_bx).max(layout.size);
269                        return LocalRef::Place(PlaceRef::alloca_size(&mut start_bx, size, layout));
270                    }
271                    _ => {}
272                };
273            }
274
275            if memory_locals.contains(local) {
276                debug!("alloc: {:?} -> place", local);
277                if layout.is_unsized() {
278                    LocalRef::UnsizedPlace(PlaceRef::alloca_unsized_indirect(&mut start_bx, layout))
279                } else {
280                    LocalRef::Place(PlaceRef::alloca(&mut start_bx, layout))
281                }
282            } else {
283                debug!("alloc: {:?} -> operand", local);
284                LocalRef::new_operand(layout)
285            }
286        };
287
288        let retptr = allocate_local(mir::RETURN_PLACE);
289        iter::once(retptr)
290            .chain(args.into_iter())
291            .chain(mir.vars_and_temps_iter().map(allocate_local))
292            .collect()
293    };
294    fx.initialize_locals(local_values);
295
296    // Apply debuginfo to the newly allocated locals.
297    fx.debug_introduce_locals(&mut start_bx, consts_debug_info.unwrap_or_default());
298
299    // If the backend supports coverage, and coverage is enabled for this function,
300    // do any necessary start-of-function codegen (e.g. locals for MC/DC bitmaps).
301    start_bx.init_coverage(instance);
302
303    // The builders will be created separately for each basic block at `codegen_block`.
304    // So drop the builder of `start_llbb` to avoid having two at the same time.
305    drop(start_bx);
306
307    let mut unreached_blocks = DenseBitSet::new_filled(mir.basic_blocks.len());
308    // Codegen the body of each reachable block using our reverse postorder list.
309    for bb in traversal_order {
310        fx.codegen_block(bb);
311        unreached_blocks.remove(bb);
312    }
313
314    // FIXME: These empty unreachable blocks are *mostly* a waste. They are occasionally
315    // targets for a SwitchInt terminator, but the reimplementation of the mono-reachable
316    // simplification in SwitchInt lowering sometimes misses cases that
317    // mono_reachable_reverse_postorder manages to figure out.
318    // The solution is to do something like post-mono GVN. But for now we have this hack.
319    for bb in unreached_blocks.iter() {
320        fx.codegen_block_as_unreachable(bb);
321    }
322}
323
324// FIXME: Move this function to mir::transform when post-mono MIR passes land.
325fn optimize_use_clone<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
326    cx: &'a Bx::CodegenCx,
327    mut mir: Body<'tcx>,
328) -> Body<'tcx> {
329    let tcx = cx.tcx();
330
331    if tcx.features().ergonomic_clones() {
332        for bb in mir.basic_blocks.as_mut() {
333            let mir::TerminatorKind::Call {
334                args,
335                destination,
336                target,
337                call_source: mir::CallSource::Use,
338                ..
339            } = &bb.terminator().kind
340            else {
341                continue;
342            };
343
344            // CallSource::Use calls always use 1 argument.
345            assert_eq!(args.len(), 1);
346            let arg = &args[0];
347
348            // These types are easily available from locals, so check that before
349            // doing DefId lookups to figure out what we're actually calling.
350            let arg_ty = arg.node.ty(&mir.local_decls, tcx);
351
352            let ty::Ref(_region, inner_ty, mir::Mutability::Not) = *arg_ty.kind() else { continue };
353
354            if !tcx.type_is_copy_modulo_regions(cx.typing_env(), inner_ty) {
355                continue;
356            }
357
358            let Some(arg_place) = arg.node.place() else { continue };
359
360            let destination_block = target.unwrap();
361
362            bb.statements.push(mir::Statement::new(
363                bb.terminator().source_info,
364                mir::StatementKind::Assign(Box::new((
365                    *destination,
366                    mir::Rvalue::Use(mir::Operand::Copy(
367                        arg_place.project_deeper(&[mir::ProjectionElem::Deref], tcx),
368                    )),
369                ))),
370            ));
371
372            bb.terminator_mut().kind = mir::TerminatorKind::Goto { target: destination_block };
373        }
374    }
375
376    mir
377}
378
379/// Produces, for each argument, a `Value` pointing at the
380/// argument's value. As arguments are places, these are always
381/// indirect.
382fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
383    bx: &mut Bx,
384    fx: &mut FunctionCx<'a, 'tcx, Bx>,
385    memory_locals: &DenseBitSet<mir::Local>,
386) -> Vec<LocalRef<'tcx, Bx::Value>> {
387    let mir = fx.mir;
388    let mut idx = 0;
389    let mut llarg_idx = fx.fn_abi.ret.is_indirect() as usize;
390
391    let mut num_untupled = None;
392
393    let codegen_fn_attrs = bx.tcx().codegen_instance_attrs(fx.instance.def);
394    if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
395        return vec![];
396    }
397
398    let args = mir
399        .args_iter()
400        .enumerate()
401        .map(|(arg_index, local)| {
402            let arg_decl = &mir.local_decls[local];
403            let arg_ty = fx.monomorphize(arg_decl.ty);
404
405            if Some(local) == mir.spread_arg {
406                // This argument (e.g., the last argument in the "rust-call" ABI)
407                // is a tuple that was spread at the ABI level and now we have
408                // to reconstruct it into a tuple local variable, from multiple
409                // individual LLVM function arguments.
410                let ty::Tuple(tupled_arg_tys) = arg_ty.kind() else {
411                    bug!("spread argument isn't a tuple?!");
412                };
413
414                let layout = bx.layout_of(arg_ty);
415
416                // FIXME: support unsized params in "rust-call" ABI
417                if layout.is_unsized() {
418                    span_bug!(
419                        arg_decl.source_info.span,
420                        "\"rust-call\" ABI does not support unsized params",
421                    );
422                }
423
424                let place = PlaceRef::alloca(bx, layout);
425                for i in 0..tupled_arg_tys.len() {
426                    let arg = &fx.fn_abi.args[idx];
427                    idx += 1;
428                    if let PassMode::Cast { pad_i32: true, .. } = arg.mode {
429                        llarg_idx += 1;
430                    }
431                    let pr_field = place.project_field(bx, i);
432                    bx.store_fn_arg(arg, &mut llarg_idx, pr_field);
433                }
434                assert_eq!(
435                    None,
436                    num_untupled.replace(tupled_arg_tys.len()),
437                    "Replaced existing num_tupled"
438                );
439
440                return LocalRef::Place(place);
441            }
442
443            if fx.fn_abi.c_variadic && arg_index == fx.fn_abi.args.len() {
444                let va_list = PlaceRef::alloca(bx, bx.layout_of(arg_ty));
445                bx.va_start(va_list.val.llval);
446
447                return LocalRef::Place(va_list);
448            }
449
450            let arg = &fx.fn_abi.args[idx];
451            idx += 1;
452            if let PassMode::Cast { pad_i32: true, .. } = arg.mode {
453                llarg_idx += 1;
454            }
455
456            if !memory_locals.contains(local) {
457                // We don't have to cast or keep the argument in the alloca.
458                // FIXME(eddyb): We should figure out how to use llvm.dbg.value instead
459                // of putting everything in allocas just so we can use llvm.dbg.declare.
460                let local = |op| LocalRef::Operand(op);
461                match arg.mode {
462                    PassMode::Ignore => {
463                        return local(OperandRef::zero_sized(arg.layout));
464                    }
465                    PassMode::Direct(_) => {
466                        let llarg = bx.get_param(llarg_idx);
467                        llarg_idx += 1;
468                        return local(OperandRef::from_immediate_or_packed_pair(
469                            bx, llarg, arg.layout,
470                        ));
471                    }
472                    PassMode::Pair(..) => {
473                        let (a, b) = (bx.get_param(llarg_idx), bx.get_param(llarg_idx + 1));
474                        llarg_idx += 2;
475
476                        return local(OperandRef {
477                            val: OperandValue::Pair(a, b),
478                            layout: arg.layout,
479                        });
480                    }
481                    _ => {}
482                }
483            }
484
485            match arg.mode {
486                // Sized indirect arguments
487                PassMode::Indirect { attrs, meta_attrs: None, on_stack: _ } => {
488                    // Don't copy an indirect argument to an alloca, the caller already put it
489                    // in a temporary alloca and gave it up.
490                    // FIXME: lifetimes
491                    if let Some(pointee_align) = attrs.pointee_align
492                        && pointee_align < arg.layout.align.abi
493                    {
494                        // ...unless the argument is underaligned, then we need to copy it to
495                        // a higher-aligned alloca.
496                        let tmp = PlaceRef::alloca(bx, arg.layout);
497                        bx.store_fn_arg(arg, &mut llarg_idx, tmp);
498                        LocalRef::Place(tmp)
499                    } else {
500                        let llarg = bx.get_param(llarg_idx);
501                        llarg_idx += 1;
502                        LocalRef::Place(PlaceRef::new_sized(llarg, arg.layout))
503                    }
504                }
505                // Unsized indirect arguments
506                PassMode::Indirect { attrs: _, meta_attrs: Some(_), on_stack: _ } => {
507                    // As the storage for the indirect argument lives during
508                    // the whole function call, we just copy the wide pointer.
509                    let llarg = bx.get_param(llarg_idx);
510                    llarg_idx += 1;
511                    let llextra = bx.get_param(llarg_idx);
512                    llarg_idx += 1;
513                    let indirect_operand = OperandValue::Pair(llarg, llextra);
514
515                    let tmp = PlaceRef::alloca_unsized_indirect(bx, arg.layout);
516                    indirect_operand.store(bx, tmp);
517                    LocalRef::UnsizedPlace(tmp)
518                }
519                _ => {
520                    let tmp = PlaceRef::alloca(bx, arg.layout);
521                    bx.store_fn_arg(arg, &mut llarg_idx, tmp);
522                    LocalRef::Place(tmp)
523                }
524            }
525        })
526        .collect::<Vec<_>>();
527
528    if fx.instance.def.requires_caller_location(bx.tcx()) {
529        let mir_args = if let Some(num_untupled) = num_untupled {
530            // Subtract off the tupled argument that gets 'expanded'
531            args.len() - 1 + num_untupled
532        } else {
533            args.len()
534        };
535        assert_eq!(
536            fx.fn_abi.args.len(),
537            mir_args + 1,
538            "#[track_caller] instance {:?} must have 1 more argument in their ABI than in their MIR",
539            fx.instance
540        );
541
542        let arg = fx.fn_abi.args.last().unwrap();
543        match arg.mode {
544            PassMode::Direct(_) => (),
545            _ => bug!("caller location must be PassMode::Direct, found {:?}", arg.mode),
546        }
547
548        fx.caller_location = Some(OperandRef {
549            val: OperandValue::Immediate(bx.get_param(llarg_idx)),
550            layout: arg.layout,
551        });
552    }
553
554    args
555}
556
557fn find_cold_blocks<'tcx>(
558    tcx: TyCtxt<'tcx>,
559    mir: &mir::Body<'tcx>,
560) -> IndexVec<mir::BasicBlock, bool> {
561    let local_decls = &mir.local_decls;
562
563    let mut cold_blocks: IndexVec<mir::BasicBlock, bool> =
564        IndexVec::from_elem(false, &mir.basic_blocks);
565
566    // Traverse all basic blocks from end of the function to the start.
567    for (bb, bb_data) in traversal::postorder(mir) {
568        let terminator = bb_data.terminator();
569
570        match terminator.kind {
571            // If a BB ends with a call to a cold function, mark it as cold.
572            mir::TerminatorKind::Call { ref func, .. }
573            | mir::TerminatorKind::TailCall { ref func, .. }
574                if let ty::FnDef(def_id, ..) = *func.ty(local_decls, tcx).kind()
575                    && let attrs = tcx.codegen_fn_attrs(def_id)
576                    && attrs.flags.contains(CodegenFnAttrFlags::COLD) =>
577            {
578                cold_blocks[bb] = true;
579                continue;
580            }
581
582            // If a BB ends with an `unreachable`, also mark it as cold.
583            mir::TerminatorKind::Unreachable => {
584                cold_blocks[bb] = true;
585                continue;
586            }
587
588            _ => {}
589        }
590
591        // If all successors of a BB are cold and there's at least one of them, mark this BB as cold
592        let mut succ = terminator.successors();
593        if let Some(first) = succ.next()
594            && cold_blocks[first]
595            && succ.all(|s| cold_blocks[s])
596        {
597            cold_blocks[bb] = true;
598        }
599    }
600
601    cold_blocks
602}
rustc_codegen_ssa/mir/mod.rs

rustc_codegen_ssa/mir/
mod.rs
