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

rustc_codegen_ssa/mir/
mod.rs
