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    /// Every time it is initialized, we have to reallocate the place
144    /// and update the wide pointer. That's the reason why it is indirect.
145    UnsizedPlace(PlaceRef<'tcx, V>),
146    /// The backend [`OperandValue`] has already been generated.
147    Operand(OperandRef<'tcx, V>),
148    /// Will be a `Self::Operand` once we get to its definition.
149    PendingOperand,
150}
151
152impl<'tcx, V: CodegenObject> LocalRef<'tcx, V> {
153    fn new_operand(layout: TyAndLayout<'tcx>) -> LocalRef<'tcx, V> {
154        if layout.is_zst() {
155            // Zero-size temporaries aren't always initialized, which
156            // doesn't matter because they don't contain data, but
157            // we need something sufficiently aligned in the operand.
158            LocalRef::Operand(OperandRef::zero_sized(layout))
159        } else {
160            LocalRef::PendingOperand
161        }
162    }
163}
164
165///////////////////////////////////////////////////////////////////////////
166
167#[instrument(level = "debug", skip(cx))]
168pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
169    cx: &'a Bx::CodegenCx,
170    instance: Instance<'tcx>,
171) {
172    assert!(!instance.args.has_infer());
173
174    let tcx = cx.tcx();
175    let llfn = cx.get_fn(instance);
176
177    let mut mir = tcx.instance_mir(instance.def);
178
179    let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
180    debug!("fn_abi: {:?}", fn_abi);
181
182    if tcx.features().ergonomic_clones() {
183        let monomorphized_mir = instance.instantiate_mir_and_normalize_erasing_regions(
184            tcx,
185            ty::TypingEnv::fully_monomorphized(),
186            ty::EarlyBinder::bind(mir.clone()),
187        );
188        mir = tcx.arena.alloc(optimize_use_clone::<Bx>(cx, monomorphized_mir));
189    }
190
191    let debug_context = cx.create_function_debug_context(instance, fn_abi, llfn, &mir);
192
193    let start_llbb = Bx::append_block(cx, llfn, "start");
194    let mut start_bx = Bx::build(cx, start_llbb);
195
196    if mir.basic_blocks.iter().any(|bb| {
197        bb.is_cleanup || matches!(bb.terminator().unwind(), Some(mir::UnwindAction::Terminate(_)))
198    }) {
199        start_bx.set_personality_fn(cx.eh_personality());
200    }
201
202    let cleanup_kinds =
203        base::wants_new_eh_instructions(tcx.sess).then(|| analyze::cleanup_kinds(&mir));
204
205    let cached_llbbs: IndexVec<mir::BasicBlock, CachedLlbb<Bx::BasicBlock>> =
206        mir.basic_blocks
207            .indices()
208            .map(|bb| {
209                if bb == mir::START_BLOCK { CachedLlbb::Some(start_llbb) } else { CachedLlbb::None }
210            })
211            .collect();
212
213    let mut fx = FunctionCx {
214        instance,
215        mir,
216        llfn,
217        fn_abi,
218        cx,
219        personality_slot: None,
220        cached_llbbs,
221        unreachable_block: None,
222        terminate_block: None,
223        cleanup_kinds,
224        landing_pads: IndexVec::from_elem(None, &mir.basic_blocks),
225        funclets: IndexVec::from_fn_n(|_| None, mir.basic_blocks.len()),
226        cold_blocks: find_cold_blocks(tcx, mir),
227        locals: locals::Locals::empty(),
228        debug_context,
229        per_local_var_debug_info: None,
230        caller_location: None,
231    };
232
233    // It may seem like we should iterate over `required_consts` to ensure they all successfully
234    // evaluate; however, the `MirUsedCollector` already did that during the collection phase of
235    // monomorphization, and if there is an error during collection then codegen never starts -- so
236    // we don't have to do it again.
237
238    let (per_local_var_debug_info, consts_debug_info) =
239        fx.compute_per_local_var_debug_info(&mut start_bx).unzip();
240    fx.per_local_var_debug_info = per_local_var_debug_info;
241
242    let traversal_order = traversal::mono_reachable_reverse_postorder(mir, tcx, instance);
243    let memory_locals = analyze::non_ssa_locals(&fx, &traversal_order);
244
245    // Allocate variable and temp allocas
246    let local_values = {
247        let args = arg_local_refs(&mut start_bx, &mut fx, &memory_locals);
248
249        let mut allocate_local = |local: Local| {
250            let decl = &mir.local_decls[local];
251            let layout = start_bx.layout_of(fx.monomorphize(decl.ty));
252            assert!(!layout.ty.has_erasable_regions());
253
254            if local == mir::RETURN_PLACE {
255                match fx.fn_abi.ret.mode {
256                    PassMode::Indirect { .. } => {
257                        debug!("alloc: {:?} (return place) -> place", local);
258                        let llretptr = start_bx.get_param(0);
259                        return LocalRef::Place(PlaceRef::new_sized(llretptr, layout));
260                    }
261                    PassMode::Cast { ref cast, .. } => {
262                        debug!("alloc: {:?} (return place) -> place", local);
263                        let size = cast.size(&start_bx).max(layout.size);
264                        return LocalRef::Place(PlaceRef::alloca_size(&mut start_bx, size, layout));
265                    }
266                    _ => {}
267                };
268            }
269
270            if memory_locals.contains(local) {
271                debug!("alloc: {:?} -> place", local);
272                if layout.is_unsized() {
273                    LocalRef::UnsizedPlace(PlaceRef::alloca_unsized_indirect(&mut start_bx, layout))
274                } else {
275                    LocalRef::Place(PlaceRef::alloca(&mut start_bx, layout))
276                }
277            } else {
278                debug!("alloc: {:?} -> operand", local);
279                LocalRef::new_operand(layout)
280            }
281        };
282
283        let retptr = allocate_local(mir::RETURN_PLACE);
284        iter::once(retptr)
285            .chain(args.into_iter())
286            .chain(mir.vars_and_temps_iter().map(allocate_local))
287            .collect()
288    };
289    fx.initialize_locals(local_values);
290
291    // Apply debuginfo to the newly allocated locals.
292    fx.debug_introduce_locals(&mut start_bx, consts_debug_info.unwrap_or_default());
293
294    // If the backend supports coverage, and coverage is enabled for this function,
295    // do any necessary start-of-function codegen (e.g. locals for MC/DC bitmaps).
296    start_bx.init_coverage(instance);
297
298    // The builders will be created separately for each basic block at `codegen_block`.
299    // So drop the builder of `start_llbb` to avoid having two at the same time.
300    drop(start_bx);
301
302    let mut unreached_blocks = DenseBitSet::new_filled(mir.basic_blocks.len());
303    // Codegen the body of each reachable block using our reverse postorder list.
304    for bb in traversal_order {
305        fx.codegen_block(bb);
306        unreached_blocks.remove(bb);
307    }
308
309    // FIXME: These empty unreachable blocks are *mostly* a waste. They are occasionally
310    // targets for a SwitchInt terminator, but the reimplementation of the mono-reachable
311    // simplification in SwitchInt lowering sometimes misses cases that
312    // mono_reachable_reverse_postorder manages to figure out.
313    // The solution is to do something like post-mono GVN. But for now we have this hack.
314    for bb in unreached_blocks.iter() {
315        fx.codegen_block_as_unreachable(bb);
316    }
317}
318
319// FIXME: Move this function to mir::transform when post-mono MIR passes land.
320fn optimize_use_clone<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
321    cx: &'a Bx::CodegenCx,
322    mut mir: Body<'tcx>,
323) -> Body<'tcx> {
324    let tcx = cx.tcx();
325
326    if tcx.features().ergonomic_clones() {
327        for bb in mir.basic_blocks.as_mut() {
328            let mir::TerminatorKind::Call {
329                args,
330                destination,
331                target,
332                call_source: mir::CallSource::Use,
333                ..
334            } = &bb.terminator().kind
335            else {
336                continue;
337            };
338
339            // CallSource::Use calls always use 1 argument.
340            assert_eq!(args.len(), 1);
341            let arg = &args[0];
342
343            // These types are easily available from locals, so check that before
344            // doing DefId lookups to figure out what we're actually calling.
345            let arg_ty = arg.node.ty(&mir.local_decls, tcx);
346
347            let ty::Ref(_region, inner_ty, mir::Mutability::Not) = *arg_ty.kind() else { continue };
348
349            if !tcx.type_is_copy_modulo_regions(cx.typing_env(), inner_ty) {
350                continue;
351            }
352
353            let Some(arg_place) = arg.node.place() else { continue };
354
355            let destination_block = target.unwrap();
356
357            bb.statements.push(mir::Statement::new(
358                bb.terminator().source_info,
359                mir::StatementKind::Assign(Box::new((
360                    *destination,
361                    mir::Rvalue::Use(mir::Operand::Copy(
362                        arg_place.project_deeper(&[mir::ProjectionElem::Deref], tcx),
363                    )),
364                ))),
365            ));
366
367            bb.terminator_mut().kind = mir::TerminatorKind::Goto { target: destination_block };
368        }
369    }
370
371    mir
372}
373
374/// Produces, for each argument, a `Value` pointing at the
375/// argument's value. As arguments are places, these are always
376/// indirect.
377fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
378    bx: &mut Bx,
379    fx: &mut FunctionCx<'a, 'tcx, Bx>,
380    memory_locals: &DenseBitSet<mir::Local>,
381) -> Vec<LocalRef<'tcx, Bx::Value>> {
382    let mir = fx.mir;
383    let mut idx = 0;
384    let mut llarg_idx = fx.fn_abi.ret.is_indirect() as usize;
385
386    let mut num_untupled = None;
387
388    let codegen_fn_attrs = bx.tcx().codegen_fn_attrs(fx.instance.def_id());
389    let naked = codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::NAKED);
390    if naked {
391        return vec![];
392    }
393
394    let args = mir
395        .args_iter()
396        .enumerate()
397        .map(|(arg_index, local)| {
398            let arg_decl = &mir.local_decls[local];
399            let arg_ty = fx.monomorphize(arg_decl.ty);
400
401            if Some(local) == mir.spread_arg {
402                // This argument (e.g., the last argument in the "rust-call" ABI)
403                // is a tuple that was spread at the ABI level and now we have
404                // to reconstruct it into a tuple local variable, from multiple
405                // individual LLVM function arguments.
406                let ty::Tuple(tupled_arg_tys) = arg_ty.kind() else {
407                    bug!("spread argument isn't a tuple?!");
408                };
409
410                let layout = bx.layout_of(arg_ty);
411
412                // FIXME: support unsized params in "rust-call" ABI
413                if layout.is_unsized() {
414                    span_bug!(
415                        arg_decl.source_info.span,
416                        "\"rust-call\" ABI does not support unsized params",
417                    );
418                }
419
420                let place = PlaceRef::alloca(bx, layout);
421                for i in 0..tupled_arg_tys.len() {
422                    let arg = &fx.fn_abi.args[idx];
423                    idx += 1;
424                    if let PassMode::Cast { pad_i32: true, .. } = arg.mode {
425                        llarg_idx += 1;
426                    }
427                    let pr_field = place.project_field(bx, i);
428                    bx.store_fn_arg(arg, &mut llarg_idx, pr_field);
429                }
430                assert_eq!(
431                    None,
432                    num_untupled.replace(tupled_arg_tys.len()),
433                    "Replaced existing num_tupled"
434                );
435
436                return LocalRef::Place(place);
437            }
438
439            if fx.fn_abi.c_variadic && arg_index == fx.fn_abi.args.len() {
440                let va_list = PlaceRef::alloca(bx, bx.layout_of(arg_ty));
441                bx.va_start(va_list.val.llval);
442
443                return LocalRef::Place(va_list);
444            }
445
446            let arg = &fx.fn_abi.args[idx];
447            idx += 1;
448            if let PassMode::Cast { pad_i32: true, .. } = arg.mode {
449                llarg_idx += 1;
450            }
451
452            if !memory_locals.contains(local) {
453                // We don't have to cast or keep the argument in the alloca.
454                // FIXME(eddyb): We should figure out how to use llvm.dbg.value instead
455                // of putting everything in allocas just so we can use llvm.dbg.declare.
456                let local = |op| LocalRef::Operand(op);
457                match arg.mode {
458                    PassMode::Ignore => {
459                        return local(OperandRef::zero_sized(arg.layout));
460                    }
461                    PassMode::Direct(_) => {
462                        let llarg = bx.get_param(llarg_idx);
463                        llarg_idx += 1;
464                        return local(OperandRef::from_immediate_or_packed_pair(
465                            bx, llarg, arg.layout,
466                        ));
467                    }
468                    PassMode::Pair(..) => {
469                        let (a, b) = (bx.get_param(llarg_idx), bx.get_param(llarg_idx + 1));
470                        llarg_idx += 2;
471
472                        return local(OperandRef {
473                            val: OperandValue::Pair(a, b),
474                            layout: arg.layout,
475                        });
476                    }
477                    _ => {}
478                }
479            }
480
481            match arg.mode {
482                // Sized indirect arguments
483                PassMode::Indirect { attrs, meta_attrs: None, on_stack: _ } => {
484                    // Don't copy an indirect argument to an alloca, the caller already put it
485                    // in a temporary alloca and gave it up.
486                    // FIXME: lifetimes
487                    if let Some(pointee_align) = attrs.pointee_align
488                        && pointee_align < arg.layout.align.abi
489                    {
490                        // ...unless the argument is underaligned, then we need to copy it to
491                        // a higher-aligned alloca.
492                        let tmp = PlaceRef::alloca(bx, arg.layout);
493                        bx.store_fn_arg(arg, &mut llarg_idx, tmp);
494                        LocalRef::Place(tmp)
495                    } else {
496                        let llarg = bx.get_param(llarg_idx);
497                        llarg_idx += 1;
498                        LocalRef::Place(PlaceRef::new_sized(llarg, arg.layout))
499                    }
500                }
501                // Unsized indirect qrguments
502                PassMode::Indirect { attrs: _, meta_attrs: Some(_), on_stack: _ } => {
503                    // As the storage for the indirect argument lives during
504                    // the whole function call, we just copy the wide pointer.
505                    let llarg = bx.get_param(llarg_idx);
506                    llarg_idx += 1;
507                    let llextra = bx.get_param(llarg_idx);
508                    llarg_idx += 1;
509                    let indirect_operand = OperandValue::Pair(llarg, llextra);
510
511                    let tmp = PlaceRef::alloca_unsized_indirect(bx, arg.layout);
512                    indirect_operand.store(bx, tmp);
513                    LocalRef::UnsizedPlace(tmp)
514                }
515                _ => {
516                    let tmp = PlaceRef::alloca(bx, arg.layout);
517                    bx.store_fn_arg(arg, &mut llarg_idx, tmp);
518                    LocalRef::Place(tmp)
519                }
520            }
521        })
522        .collect::<Vec<_>>();
523
524    if fx.instance.def.requires_caller_location(bx.tcx()) {
525        let mir_args = if let Some(num_untupled) = num_untupled {
526            // Subtract off the tupled argument that gets 'expanded'
527            args.len() - 1 + num_untupled
528        } else {
529            args.len()
530        };
531        assert_eq!(
532            fx.fn_abi.args.len(),
533            mir_args + 1,
534            "#[track_caller] instance {:?} must have 1 more argument in their ABI than in their MIR",
535            fx.instance
536        );
537
538        let arg = fx.fn_abi.args.last().unwrap();
539        match arg.mode {
540            PassMode::Direct(_) => (),
541            _ => bug!("caller location must be PassMode::Direct, found {:?}", arg.mode),
542        }
543
544        fx.caller_location = Some(OperandRef {
545            val: OperandValue::Immediate(bx.get_param(llarg_idx)),
546            layout: arg.layout,
547        });
548    }
549
550    args
551}
552
553fn find_cold_blocks<'tcx>(
554    tcx: TyCtxt<'tcx>,
555    mir: &mir::Body<'tcx>,
556) -> IndexVec<mir::BasicBlock, bool> {
557    let local_decls = &mir.local_decls;
558
559    let mut cold_blocks: IndexVec<mir::BasicBlock, bool> =
560        IndexVec::from_elem(false, &mir.basic_blocks);
561
562    // Traverse all basic blocks from end of the function to the start.
563    for (bb, bb_data) in traversal::postorder(mir) {
564        let terminator = bb_data.terminator();
565
566        match terminator.kind {
567            // If a BB ends with a call to a cold function, mark it as cold.
568            mir::TerminatorKind::Call { ref func, .. }
569            | mir::TerminatorKind::TailCall { ref func, .. }
570                if let ty::FnDef(def_id, ..) = *func.ty(local_decls, tcx).kind()
571                    && let attrs = tcx.codegen_fn_attrs(def_id)
572                    && attrs.flags.contains(CodegenFnAttrFlags::COLD) =>
573            {
574                cold_blocks[bb] = true;
575                continue;
576            }
577
578            // If a BB ends with an `unreachable`, also mark it as cold.
579            mir::TerminatorKind::Unreachable => {
580                cold_blocks[bb] = true;
581                continue;
582            }
583
584            _ => {}
585        }
586
587        // If all successors of a BB are cold and there's at least one of them, mark this BB as cold
588        let mut succ = terminator.successors();
589        if let Some(first) = succ.next()
590            && cold_blocks[first]
591            && succ.all(|s| cold_blocks[s])
592        {
593            cold_blocks[bb] = true;
594        }
595    }
596
597    cold_blocks
598}
rustc_codegen_ssa/mir/mod.rs

rustc_codegen_ssa/mir/
mod.rs
