1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
12 Managing the scope stack. The scopes are tied to lexical scopes, so as
13 we descend the HAIR, we push a scope on the stack, build its
14 contents, and then pop it off. Every scope is named by a
19 When pushing a new scope, we record the current point in the graph (a
20 basic block); this marks the entry to the scope. We then generate more
21 stuff in the control-flow graph. Whenever the scope is exited, either
22 via a `break` or `return` or just by fallthrough, that marks an exit
23 from the scope. Each lexical scope thus corresponds to a single-entry,
24 multiple-exit (SEME) region in the control-flow graph.
26 For now, we keep a mapping from each `region::Scope` to its
27 corresponding SEME region for later reference (see caveat in next
28 paragraph). This is because region scopes are tied to
29 them. Eventually, when we shift to non-lexical lifetimes, there should
30 be no need to remember this mapping.
32 There is one additional wrinkle, actually, that I wanted to hide from
33 you but duty compels me to mention. In the course of building
34 matches, it sometimes happen that certain code (namely guards) gets
35 executed multiple times. This means that the scope lexical scope may
36 in fact correspond to multiple, disjoint SEME regions. So in fact our
37 mapping is from one scope to a vector of SEME regions.
41 The primary purpose for scopes is to insert drops: while building
42 the contents, we also accumulate places that need to be dropped upon
43 exit from each scope. This is done by calling `schedule_drop`. Once a
44 drop is scheduled, whenever we branch out we will insert drops of all
45 those places onto the outgoing edge. Note that we don't know the full
46 set of scheduled drops up front, and so whenever we exit from the
47 scope we only drop the values scheduled thus far. For example, consider
48 the scope S corresponding to this loop:
59 When processing the `let x`, we will add one drop to the scope for
60 `x`. The break will then insert a drop for `x`. When we process `let
61 y`, we will add another drop (in fact, to a subscope, but let's ignore
62 that for now); any later drops would also drop `y`.
66 There are numerous "normal" ways to early exit a scope: `break`,
67 `continue`, `return` (panics are handled separately). Whenever an
68 early exit occurs, the method `exit_scope` is called. It is given the
69 current point in execution where the early exit occurs, as well as the
70 scope you want to branch to (note that all early exits from to some
71 other enclosing scope). `exit_scope` will record this exit point and
74 Panics are handled in a similar fashion, except that a panic always
75 returns out to the `DIVERGE_BLOCK`. To trigger a panic, simply call
76 `panic(p)` with the current point `p`. Or else you can call
77 `diverge_cleanup`, which will produce a block that you can branch to
78 which does the appropriate cleanup and then diverges. `panic(p)`
79 simply calls `diverge_cleanup()` and adds an edge from `p` to the
84 In addition to the normal scope stack, we track a loop scope stack
85 that contains only loops. It tracks where a `break` and `continue`
90 use build::{BlockAnd, BlockAndExtension, Builder, CFG};
92 use rustc::middle::region;
93 use rustc::ty::{Ty, TyCtxt};
95 use rustc::hir::def_id::LOCAL_CRATE;
97 use syntax_pos::{Span};
98 use rustc_data_structures::indexed_vec::Idx;
99 use rustc_data_structures::fx::FxHashMap;
102 pub struct Scope<'tcx> {
103 /// The source scope this scope was created in.
104 source_scope: SourceScope,
106 /// the region span of this scope within source code.
107 region_scope: region::Scope,
109 /// the span of that region_scope
110 region_scope_span: Span,
112 /// Whether there's anything to do for the cleanup path, that is,
113 /// when unwinding through this scope. This includes destructors,
114 /// but not StorageDead statements, which don't get emitted at all
115 /// for unwinding, for several reasons:
116 /// * clang doesn't emit llvm.lifetime.end for C++ unwinding
117 /// * LLVM's memory dependency analysis can't handle it atm
118 /// * polluting the cleanup MIR with StorageDead creates
119 /// landing pads even though there's no actual destructors
120 /// * freeing up stack space has no effect during unwinding
123 /// set of places to drop when exiting this scope. This starts
124 /// out empty but grows as variables are declared during the
125 /// building process. This is a stack, so we always drop from the
126 /// end of the vector (top of the stack) first.
127 drops: Vec<DropData<'tcx>>,
129 /// The cache for drop chain on “normal” exit into a particular BasicBlock.
130 cached_exits: FxHashMap<(BasicBlock, region::Scope), BasicBlock>,
132 /// The cache for drop chain on "generator drop" exit.
133 cached_generator_drop: Option<BasicBlock>,
135 /// The cache for drop chain on "unwind" exit.
136 cached_unwind: CachedBlock,
140 struct DropData<'tcx> {
141 /// span where drop obligation was incurred (typically where place was declared)
145 location: Place<'tcx>,
147 /// Whether this is a value Drop or a StorageDead.
151 #[derive(Debug, Default, Clone, Copy)]
152 pub(crate) struct CachedBlock {
153 /// The cached block for the cleanups-on-diverge path. This block
154 /// contains code to run the current drop and all the preceding
155 /// drops (i.e. those having lower index in Drop’s Scope drop
157 unwind: Option<BasicBlock>,
159 /// The cached block for unwinds during cleanups-on-generator-drop path
161 /// This is split from the standard unwind path here to prevent drop
162 /// elaboration from creating drop flags that would have to be captured
163 /// by the generator. I'm not sure how important this optimization is,
165 generator_drop: Option<BasicBlock>,
169 pub(crate) enum DropKind {
171 cached_block: CachedBlock,
176 #[derive(Clone, Debug)]
177 pub struct BreakableScope<'tcx> {
178 /// Region scope of the loop
179 pub region_scope: region::Scope,
180 /// Where the body of the loop begins. `None` if block
181 pub continue_block: Option<BasicBlock>,
182 /// Block to branch into when the loop or block terminates (either by being `break`-en out
183 /// from, or by having its condition to become false)
184 pub break_block: BasicBlock,
185 /// The destination of the loop/block expression itself (i.e. where to put the result of a
186 /// `break` expression)
187 pub break_destination: Place<'tcx>,
191 fn invalidate(&mut self) {
192 self.generator_drop = None;
196 fn get(&self, generator_drop: bool) -> Option<BasicBlock> {
204 fn ref_mut(&mut self, generator_drop: bool) -> &mut Option<BasicBlock> {
206 &mut self.generator_drop
214 fn may_panic(&self) -> bool {
216 DropKind::Value { .. } => true,
217 DropKind::Storage => false
222 impl<'tcx> Scope<'tcx> {
223 /// Invalidate all the cached blocks in the scope.
225 /// Should always be run for all inner scopes when a drop is pushed into some scope enclosing a
226 /// larger extent of code.
228 /// `storage_only` controls whether to invalidate only drop paths run `StorageDead`.
229 /// `this_scope_only` controls whether to invalidate only drop paths that refer to the current
230 /// top-of-scope (as opposed to dependent scopes).
231 fn invalidate_cache(&mut self, storage_only: bool, this_scope_only: bool) {
232 // FIXME: maybe do shared caching of `cached_exits` etc. to handle functions
233 // with lots of `try!`?
235 // cached exits drop storage and refer to the top-of-scope
236 self.cached_exits.clear();
239 // the current generator drop and unwind ignore
240 // storage but refer to top-of-scope
241 self.cached_generator_drop = None;
242 self.cached_unwind.invalidate();
245 if !storage_only && !this_scope_only {
246 for dropdata in &mut self.drops {
247 if let DropKind::Value { ref mut cached_block } = dropdata.kind {
248 cached_block.invalidate();
254 /// Given a span and this scope's source scope, make a SourceInfo.
255 fn source_info(&self, span: Span) -> SourceInfo {
258 scope: self.source_scope
263 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
264 // Adding and removing scopes
265 // ==========================
266 /// Start a breakable scope, which tracks where `continue` and `break`
267 /// should branch to. See module comment for more details.
269 /// Returns the might_break attribute of the BreakableScope used.
270 pub fn in_breakable_scope<F, R>(&mut self,
271 loop_block: Option<BasicBlock>,
272 break_block: BasicBlock,
273 break_destination: Place<'tcx>,
275 where F: FnOnce(&mut Builder<'a, 'gcx, 'tcx>) -> R
277 let region_scope = self.topmost_scope();
278 let scope = BreakableScope {
280 continue_block: loop_block,
284 self.breakable_scopes.push(scope);
286 let breakable_scope = self.breakable_scopes.pop().unwrap();
287 assert!(breakable_scope.region_scope == region_scope);
291 pub fn in_opt_scope<F, R>(&mut self,
292 opt_scope: Option<(region::Scope, SourceInfo)>,
293 mut block: BasicBlock,
296 where F: FnOnce(&mut Builder<'a, 'gcx, 'tcx>) -> BlockAnd<R>
298 debug!("in_opt_scope(opt_scope={:?}, block={:?})", opt_scope, block);
299 if let Some(region_scope) = opt_scope { self.push_scope(region_scope); }
300 let rv = unpack!(block = f(self));
301 if let Some(region_scope) = opt_scope {
302 unpack!(block = self.pop_scope(region_scope, block));
304 debug!("in_scope: exiting opt_scope={:?} block={:?}", opt_scope, block);
308 /// Convenience wrapper that pushes a scope and then executes `f`
309 /// to build its contents, popping the scope afterwards.
310 pub fn in_scope<F, R>(&mut self,
311 region_scope: (region::Scope, SourceInfo),
312 lint_level: LintLevel,
313 mut block: BasicBlock,
316 where F: FnOnce(&mut Builder<'a, 'gcx, 'tcx>) -> BlockAnd<R>
318 debug!("in_scope(region_scope={:?}, block={:?})", region_scope, block);
319 let source_scope = self.source_scope;
320 let tcx = self.hir.tcx();
321 if let LintLevel::Explicit(node_id) = lint_level {
322 let same_lint_scopes = tcx.dep_graph.with_ignore(|| {
323 let sets = tcx.lint_levels(LOCAL_CRATE);
325 tcx.hir.definitions().node_to_hir_id(
326 self.source_scope_local_data[source_scope].lint_root
329 tcx.hir.definitions().node_to_hir_id(node_id);
330 sets.lint_level_set(parent_hir_id) ==
331 sets.lint_level_set(current_hir_id)
334 if !same_lint_scopes {
336 self.new_source_scope(region_scope.1.span, lint_level,
340 self.push_scope(region_scope);
341 let rv = unpack!(block = f(self));
342 unpack!(block = self.pop_scope(region_scope, block));
343 self.source_scope = source_scope;
344 debug!("in_scope: exiting region_scope={:?} block={:?}", region_scope, block);
348 /// Push a scope onto the stack. You can then build code in this
349 /// scope and call `pop_scope` afterwards. Note that these two
350 /// calls must be paired; using `in_scope` as a convenience
351 /// wrapper maybe preferable.
352 pub fn push_scope(&mut self, region_scope: (region::Scope, SourceInfo)) {
353 debug!("push_scope({:?})", region_scope);
354 let vis_scope = self.source_scope;
355 self.scopes.push(Scope {
356 source_scope: vis_scope,
357 region_scope: region_scope.0,
358 region_scope_span: region_scope.1.span,
359 needs_cleanup: false,
361 cached_generator_drop: None,
362 cached_exits: FxHashMap(),
363 cached_unwind: CachedBlock::default(),
367 /// Pops a scope, which should have region scope `region_scope`,
368 /// adding any drops onto the end of `block` that are needed.
369 /// This must match 1-to-1 with `push_scope`.
370 pub fn pop_scope(&mut self,
371 region_scope: (region::Scope, SourceInfo),
372 mut block: BasicBlock)
374 debug!("pop_scope({:?}, {:?})", region_scope, block);
375 // If we are emitting a `drop` statement, we need to have the cached
376 // diverge cleanup pads ready in case that drop panics.
378 self.scopes.last().unwrap().drops.iter().any(|s| s.kind.may_panic());
380 self.diverge_cleanup();
382 let scope = self.scopes.pop().unwrap();
383 assert_eq!(scope.region_scope, region_scope.0);
385 self.cfg.push_end_region(self.hir.tcx(), block, region_scope.1, scope.region_scope);
386 let resume_block = self.resume_block();
387 unpack!(block = build_scope_drops(&mut self.cfg,
399 /// Branch out of `block` to `target`, exiting all scopes up to
400 /// and including `region_scope`. This will insert whatever drops are
401 /// needed, as well as tracking this exit for the SEME region. See
402 /// module comment for details.
403 pub fn exit_scope(&mut self,
405 region_scope: (region::Scope, SourceInfo),
406 mut block: BasicBlock,
407 target: BasicBlock) {
408 debug!("exit_scope(region_scope={:?}, block={:?}, target={:?})",
409 region_scope, block, target);
410 let scope_count = 1 + self.scopes.iter().rev()
411 .position(|scope| scope.region_scope == region_scope.0)
413 span_bug!(span, "region_scope {:?} does not enclose", region_scope)
415 let len = self.scopes.len();
416 assert!(scope_count < len, "should not use `exit_scope` to pop ALL scopes");
418 // If we are emitting a `drop` statement, we need to have the cached
419 // diverge cleanup pads ready in case that drop panics.
420 let may_panic = self.scopes[(len - scope_count)..].iter()
421 .any(|s| s.drops.iter().any(|s| s.kind.may_panic()));
423 self.diverge_cleanup();
427 let resume_block = self.resume_block();
428 let mut rest = &mut self.scopes[(len - scope_count)..];
429 while let Some((scope, rest_)) = {rest}.split_last_mut() {
431 block = if let Some(&e) = scope.cached_exits.get(&(target, region_scope.0)) {
432 self.cfg.terminate(block, scope.source_info(span),
433 TerminatorKind::Goto { target: e });
436 let b = self.cfg.start_new_block();
437 self.cfg.terminate(block, scope.source_info(span),
438 TerminatorKind::Goto { target: b });
439 scope.cached_exits.insert((target, region_scope.0), b);
443 // End all regions for scopes out of which we are breaking.
444 self.cfg.push_end_region(self.hir.tcx(), block, region_scope.1, scope.region_scope);
446 unpack!(block = build_scope_drops(&mut self.cfg,
455 let scope = &self.scopes[len - scope_count];
456 self.cfg.terminate(block, scope.source_info(span),
457 TerminatorKind::Goto { target: target });
460 /// Creates a path that performs all required cleanup for dropping a generator.
462 /// This path terminates in GeneratorDrop. Returns the start of the path.
463 /// None indicates there’s no cleanup to do at this point.
464 pub fn generator_drop_cleanup(&mut self) -> Option<BasicBlock> {
465 if !self.scopes.iter().any(|scope| scope.needs_cleanup) {
470 self.diverge_cleanup_gen(true);
472 let src_info = self.scopes[0].source_info(self.fn_span);
473 let mut block = self.cfg.start_new_block();
475 let resume_block = self.resume_block();
476 let mut rest = &mut self.scopes[..];
478 while let Some((scope, rest_)) = {rest}.split_last_mut() {
480 if !scope.needs_cleanup {
483 block = if let Some(b) = scope.cached_generator_drop {
484 self.cfg.terminate(block, src_info,
485 TerminatorKind::Goto { target: b });
488 let b = self.cfg.start_new_block();
489 scope.cached_generator_drop = Some(b);
490 self.cfg.terminate(block, src_info,
491 TerminatorKind::Goto { target: b });
495 // End all regions for scopes out of which we are breaking.
496 self.cfg.push_end_region(self.hir.tcx(), block, src_info, scope.region_scope);
498 unpack!(block = build_scope_drops(&mut self.cfg,
507 self.cfg.terminate(block, src_info, TerminatorKind::GeneratorDrop);
512 /// Creates a new source scope, nested in the current one.
513 pub fn new_source_scope(&mut self,
515 lint_level: LintLevel,
516 safety: Option<Safety>) -> SourceScope {
517 let parent = self.source_scope;
518 debug!("new_source_scope({:?}, {:?}, {:?}) - parent({:?})={:?}",
519 span, lint_level, safety,
520 parent, self.source_scope_local_data.get(parent));
521 let scope = self.source_scopes.push(SourceScopeData {
523 parent_scope: Some(parent),
525 let scope_local_data = SourceScopeLocalData {
526 lint_root: if let LintLevel::Explicit(lint_root) = lint_level {
529 self.source_scope_local_data[parent].lint_root
531 safety: safety.unwrap_or_else(|| {
532 self.source_scope_local_data[parent].safety
535 self.source_scope_local_data.push(scope_local_data);
541 /// Finds the breakable scope for a given label. This is used for
542 /// resolving `break` and `continue`.
543 pub fn find_breakable_scope(&self,
545 label: region::Scope)
546 -> &BreakableScope<'tcx> {
547 // find the loop-scope with the correct id
548 self.breakable_scopes.iter()
550 .filter(|breakable_scope| breakable_scope.region_scope == label)
552 .unwrap_or_else(|| span_bug!(span, "no enclosing breakable scope found"))
555 /// Given a span and the current source scope, make a SourceInfo.
556 pub fn source_info(&self, span: Span) -> SourceInfo {
559 scope: self.source_scope
563 /// Returns the `region::Scope` of the scope which should be exited by a
565 pub fn region_scope_of_return_scope(&self) -> region::Scope {
566 // The outermost scope (`scopes[0]`) will be the `CallSiteScope`.
567 // We want `scopes[1]`, which is the `ParameterScope`.
568 assert!(self.scopes.len() >= 2);
569 assert!(match self.scopes[1].region_scope.data() {
570 region::ScopeData::Arguments(_) => true,
573 self.scopes[1].region_scope
576 /// Returns the topmost active scope, which is known to be alive until
577 /// the next scope expression.
578 pub fn topmost_scope(&self) -> region::Scope {
579 self.scopes.last().expect("topmost_scope: no scopes present").region_scope
582 /// Returns the scope that we should use as the lifetime of an
583 /// operand. Basically, an operand must live until it is consumed.
584 /// This is similar to, but not quite the same as, the temporary
585 /// scope (which can be larger or smaller).
589 /// let x = foo(bar(X, Y));
591 /// We wish to pop the storage for X and Y after `bar()` is
592 /// called, not after the whole `let` is completed.
594 /// As another example, if the second argument diverges:
596 /// foo(Box::new(2), panic!())
598 /// We would allocate the box but then free it on the unwinding
599 /// path; we would also emit a free on the 'success' path from
600 /// panic, but that will turn out to be removed as dead-code.
602 /// When building statics/constants, returns `None` since
603 /// intermediate values do not have to be dropped in that case.
604 pub fn local_scope(&self) -> Option<region::Scope> {
605 match self.hir.body_owner_kind {
606 hir::BodyOwnerKind::Const |
607 hir::BodyOwnerKind::Static(_) =>
608 // No need to free storage in this context.
610 hir::BodyOwnerKind::Fn =>
611 Some(self.topmost_scope()),
615 // Schedule an abort block - this is used for some ABIs that cannot unwind
616 pub fn schedule_abort(&mut self) -> BasicBlock {
617 self.scopes[0].needs_cleanup = true;
618 let abortblk = self.cfg.start_new_cleanup_block();
619 let source_info = self.scopes[0].source_info(self.fn_span);
620 self.cfg.terminate(abortblk, source_info, TerminatorKind::Abort);
621 self.cached_resume_block = Some(abortblk);
625 pub fn schedule_drop_storage_and_value(
628 region_scope: region::Scope,
633 span, region_scope, place, place_ty,
637 span, region_scope, place, place_ty,
639 cached_block: CachedBlock::default(),
646 /// Indicates that `place` should be dropped on exit from
649 /// When called with `DropKind::Storage`, `place` should be a local
650 /// with an index higher than the current `self.arg_count`.
651 pub fn schedule_drop(
654 region_scope: region::Scope,
659 let needs_drop = self.hir.needs_drop(place_ty);
661 DropKind::Value { .. } => if !needs_drop { return },
662 DropKind::Storage => {
664 Place::Local(index) => if index.index() <= self.arg_count {
666 span, "`schedule_drop` called with index {} and arg_count {}",
672 span, "`schedule_drop` called with non-`Local` place {:?}", place
678 for scope in self.scopes.iter_mut().rev() {
679 let this_scope = scope.region_scope == region_scope;
680 // When building drops, we try to cache chains of drops in such a way so these drops
681 // could be reused by the drops which would branch into the cached (already built)
682 // blocks. This, however, means that whenever we add a drop into a scope which already
683 // had some blocks built (and thus, cached) for it, we must invalidate all caches which
684 // might branch into the scope which had a drop just added to it. This is necessary,
685 // because otherwise some other code might use the cache to branch into already built
686 // chain of drops, essentially ignoring the newly added drop.
688 // For example consider there’s two scopes with a drop in each. These are built and
689 // thus the caches are filled:
691 // +--------------------------------------------------------+
692 // | +---------------------------------+ |
693 // | | +--------+ +-------------+ | +---------------+ |
694 // | | | return | <-+ | drop(outer) | <-+ | drop(middle) | |
695 // | | +--------+ +-------------+ | +---------------+ |
696 // | +------------|outer_scope cache|--+ |
697 // +------------------------------|middle_scope cache|------+
699 // Now, a new, inner-most scope is added along with a new drop into both inner-most and
700 // outer-most scopes:
702 // +------------------------------------------------------------+
703 // | +----------------------------------+ |
704 // | | +--------+ +-------------+ | +---------------+ | +-------------+
705 // | | | return | <+ | drop(new) | <-+ | drop(middle) | <--+| drop(inner) |
706 // | | +--------+ | | drop(outer) | | +---------------+ | +-------------+
707 // | | +-+ +-------------+ | |
708 // | +---|invalid outer_scope cache|----+ |
709 // +----=----------------|invalid middle_scope cache|-----------+
711 // If, when adding `drop(new)` we do not invalidate the cached blocks for both
712 // outer_scope and middle_scope, then, when building drops for the inner (right-most)
713 // scope, the old, cached blocks, without `drop(new)` will get used, producing the
716 // The cache and its invalidation for unwind branch is somewhat special. The cache is
717 // per-drop, rather than per scope, which has a several different implications. Adding
718 // a new drop into a scope will not invalidate cached blocks of the prior drops in the
719 // scope. That is true, because none of the already existing drops will have an edge
720 // into a block with the newly added drop.
722 // Note that this code iterates scopes from the inner-most to the outer-most,
723 // invalidating caches of each scope visited. This way bare minimum of the
724 // caches gets invalidated. i.e. if a new drop is added into the middle scope, the
725 // cache of outer scpoe stays intact.
726 scope.invalidate_cache(!needs_drop, this_scope);
728 if let DropKind::Value { .. } = drop_kind {
729 scope.needs_cleanup = true;
732 let region_scope_span = region_scope.span(self.hir.tcx(),
733 &self.hir.region_scope_tree);
734 // Attribute scope exit drops to scope's closing brace.
735 let scope_end = self.hir.tcx().sess.codemap().end_point(region_scope_span);
737 scope.drops.push(DropData {
739 location: place.clone(),
745 span_bug!(span, "region scope {:?} not in scope to drop {:?}", region_scope, place);
750 /// Creates a path that performs all required cleanup for unwinding.
752 /// This path terminates in Resume. Returns the start of the path.
753 /// See module comment for more details. None indicates there’s no
754 /// cleanup to do at this point.
755 pub fn diverge_cleanup(&mut self) -> BasicBlock {
756 self.diverge_cleanup_gen(false)
759 fn resume_block(&mut self) -> BasicBlock {
760 if let Some(target) = self.cached_resume_block {
763 let resumeblk = self.cfg.start_new_cleanup_block();
764 self.cfg.terminate(resumeblk,
766 scope: OUTERMOST_SOURCE_SCOPE,
769 TerminatorKind::Resume);
770 self.cached_resume_block = Some(resumeblk);
775 fn diverge_cleanup_gen(&mut self, generator_drop: bool) -> BasicBlock {
776 // To start, create the resume terminator.
777 let mut target = self.resume_block();
779 let Builder { ref mut cfg, ref mut scopes, .. } = *self;
781 // Build up the drops in **reverse** order. The end result will
784 // scopes[n] -> scopes[n-1] -> ... -> scopes[0]
786 // However, we build this in **reverse order**. That is, we
787 // process scopes[0], then scopes[1], etc, pointing each one at
788 // the result generates from the one before. Along the way, we
789 // store caches. If everything is cached, we'll just walk right
790 // to left reading the cached results but never created anything.
792 if scopes.iter().any(|scope| scope.needs_cleanup) {
793 for scope in scopes.iter_mut() {
794 target = build_diverge_scope(self.hir.tcx(), cfg, scope.region_scope_span,
795 scope, target, generator_drop);
802 /// Utility function for *non*-scope code to build their own drops
803 pub fn build_drop(&mut self,
806 location: Place<'tcx>,
807 ty: Ty<'tcx>) -> BlockAnd<()> {
808 if !self.hir.needs_drop(ty) {
811 let source_info = self.source_info(span);
812 let next_target = self.cfg.start_new_block();
813 let diverge_target = self.diverge_cleanup();
814 self.cfg.terminate(block, source_info,
815 TerminatorKind::Drop {
818 unwind: Some(diverge_target),
823 /// Utility function for *non*-scope code to build their own drops
824 pub fn build_drop_and_replace(&mut self,
827 location: Place<'tcx>,
828 value: Operand<'tcx>) -> BlockAnd<()> {
829 let source_info = self.source_info(span);
830 let next_target = self.cfg.start_new_block();
831 let diverge_target = self.diverge_cleanup();
832 self.cfg.terminate(block, source_info,
833 TerminatorKind::DropAndReplace {
837 unwind: Some(diverge_target),
842 /// Create an Assert terminator and return the success block.
843 /// If the boolean condition operand is not the expected value,
844 /// a runtime panic will be caused with the given message.
845 pub fn assert(&mut self, block: BasicBlock,
848 msg: AssertMessage<'tcx>,
851 let source_info = self.source_info(span);
853 let success_block = self.cfg.start_new_block();
854 let cleanup = self.diverge_cleanup();
856 self.cfg.terminate(block, source_info,
857 TerminatorKind::Assert {
861 target: success_block,
862 cleanup: Some(cleanup),
869 /// Builds drops for pop_scope and exit_scope.
870 fn build_scope_drops<'tcx>(cfg: &mut CFG<'tcx>,
871 resume_block: BasicBlock,
873 earlier_scopes: &[Scope<'tcx>],
874 mut block: BasicBlock,
876 generator_drop: bool)
878 debug!("build_scope_drops({:?} -> {:?})", block, scope);
879 let mut iter = scope.drops.iter().rev();
880 while let Some(drop_data) = iter.next() {
881 let source_info = scope.source_info(drop_data.span);
882 match drop_data.kind {
883 DropKind::Value { .. } => {
884 // Try to find the next block with its cached block for us to
885 // diverge into, either a previous block in this current scope or
886 // the top of the previous scope.
888 // If it wasn't for EndRegion, we could just chain all the DropData
889 // together and pick the first DropKind::Value. Please do that
890 // when we replace EndRegion with NLL.
891 let on_diverge = iter.clone().filter_map(|dd| {
893 DropKind::Value { cached_block } => Some(cached_block),
894 DropKind::Storage => None
896 }).next().or_else(|| {
897 if earlier_scopes.iter().any(|scope| scope.needs_cleanup) {
898 // If *any* scope requires cleanup code to be run,
899 // we must use the cached unwind from the *topmost*
900 // scope, to ensure all EndRegions from surrounding
901 // scopes are executed before the drop code runs.
902 Some(earlier_scopes.last().unwrap().cached_unwind)
904 // We don't need any further cleanup, so return None
905 // to avoid creating a landing pad. We can skip
906 // EndRegions because all local regions end anyway
907 // when the function unwinds.
909 // This is an important optimization because LLVM is
910 // terrible at optimizing landing pads. FIXME: I think
911 // it would be cleaner and better to do this optimization
912 // in SimplifyCfg instead of here.
917 let on_diverge = on_diverge.map(|cached_block| {
918 cached_block.get(generator_drop).unwrap_or_else(|| {
919 span_bug!(drop_data.span, "cached block not present?")
923 let next = cfg.start_new_block();
924 cfg.terminate(block, source_info, TerminatorKind::Drop {
925 location: drop_data.location.clone(),
927 unwind: Some(on_diverge.unwrap_or(resume_block))
931 DropKind::Storage => {
932 // We do not need to emit StorageDead for generator drops
937 // Drop the storage for both value and storage drops.
938 // Only temps and vars need their storage dead.
939 match drop_data.location {
940 Place::Local(index) if index.index() > arg_count => {
941 cfg.push(block, Statement {
943 kind: StatementKind::StorageDead(index)
954 fn build_diverge_scope<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
957 scope: &mut Scope<'tcx>,
958 mut target: BasicBlock,
959 generator_drop: bool)
962 // Build up the drops in **reverse** order. The end result will
965 // [EndRegion Block] -> [drops[n]] -...-> [drops[0]] -> [Free] -> [target]
967 // +---------------------------------------------------------+
970 // The code in this function reads from right to left. At each
971 // point, we check for cached blocks representing the
972 // remainder. If everything is cached, we'll just walk right to
973 // left reading the cached results but never create anything.
975 let source_scope = scope.source_scope;
976 let source_info = |span| SourceInfo {
981 // Next, build up the drops. Here we iterate the vector in
982 // *forward* order, so that we generate drops[0] first (right to
983 // left in diagram above).
984 for (j, drop_data) in scope.drops.iter_mut().enumerate() {
985 debug!("build_diverge_scope drop_data[{}]: {:?}", j, drop_data);
986 // Only full value drops are emitted in the diverging path,
989 // Note: This may not actually be what we desire (are we
990 // "freeing" stack storage as we unwind, or merely observing a
991 // frozen stack)? In particular, the intent may have been to
992 // match the behavior of clang, but on inspection eddyb says
993 // this is not what clang does.
994 let cached_block = match drop_data.kind {
995 DropKind::Value { ref mut cached_block } => cached_block.ref_mut(generator_drop),
996 DropKind::Storage => continue
998 target = if let Some(cached_block) = *cached_block {
1001 let block = cfg.start_new_cleanup_block();
1002 cfg.terminate(block, source_info(drop_data.span),
1003 TerminatorKind::Drop {
1004 location: drop_data.location.clone(),
1008 *cached_block = Some(block);
1013 // Finally, push the EndRegion block, used by mir-borrowck, and set
1014 // `cached_unwind` to point to it (Block becomes trivial goto after
1015 // pass that removes all EndRegions).
1017 let cached_block = scope.cached_unwind.ref_mut(generator_drop);
1018 if let Some(cached_block) = *cached_block {
1021 let block = cfg.start_new_cleanup_block();
1022 cfg.push_end_region(tcx, block, source_info(span), scope.region_scope);
1023 cfg.terminate(block, source_info(span), TerminatorKind::Goto { target: target });
1024 *cached_block = Some(block);
1029 debug!("build_diverge_scope({:?}, {:?}) = {:?}", scope, span, target);