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, translate ite
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 translating
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 translating
42 the contents, we also accumulate lvalues 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 lvalues 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};
94 use rustc::hir::def_id::LOCAL_CRATE;
96 use rustc::mir::transform::MirSource;
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 visibility scope this scope was created in.
104 visibility_scope: VisibilityScope,
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 lvalues 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 lvalue was declared)
145 location: Lvalue<'tcx>,
147 /// Whether this is a full value Drop, or just a StorageDead.
151 #[derive(Debug, Default, Clone, Copy)]
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>,
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: Lvalue<'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 visibility scope, make a SourceInfo.
255 fn source_info(&self, span: Span) -> SourceInfo {
258 scope: self.visibility_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: Lvalue<'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 visibility_scope = self.visibility_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.visibility_scope_info[visibility_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 {
335 self.visibility_scope =
336 self.new_visibility_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.visibility_scope = visibility_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.visibility_scope;
355 self.scopes.push(Scope {
356 visibility_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 unpack!(block = build_scope_drops(&mut self.cfg,
397 /// Branch out of `block` to `target`, exiting all scopes up to
398 /// and including `region_scope`. This will insert whatever drops are
399 /// needed, as well as tracking this exit for the SEME region. See
400 /// module comment for details.
401 pub fn exit_scope(&mut self,
403 region_scope: (region::Scope, SourceInfo),
404 mut block: BasicBlock,
405 target: BasicBlock) {
406 debug!("exit_scope(region_scope={:?}, block={:?}, target={:?})",
407 region_scope, block, target);
408 let scope_count = 1 + self.scopes.iter().rev()
409 .position(|scope| scope.region_scope == region_scope.0)
411 span_bug!(span, "region_scope {:?} does not enclose", region_scope)
413 let len = self.scopes.len();
414 assert!(scope_count < len, "should not use `exit_scope` to pop ALL scopes");
416 // If we are emitting a `drop` statement, we need to have the cached
417 // diverge cleanup pads ready in case that drop panics.
418 let may_panic = self.scopes[(len - scope_count)..].iter()
419 .any(|s| s.drops.iter().any(|s| s.kind.may_panic()));
421 self.diverge_cleanup();
425 let mut rest = &mut self.scopes[(len - scope_count)..];
426 while let Some((scope, rest_)) = {rest}.split_last_mut() {
428 block = if let Some(&e) = scope.cached_exits.get(&(target, region_scope.0)) {
429 self.cfg.terminate(block, scope.source_info(span),
430 TerminatorKind::Goto { target: e });
433 let b = self.cfg.start_new_block();
434 self.cfg.terminate(block, scope.source_info(span),
435 TerminatorKind::Goto { target: b });
436 scope.cached_exits.insert((target, region_scope.0), b);
440 // End all regions for scopes out of which we are breaking.
441 self.cfg.push_end_region(self.hir.tcx(), block, region_scope.1, scope.region_scope);
443 unpack!(block = build_scope_drops(&mut self.cfg,
451 let scope = &self.scopes[len - scope_count];
452 self.cfg.terminate(block, scope.source_info(span),
453 TerminatorKind::Goto { target: target });
456 /// Creates a path that performs all required cleanup for dropping a generator.
458 /// This path terminates in GeneratorDrop. Returns the start of the path.
459 /// None indicates there’s no cleanup to do at this point.
460 pub fn generator_drop_cleanup(&mut self) -> Option<BasicBlock> {
461 if !self.scopes.iter().any(|scope| scope.needs_cleanup) {
466 self.diverge_cleanup_gen(true);
468 let src_info = self.scopes[0].source_info(self.fn_span);
469 let mut block = self.cfg.start_new_block();
471 let mut rest = &mut self.scopes[..];
473 while let Some((scope, rest_)) = {rest}.split_last_mut() {
475 if !scope.needs_cleanup {
478 block = if let Some(b) = scope.cached_generator_drop {
479 self.cfg.terminate(block, src_info,
480 TerminatorKind::Goto { target: b });
483 let b = self.cfg.start_new_block();
484 scope.cached_generator_drop = Some(b);
485 self.cfg.terminate(block, src_info,
486 TerminatorKind::Goto { target: b });
490 // End all regions for scopes out of which we are breaking.
491 self.cfg.push_end_region(self.hir.tcx(), block, src_info, scope.region_scope);
493 unpack!(block = build_scope_drops(&mut self.cfg,
501 self.cfg.terminate(block, src_info, TerminatorKind::GeneratorDrop);
506 /// Creates a new visibility scope, nested in the current one.
507 pub fn new_visibility_scope(&mut self,
509 lint_level: LintLevel,
510 safety: Option<Safety>) -> VisibilityScope {
511 let parent = self.visibility_scope;
512 debug!("new_visibility_scope({:?}, {:?}, {:?}) - parent({:?})={:?}",
513 span, lint_level, safety,
514 parent, self.visibility_scope_info.get(parent));
515 let scope = self.visibility_scopes.push(VisibilityScopeData {
517 parent_scope: Some(parent),
519 let scope_info = VisibilityScopeInfo {
520 lint_root: if let LintLevel::Explicit(lint_root) = lint_level {
523 self.visibility_scope_info[parent].lint_root
525 safety: safety.unwrap_or_else(|| {
526 self.visibility_scope_info[parent].safety
529 self.visibility_scope_info.push(scope_info);
535 /// Finds the breakable scope for a given label. This is used for
536 /// resolving `break` and `continue`.
537 pub fn find_breakable_scope(&mut self,
539 label: region::Scope)
540 -> &mut BreakableScope<'tcx> {
541 // find the loop-scope with the correct id
542 self.breakable_scopes.iter_mut()
544 .filter(|breakable_scope| breakable_scope.region_scope == label)
546 .unwrap_or_else(|| span_bug!(span, "no enclosing breakable scope found"))
549 /// Given a span and the current visibility scope, make a SourceInfo.
550 pub fn source_info(&self, span: Span) -> SourceInfo {
553 scope: self.visibility_scope
557 /// Returns the `region::Scope` of the scope which should be exited by a
559 pub fn region_scope_of_return_scope(&self) -> region::Scope {
560 // The outermost scope (`scopes[0]`) will be the `CallSiteScope`.
561 // We want `scopes[1]`, which is the `ParameterScope`.
562 assert!(self.scopes.len() >= 2);
563 assert!(match self.scopes[1].region_scope.data() {
564 region::ScopeData::Arguments(_) => true,
567 self.scopes[1].region_scope
570 /// Returns the topmost active scope, which is known to be alive until
571 /// the next scope expression.
572 pub fn topmost_scope(&self) -> region::Scope {
573 self.scopes.last().expect("topmost_scope: no scopes present").region_scope
576 /// Returns the scope that we should use as the lifetime of an
577 /// operand. Basically, an operand must live until it is consumed.
578 /// This is similar to, but not quite the same as, the temporary
579 /// scope (which can be larger or smaller).
583 /// let x = foo(bar(X, Y));
585 /// We wish to pop the storage for X and Y after `bar()` is
586 /// called, not after the whole `let` is completed.
588 /// As another example, if the second argument diverges:
590 /// foo(Box::new(2), panic!())
592 /// We would allocate the box but then free it on the unwinding
593 /// path; we would also emit a free on the 'success' path from
594 /// panic, but that will turn out to be removed as dead-code.
596 /// When building statics/constants, returns `None` since
597 /// intermediate values do not have to be dropped in that case.
598 pub fn local_scope(&self) -> Option<region::Scope> {
600 MirSource::Const(_) |
601 MirSource::Static(..) =>
602 // No need to free storage in this context.
605 Some(self.topmost_scope()),
606 MirSource::Promoted(..) |
607 MirSource::GeneratorDrop(..) =>
614 /// Indicates that `lvalue` should be dropped on exit from
616 pub fn schedule_drop(&mut self,
618 region_scope: region::Scope,
619 lvalue: &Lvalue<'tcx>,
620 lvalue_ty: Ty<'tcx>) {
621 let needs_drop = self.hir.needs_drop(lvalue_ty);
622 let drop_kind = if needs_drop {
623 DropKind::Value { cached_block: CachedBlock::default() }
625 // Only temps and vars need their storage dead.
627 Lvalue::Local(index) if index.index() > self.arg_count => DropKind::Storage,
632 for scope in self.scopes.iter_mut().rev() {
633 let this_scope = scope.region_scope == region_scope;
634 // When building drops, we try to cache chains of drops in such a way so these drops
635 // could be reused by the drops which would branch into the cached (already built)
636 // blocks. This, however, means that whenever we add a drop into a scope which already
637 // had some blocks built (and thus, cached) for it, we must invalidate all caches which
638 // might branch into the scope which had a drop just added to it. This is necessary,
639 // because otherwise some other code might use the cache to branch into already built
640 // chain of drops, essentially ignoring the newly added drop.
642 // For example consider there’s two scopes with a drop in each. These are built and
643 // thus the caches are filled:
645 // +--------------------------------------------------------+
646 // | +---------------------------------+ |
647 // | | +--------+ +-------------+ | +---------------+ |
648 // | | | return | <-+ | drop(outer) | <-+ | drop(middle) | |
649 // | | +--------+ +-------------+ | +---------------+ |
650 // | +------------|outer_scope cache|--+ |
651 // +------------------------------|middle_scope cache|------+
653 // Now, a new, inner-most scope is added along with a new drop into both inner-most and
654 // outer-most scopes:
656 // +------------------------------------------------------------+
657 // | +----------------------------------+ |
658 // | | +--------+ +-------------+ | +---------------+ | +-------------+
659 // | | | return | <+ | drop(new) | <-+ | drop(middle) | <--+| drop(inner) |
660 // | | +--------+ | | drop(outer) | | +---------------+ | +-------------+
661 // | | +-+ +-------------+ | |
662 // | +---|invalid outer_scope cache|----+ |
663 // +----=----------------|invalid middle_scope cache|-----------+
665 // If, when adding `drop(new)` we do not invalidate the cached blocks for both
666 // outer_scope and middle_scope, then, when building drops for the inner (right-most)
667 // scope, the old, cached blocks, without `drop(new)` will get used, producing the
670 // The cache and its invalidation for unwind branch is somewhat special. The cache is
671 // per-drop, rather than per scope, which has a several different implications. Adding
672 // a new drop into a scope will not invalidate cached blocks of the prior drops in the
673 // scope. That is true, because none of the already existing drops will have an edge
674 // into a block with the newly added drop.
676 // Note that this code iterates scopes from the inner-most to the outer-most,
677 // invalidating caches of each scope visited. This way bare minimum of the
678 // caches gets invalidated. i.e. if a new drop is added into the middle scope, the
679 // cache of outer scpoe stays intact.
680 scope.invalidate_cache(!needs_drop, this_scope);
682 if let DropKind::Value { .. } = drop_kind {
683 scope.needs_cleanup = true;
685 let region_scope_span = region_scope.span(self.hir.tcx(),
686 &self.hir.region_scope_tree);
687 // Attribute scope exit drops to scope's closing brace
688 let scope_end = region_scope_span.with_lo(region_scope_span.hi());
689 scope.drops.push(DropData {
691 location: lvalue.clone(),
697 span_bug!(span, "region scope {:?} not in scope to drop {:?}", region_scope, lvalue);
702 /// Creates a path that performs all required cleanup for unwinding.
704 /// This path terminates in Resume. Returns the start of the path.
705 /// See module comment for more details. None indicates there’s no
706 /// cleanup to do at this point.
707 pub fn diverge_cleanup(&mut self) -> Option<BasicBlock> {
708 self.diverge_cleanup_gen(false)
711 fn diverge_cleanup_gen(&mut self, generator_drop: bool) -> Option<BasicBlock> {
712 if !self.scopes.iter().any(|scope| scope.needs_cleanup) {
715 assert!(!self.scopes.is_empty()); // or `any` above would be false
717 let Builder { ref mut cfg, ref mut scopes,
718 ref mut cached_resume_block, .. } = *self;
720 // Build up the drops in **reverse** order. The end result will
723 // scopes[n] -> scopes[n-1] -> ... -> scopes[0]
725 // However, we build this in **reverse order**. That is, we
726 // process scopes[0], then scopes[1], etc, pointing each one at
727 // the result generates from the one before. Along the way, we
728 // store caches. If everything is cached, we'll just walk right
729 // to left reading the cached results but never created anything.
731 // To start, create the resume terminator.
732 let mut target = if let Some(target) = *cached_resume_block {
735 let resumeblk = cfg.start_new_cleanup_block();
736 cfg.terminate(resumeblk,
737 scopes[0].source_info(self.fn_span),
738 TerminatorKind::Resume);
739 *cached_resume_block = Some(resumeblk);
743 for scope in scopes.iter_mut() {
744 target = build_diverge_scope(self.hir.tcx(), cfg, scope.region_scope_span,
745 scope, target, generator_drop);
750 /// Utility function for *non*-scope code to build their own drops
751 pub fn build_drop(&mut self,
754 location: Lvalue<'tcx>,
755 ty: Ty<'tcx>) -> BlockAnd<()> {
756 if !self.hir.needs_drop(ty) {
759 let source_info = self.source_info(span);
760 let next_target = self.cfg.start_new_block();
761 let diverge_target = self.diverge_cleanup();
762 self.cfg.terminate(block, source_info,
763 TerminatorKind::Drop {
766 unwind: diverge_target,
771 /// Utility function for *non*-scope code to build their own drops
772 pub fn build_drop_and_replace(&mut self,
775 location: Lvalue<'tcx>,
776 value: Operand<'tcx>) -> BlockAnd<()> {
777 let source_info = self.source_info(span);
778 let next_target = self.cfg.start_new_block();
779 let diverge_target = self.diverge_cleanup();
780 self.cfg.terminate(block, source_info,
781 TerminatorKind::DropAndReplace {
785 unwind: diverge_target,
790 /// Create an Assert terminator and return the success block.
791 /// If the boolean condition operand is not the expected value,
792 /// a runtime panic will be caused with the given message.
793 pub fn assert(&mut self, block: BasicBlock,
796 msg: AssertMessage<'tcx>,
799 let source_info = self.source_info(span);
801 let success_block = self.cfg.start_new_block();
802 let cleanup = self.diverge_cleanup();
804 self.cfg.terminate(block, source_info,
805 TerminatorKind::Assert {
809 target: success_block,
817 /// Builds drops for pop_scope and exit_scope.
818 fn build_scope_drops<'tcx>(cfg: &mut CFG<'tcx>,
820 earlier_scopes: &[Scope<'tcx>],
821 mut block: BasicBlock,
823 generator_drop: bool)
825 debug!("build_scope_drops({:?} -> {:?})", block, scope);
826 let mut iter = scope.drops.iter().rev();
827 while let Some(drop_data) = iter.next() {
828 let source_info = scope.source_info(drop_data.span);
829 match drop_data.kind {
830 DropKind::Value { .. } => {
831 // Try to find the next block with its cached block for us to
832 // diverge into, either a previous block in this current scope or
833 // the top of the previous scope.
835 // If it wasn't for EndRegion, we could just chain all the DropData
836 // together and pick the first DropKind::Value. Please do that
837 // when we replace EndRegion with NLL.
838 let on_diverge = iter.clone().filter_map(|dd| {
840 DropKind::Value { cached_block } => Some(cached_block),
841 DropKind::Storage => None
843 }).next().or_else(|| {
844 if earlier_scopes.iter().any(|scope| scope.needs_cleanup) {
845 // If *any* scope requires cleanup code to be run,
846 // we must use the cached unwind from the *topmost*
847 // scope, to ensure all EndRegions from surrounding
848 // scopes are executed before the drop code runs.
849 Some(earlier_scopes.last().unwrap().cached_unwind)
851 // We don't need any further cleanup, so return None
852 // to avoid creating a landing pad. We can skip
853 // EndRegions because all local regions end anyway
854 // when the function unwinds.
856 // This is an important optimization because LLVM is
857 // terrible at optimizing landing pads. FIXME: I think
858 // it would be cleaner and better to do this optimization
859 // in SimplifyCfg instead of here.
864 let on_diverge = on_diverge.map(|cached_block| {
865 cached_block.get(generator_drop).unwrap_or_else(|| {
866 span_bug!(drop_data.span, "cached block not present?")
870 let next = cfg.start_new_block();
871 cfg.terminate(block, source_info, TerminatorKind::Drop {
872 location: drop_data.location.clone(),
878 DropKind::Storage => {}
881 // We do not need to emit StorageDead for generator drops
886 // Drop the storage for both value and storage drops.
887 // Only temps and vars need their storage dead.
888 match drop_data.location {
889 Lvalue::Local(index) if index.index() > arg_count => {
890 cfg.push(block, Statement {
892 kind: StatementKind::StorageDead(index)
901 fn build_diverge_scope<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
904 scope: &mut Scope<'tcx>,
905 mut target: BasicBlock,
906 generator_drop: bool)
909 // Build up the drops in **reverse** order. The end result will
912 // [EndRegion Block] -> [drops[n]] -...-> [drops[0]] -> [Free] -> [target]
914 // +---------------------------------------------------------+
917 // The code in this function reads from right to left. At each
918 // point, we check for cached blocks representing the
919 // remainder. If everything is cached, we'll just walk right to
920 // left reading the cached results but never create anything.
922 let visibility_scope = scope.visibility_scope;
923 let source_info = |span| SourceInfo {
925 scope: visibility_scope
928 // Next, build up the drops. Here we iterate the vector in
929 // *forward* order, so that we generate drops[0] first (right to
930 // left in diagram above).
931 for (j, drop_data) in scope.drops.iter_mut().enumerate() {
932 debug!("build_diverge_scope drop_data[{}]: {:?}", j, drop_data);
933 // Only full value drops are emitted in the diverging path,
936 // Note: This may not actually be what we desire (are we
937 // "freeing" stack storage as we unwind, or merely observing a
938 // frozen stack)? In particular, the intent may have been to
939 // match the behavior of clang, but on inspection eddyb says
940 // this is not what clang does.
941 let cached_block = match drop_data.kind {
942 DropKind::Value { ref mut cached_block } => cached_block.ref_mut(generator_drop),
943 DropKind::Storage => continue
945 target = if let Some(cached_block) = *cached_block {
948 let block = cfg.start_new_cleanup_block();
949 cfg.terminate(block, source_info(drop_data.span),
950 TerminatorKind::Drop {
951 location: drop_data.location.clone(),
955 *cached_block = Some(block);
960 // Finally, push the EndRegion block, used by mir-borrowck, and set
961 // `cached_unwind` to point to it (Block becomes trivial goto after
962 // pass that removes all EndRegions).
964 let cached_block = scope.cached_unwind.ref_mut(generator_drop);
965 if let Some(cached_block) = *cached_block {
968 let block = cfg.start_new_cleanup_block();
969 cfg.push_end_region(tcx, block, source_info(span), scope.region_scope);
970 cfg.terminate(block, source_info(span), TerminatorKind::Goto { target: target });
971 *cached_block = Some(block);
976 debug!("build_diverge_scope({:?}, {:?}) = {:?}", scope, span, target);