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1 //===-- EHScopeStack.h - Stack for cleanup IR generation --------*- C++ -*-===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // These classes should be the minimum interface required for other parts of
11 // CodeGen to emit cleanups.  The implementation is in CGCleanup.cpp and other
12 // implemenentation details that are not widely needed are in CGCleanup.h.
13 //
14 //===----------------------------------------------------------------------===//
15 
16 #ifndef LLVM_CLANG_LIB_CODEGEN_EHSCOPESTACK_H
17 #define LLVM_CLANG_LIB_CODEGEN_EHSCOPESTACK_H
18 
19 #include "clang/Basic/LLVM.h"
20 #include "llvm/ADT/STLExtras.h"
21 #include "llvm/ADT/SmallVector.h"
22 #include "llvm/IR/BasicBlock.h"
23 #include "llvm/IR/Instructions.h"
24 #include "llvm/IR/Value.h"
25 
26 namespace clang {
27 namespace CodeGen {
28 
29 class CodeGenFunction;
30 
31 /// A branch fixup.  These are required when emitting a goto to a
32 /// label which hasn't been emitted yet.  The goto is optimistically
33 /// emitted as a branch to the basic block for the label, and (if it
34 /// occurs in a scope with non-trivial cleanups) a fixup is added to
35 /// the innermost cleanup.  When a (normal) cleanup is popped, any
36 /// unresolved fixups in that scope are threaded through the cleanup.
37 struct BranchFixup {
38   /// The block containing the terminator which needs to be modified
39   /// into a switch if this fixup is resolved into the current scope.
40   /// If null, LatestBranch points directly to the destination.
41   llvm::BasicBlock *OptimisticBranchBlock;
42 
43   /// The ultimate destination of the branch.
44   ///
45   /// This can be set to null to indicate that this fixup was
46   /// successfully resolved.
47   llvm::BasicBlock *Destination;
48 
49   /// The destination index value.
50   unsigned DestinationIndex;
51 
52   /// The initial branch of the fixup.
53   llvm::BranchInst *InitialBranch;
54 };
55 
56 template <class T> struct InvariantValue {
57   typedef T type;
58   typedef T saved_type;
needsSavingInvariantValue59   static bool needsSaving(type value) { return false; }
saveInvariantValue60   static saved_type save(CodeGenFunction &CGF, type value) { return value; }
restoreInvariantValue61   static type restore(CodeGenFunction &CGF, saved_type value) { return value; }
62 };
63 
64 /// A metaprogramming class for ensuring that a value will dominate an
65 /// arbitrary position in a function.
66 template <class T> struct DominatingValue : InvariantValue<T> {};
67 
68 template <class T, bool mightBeInstruction =
69             std::is_base_of<llvm::Value, T>::value &&
70             !std::is_base_of<llvm::Constant, T>::value &&
71             !std::is_base_of<llvm::BasicBlock, T>::value>
72 struct DominatingPointer;
73 template <class T> struct DominatingPointer<T,false> : InvariantValue<T*> {};
74 // template <class T> struct DominatingPointer<T,true> at end of file
75 
76 template <class T> struct DominatingValue<T*> : DominatingPointer<T> {};
77 
78 enum CleanupKind : unsigned {
79   /// Denotes a cleanup that should run when a scope is exited using exceptional
80   /// control flow (a throw statement leading to stack unwinding, ).
81   EHCleanup = 0x1,
82 
83   /// Denotes a cleanup that should run when a scope is exited using normal
84   /// control flow (falling off the end of the scope, return, goto, ...).
85   NormalCleanup = 0x2,
86 
87   NormalAndEHCleanup = EHCleanup | NormalCleanup,
88 
89   InactiveCleanup = 0x4,
90   InactiveEHCleanup = EHCleanup | InactiveCleanup,
91   InactiveNormalCleanup = NormalCleanup | InactiveCleanup,
92   InactiveNormalAndEHCleanup = NormalAndEHCleanup | InactiveCleanup,
93 
94   LifetimeMarker = 0x8,
95   NormalEHLifetimeMarker = LifetimeMarker | NormalAndEHCleanup,
96 };
97 
98 /// A stack of scopes which respond to exceptions, including cleanups
99 /// and catch blocks.
100 class EHScopeStack {
101 public:
102   /* Should switch to alignof(uint64_t) instead of 8, when EHCleanupScope can */
103   enum { ScopeStackAlignment = 8 };
104 
105   /// A saved depth on the scope stack.  This is necessary because
106   /// pushing scopes onto the stack invalidates iterators.
107   class stable_iterator {
108     friend class EHScopeStack;
109 
110     /// Offset from StartOfData to EndOfBuffer.
111     ptrdiff_t Size;
112 
113     stable_iterator(ptrdiff_t Size) : Size(Size) {}
114 
115   public:
116     static stable_iterator invalid() { return stable_iterator(-1); }
117     stable_iterator() : Size(-1) {}
118 
119     bool isValid() const { return Size >= 0; }
120 
121     /// Returns true if this scope encloses I.
122     /// Returns false if I is invalid.
123     /// This scope must be valid.
124     bool encloses(stable_iterator I) const { return Size <= I.Size; }
125 
126     /// Returns true if this scope strictly encloses I: that is,
127     /// if it encloses I and is not I.
128     /// Returns false is I is invalid.
129     /// This scope must be valid.
130     bool strictlyEncloses(stable_iterator I) const { return Size < I.Size; }
131 
132     friend bool operator==(stable_iterator A, stable_iterator B) {
133       return A.Size == B.Size;
134     }
135     friend bool operator!=(stable_iterator A, stable_iterator B) {
136       return A.Size != B.Size;
137     }
138   };
139 
140   /// Information for lazily generating a cleanup.  Subclasses must be
141   /// POD-like: cleanups will not be destructed, and they will be
142   /// allocated on the cleanup stack and freely copied and moved
143   /// around.
144   ///
145   /// Cleanup implementations should generally be declared in an
146   /// anonymous namespace.
147   class Cleanup {
148     // Anchor the construction vtable.
149     virtual void anchor();
150 
151   protected:
152     ~Cleanup() = default;
153 
154   public:
155     Cleanup(const Cleanup &) = default;
156     Cleanup(Cleanup &&) {}
157     Cleanup() = default;
158 
159     /// Generation flags.
160     class Flags {
161       enum {
162         F_IsForEH             = 0x1,
163         F_IsNormalCleanupKind = 0x2,
164         F_IsEHCleanupKind     = 0x4
165       };
166       unsigned flags;
167 
168     public:
169       Flags() : flags(0) {}
170 
171       /// isForEH - true if the current emission is for an EH cleanup.
172       bool isForEHCleanup() const { return flags & F_IsForEH; }
173       bool isForNormalCleanup() const { return !isForEHCleanup(); }
174       void setIsForEHCleanup() { flags |= F_IsForEH; }
175 
176       bool isNormalCleanupKind() const { return flags & F_IsNormalCleanupKind; }
177       void setIsNormalCleanupKind() { flags |= F_IsNormalCleanupKind; }
178 
179       /// isEHCleanupKind - true if the cleanup was pushed as an EH
180       /// cleanup.
181       bool isEHCleanupKind() const { return flags & F_IsEHCleanupKind; }
182       void setIsEHCleanupKind() { flags |= F_IsEHCleanupKind; }
183     };
184 
185 
186     /// Emit the cleanup.  For normal cleanups, this is run in the
187     /// same EH context as when the cleanup was pushed, i.e. the
188     /// immediately-enclosing context of the cleanup scope.  For
189     /// EH cleanups, this is run in a terminate context.
190     ///
191     // \param flags cleanup kind.
192     virtual void Emit(CodeGenFunction &CGF, Flags flags) = 0;
193   };
194 
195   /// ConditionalCleanup stores the saved form of its parameters,
196   /// then restores them and performs the cleanup.
197   template <class T, class... As>
198   class ConditionalCleanup final : public Cleanup {
199     typedef std::tuple<typename DominatingValue<As>::saved_type...> SavedTuple;
200     SavedTuple Saved;
201 
202     template <std::size_t... Is>
203     T restore(CodeGenFunction &CGF, llvm::index_sequence<Is...>) {
204       // It's important that the restores are emitted in order. The braced init
205       // list guarentees that.
206       return T{DominatingValue<As>::restore(CGF, std::get<Is>(Saved))...};
207     }
208 
209     void Emit(CodeGenFunction &CGF, Flags flags) override {
210       restore(CGF, llvm::index_sequence_for<As...>()).Emit(CGF, flags);
211     }
212 
213   public:
214     ConditionalCleanup(typename DominatingValue<As>::saved_type... A)
215         : Saved(A...) {}
216 
217     ConditionalCleanup(SavedTuple Tuple) : Saved(std::move(Tuple)) {}
218   };
219 
220 private:
221   // The implementation for this class is in CGException.h and
222   // CGException.cpp; the definition is here because it's used as a
223   // member of CodeGenFunction.
224 
225   /// The start of the scope-stack buffer, i.e. the allocated pointer
226   /// for the buffer.  All of these pointers are either simultaneously
227   /// null or simultaneously valid.
228   char *StartOfBuffer;
229 
230   /// The end of the buffer.
231   char *EndOfBuffer;
232 
233   /// The first valid entry in the buffer.
234   char *StartOfData;
235 
236   /// The innermost normal cleanup on the stack.
237   stable_iterator InnermostNormalCleanup;
238 
239   /// The innermost EH scope on the stack.
240   stable_iterator InnermostEHScope;
241 
242   /// The current set of branch fixups.  A branch fixup is a jump to
243   /// an as-yet unemitted label, i.e. a label for which we don't yet
244   /// know the EH stack depth.  Whenever we pop a cleanup, we have
245   /// to thread all the current branch fixups through it.
246   ///
247   /// Fixups are recorded as the Use of the respective branch or
248   /// switch statement.  The use points to the final destination.
249   /// When popping out of a cleanup, these uses are threaded through
250   /// the cleanup and adjusted to point to the new cleanup.
251   ///
252   /// Note that branches are allowed to jump into protected scopes
253   /// in certain situations;  e.g. the following code is legal:
254   ///     struct A { ~A(); }; // trivial ctor, non-trivial dtor
255   ///     goto foo;
256   ///     A a;
257   ///    foo:
258   ///     bar();
259   SmallVector<BranchFixup, 8> BranchFixups;
260 
261   char *allocate(size_t Size);
262   void deallocate(size_t Size);
263 
264   void *pushCleanup(CleanupKind K, size_t DataSize);
265 
266 public:
267   EHScopeStack() : StartOfBuffer(nullptr), EndOfBuffer(nullptr),
268                    StartOfData(nullptr), InnermostNormalCleanup(stable_end()),
269                    InnermostEHScope(stable_end()) {}
270   ~EHScopeStack() { delete[] StartOfBuffer; }
271 
272   /// Push a lazily-created cleanup on the stack.
273   template <class T, class... As> void pushCleanup(CleanupKind Kind, As... A) {
274     static_assert(llvm::AlignOf<T>::Alignment <= ScopeStackAlignment,
275                   "Cleanup's alignment is too large.");
276     void *Buffer = pushCleanup(Kind, sizeof(T));
277     Cleanup *Obj = new (Buffer) T(A...);
278     (void) Obj;
279   }
280 
281   /// Push a lazily-created cleanup on the stack. Tuple version.
282   template <class T, class... As>
283   void pushCleanupTuple(CleanupKind Kind, std::tuple<As...> A) {
284     static_assert(llvm::AlignOf<T>::Alignment <= ScopeStackAlignment,
285                   "Cleanup's alignment is too large.");
286     void *Buffer = pushCleanup(Kind, sizeof(T));
287     Cleanup *Obj = new (Buffer) T(std::move(A));
288     (void) Obj;
289   }
290 
291   // Feel free to add more variants of the following:
292 
293   /// Push a cleanup with non-constant storage requirements on the
294   /// stack.  The cleanup type must provide an additional static method:
295   ///   static size_t getExtraSize(size_t);
296   /// The argument to this method will be the value N, which will also
297   /// be passed as the first argument to the constructor.
298   ///
299   /// The data stored in the extra storage must obey the same
300   /// restrictions as normal cleanup member data.
301   ///
302   /// The pointer returned from this method is valid until the cleanup
303   /// stack is modified.
304   template <class T, class... As>
305   T *pushCleanupWithExtra(CleanupKind Kind, size_t N, As... A) {
306     static_assert(llvm::AlignOf<T>::Alignment <= ScopeStackAlignment,
307                   "Cleanup's alignment is too large.");
308     void *Buffer = pushCleanup(Kind, sizeof(T) + T::getExtraSize(N));
309     return new (Buffer) T(N, A...);
310   }
311 
312   void pushCopyOfCleanup(CleanupKind Kind, const void *Cleanup, size_t Size) {
313     void *Buffer = pushCleanup(Kind, Size);
314     std::memcpy(Buffer, Cleanup, Size);
315   }
316 
317   /// Pops a cleanup scope off the stack.  This is private to CGCleanup.cpp.
318   void popCleanup();
319 
320   /// Push a set of catch handlers on the stack.  The catch is
321   /// uninitialized and will need to have the given number of handlers
322   /// set on it.
323   class EHCatchScope *pushCatch(unsigned NumHandlers);
324 
325   /// Pops a catch scope off the stack.  This is private to CGException.cpp.
326   void popCatch();
327 
328   /// Push an exceptions filter on the stack.
329   class EHFilterScope *pushFilter(unsigned NumFilters);
330 
331   /// Pops an exceptions filter off the stack.
332   void popFilter();
333 
334   /// Push a terminate handler on the stack.
335   void pushTerminate();
336 
337   /// Pops a terminate handler off the stack.
338   void popTerminate();
339 
340   // Returns true iff the current scope is either empty or contains only
341   // lifetime markers, i.e. no real cleanup code
342   bool containsOnlyLifetimeMarkers(stable_iterator Old) const;
343 
344   /// Determines whether the exception-scopes stack is empty.
345   bool empty() const { return StartOfData == EndOfBuffer; }
346 
347   bool requiresLandingPad() const;
348 
349   /// Determines whether there are any normal cleanups on the stack.
350   bool hasNormalCleanups() const {
351     return InnermostNormalCleanup != stable_end();
352   }
353 
354   /// Returns the innermost normal cleanup on the stack, or
355   /// stable_end() if there are no normal cleanups.
356   stable_iterator getInnermostNormalCleanup() const {
357     return InnermostNormalCleanup;
358   }
359   stable_iterator getInnermostActiveNormalCleanup() const;
360 
361   stable_iterator getInnermostEHScope() const {
362     return InnermostEHScope;
363   }
364 
365 
366   /// An unstable reference to a scope-stack depth.  Invalidated by
367   /// pushes but not pops.
368   class iterator;
369 
370   /// Returns an iterator pointing to the innermost EH scope.
371   iterator begin() const;
372 
373   /// Returns an iterator pointing to the outermost EH scope.
374   iterator end() const;
375 
376   /// Create a stable reference to the top of the EH stack.  The
377   /// returned reference is valid until that scope is popped off the
378   /// stack.
379   stable_iterator stable_begin() const {
380     return stable_iterator(EndOfBuffer - StartOfData);
381   }
382 
383   /// Create a stable reference to the bottom of the EH stack.
384   static stable_iterator stable_end() {
385     return stable_iterator(0);
386   }
387 
388   /// Translates an iterator into a stable_iterator.
389   stable_iterator stabilize(iterator it) const;
390 
391   /// Turn a stable reference to a scope depth into a unstable pointer
392   /// to the EH stack.
393   iterator find(stable_iterator save) const;
394 
395   /// Add a branch fixup to the current cleanup scope.
396   BranchFixup &addBranchFixup() {
397     assert(hasNormalCleanups() && "adding fixup in scope without cleanups");
398     BranchFixups.push_back(BranchFixup());
399     return BranchFixups.back();
400   }
401 
402   unsigned getNumBranchFixups() const { return BranchFixups.size(); }
403   BranchFixup &getBranchFixup(unsigned I) {
404     assert(I < getNumBranchFixups());
405     return BranchFixups[I];
406   }
407 
408   /// Pops lazily-removed fixups from the end of the list.  This
409   /// should only be called by procedures which have just popped a
410   /// cleanup or resolved one or more fixups.
411   void popNullFixups();
412 
413   /// Clears the branch-fixups list.  This should only be called by
414   /// ResolveAllBranchFixups.
415   void clearFixups() { BranchFixups.clear(); }
416 };
417 
418 } // namespace CodeGen
419 } // namespace clang
420 
421 #endif
422