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//===-- EHScopeStack.h - Stack for cleanup IR generation --------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// These classes should be the minimum interface required for other parts of
// CodeGen to emit cleanups. The implementation is in CGCleanup.cpp and other
// implemenentation details that are not widely needed are in CGCleanup.h.
#include "clang/Basic/LLVM.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Value.h"
namespace clang {
namespace CodeGen {
class CodeGenFunction;
/// A branch fixup. These are required when emitting a goto to a
/// label which hasn't been emitted yet. The goto is optimistically
/// emitted as a branch to the basic block for the label, and (if it
/// occurs in a scope with non-trivial cleanups) a fixup is added to
/// the innermost cleanup. When a (normal) cleanup is popped, any
/// unresolved fixups in that scope are threaded through the cleanup.
struct BranchFixup {
/// The block containing the terminator which needs to be modified
/// into a switch if this fixup is resolved into the current scope.
/// If null, LatestBranch points directly to the destination.
llvm::BasicBlock *OptimisticBranchBlock;
/// The ultimate destination of the branch.
/// This can be set to null to indicate that this fixup was
/// successfully resolved.
llvm::BasicBlock *Destination;
/// The destination index value.
unsigned DestinationIndex;
/// The initial branch of the fixup.
llvm::BranchInst *InitialBranch;
template <class T> struct InvariantValue {
typedef T type;
typedef T saved_type;
static bool needsSaving(type value) { return false; }
static saved_type save(CodeGenFunction &CGF, type value) { return value; }
static type restore(CodeGenFunction &CGF, saved_type value) { return value; }
/// A metaprogramming class for ensuring that a value will dominate an
/// arbitrary position in a function.
template <class T> struct DominatingValue : InvariantValue<T> {};
template <class T, bool mightBeInstruction =
std::is_base_of<llvm::Value, T>::value &&
!std::is_base_of<llvm::Constant, T>::value &&
!std::is_base_of<llvm::BasicBlock, T>::value>
struct DominatingPointer;
template <class T> struct DominatingPointer<T,false> : InvariantValue<T*> {};
// template <class T> struct DominatingPointer<T,true> at end of file
template <class T> struct DominatingValue<T*> : DominatingPointer<T> {};
enum CleanupKind : unsigned {
/// Denotes a cleanup that should run when a scope is exited using exceptional
/// control flow (a throw statement leading to stack unwinding, ).
EHCleanup = 0x1,
/// Denotes a cleanup that should run when a scope is exited using normal
/// control flow (falling off the end of the scope, return, goto, ...).
NormalCleanup = 0x2,
NormalAndEHCleanup = EHCleanup | NormalCleanup,
LifetimeMarker = 0x8,
NormalEHLifetimeMarker = LifetimeMarker | NormalAndEHCleanup,
/// A stack of scopes which respond to exceptions, including cleanups
/// and catch blocks.
class EHScopeStack {
/* Should switch to alignof(uint64_t) instead of 8, when EHCleanupScope can */
enum { ScopeStackAlignment = 8 };
/// A saved depth on the scope stack. This is necessary because
/// pushing scopes onto the stack invalidates iterators.
class stable_iterator {
friend class EHScopeStack;
/// Offset from StartOfData to EndOfBuffer.
ptrdiff_t Size;
stable_iterator(ptrdiff_t Size) : Size(Size) {}
static stable_iterator invalid() { return stable_iterator(-1); }
stable_iterator() : Size(-1) {}
bool isValid() const { return Size >= 0; }
/// Returns true if this scope encloses I.
/// Returns false if I is invalid.
/// This scope must be valid.
bool encloses(stable_iterator I) const { return Size <= I.Size; }
/// Returns true if this scope strictly encloses I: that is,
/// if it encloses I and is not I.
/// Returns false is I is invalid.
/// This scope must be valid.
bool strictlyEncloses(stable_iterator I) const { return Size < I.Size; }
friend bool operator==(stable_iterator A, stable_iterator B) {
return A.Size == B.Size;
friend bool operator!=(stable_iterator A, stable_iterator B) {
return A.Size != B.Size;
/// Information for lazily generating a cleanup. Subclasses must be
/// POD-like: cleanups will not be destructed, and they will be
/// allocated on the cleanup stack and freely copied and moved
/// around.
/// Cleanup implementations should generally be declared in an
/// anonymous namespace.
class Cleanup {
// Anchor the construction vtable.
virtual void anchor();
~Cleanup() = default;
Cleanup(const Cleanup &) = default;
Cleanup(Cleanup &&) {}
Cleanup() = default;
virtual bool isRedundantBeforeReturn() { return false; }
/// Generation flags.
class Flags {
enum {
F_IsForEH = 0x1,
F_IsNormalCleanupKind = 0x2,
F_IsEHCleanupKind = 0x4,
F_HasExitSwitch = 0x8,
unsigned flags;
Flags() : flags(0) {}
/// isForEH - true if the current emission is for an EH cleanup.
bool isForEHCleanup() const { return flags & F_IsForEH; }
bool isForNormalCleanup() const { return !isForEHCleanup(); }
void setIsForEHCleanup() { flags |= F_IsForEH; }
bool isNormalCleanupKind() const { return flags & F_IsNormalCleanupKind; }
void setIsNormalCleanupKind() { flags |= F_IsNormalCleanupKind; }
/// isEHCleanupKind - true if the cleanup was pushed as an EH
/// cleanup.
bool isEHCleanupKind() const { return flags & F_IsEHCleanupKind; }
void setIsEHCleanupKind() { flags |= F_IsEHCleanupKind; }
bool hasExitSwitch() const { return flags & F_HasExitSwitch; }
void setHasExitSwitch() { flags |= F_HasExitSwitch; }
/// Emit the cleanup. For normal cleanups, this is run in the
/// same EH context as when the cleanup was pushed, i.e. the
/// immediately-enclosing context of the cleanup scope. For
/// EH cleanups, this is run in a terminate context.
// \param flags cleanup kind.
virtual void Emit(CodeGenFunction &CGF, Flags flags) = 0;
/// ConditionalCleanup stores the saved form of its parameters,
/// then restores them and performs the cleanup.
template <class T, class... As>
class ConditionalCleanup final : public Cleanup {
typedef std::tuple<typename DominatingValue<As>::saved_type...> SavedTuple;
SavedTuple Saved;
template <std::size_t... Is>
T restore(CodeGenFunction &CGF, std::index_sequence<Is...>) {
// It's important that the restores are emitted in order. The braced init
// list guarantees that.
return T{DominatingValue<As>::restore(CGF, std::get<Is>(Saved))...};
void Emit(CodeGenFunction &CGF, Flags flags) override {
restore(CGF, std::index_sequence_for<As...>()).Emit(CGF, flags);
ConditionalCleanup(typename DominatingValue<As>::saved_type... A)
: Saved(A...) {}
ConditionalCleanup(SavedTuple Tuple) : Saved(std::move(Tuple)) {}
// The implementation for this class is in CGException.h and
// CGException.cpp; the definition is here because it's used as a
// member of CodeGenFunction.
/// The start of the scope-stack buffer, i.e. the allocated pointer
/// for the buffer. All of these pointers are either simultaneously
/// null or simultaneously valid.
char *StartOfBuffer;
/// The end of the buffer.
char *EndOfBuffer;
/// The first valid entry in the buffer.
char *StartOfData;
/// The innermost normal cleanup on the stack.
stable_iterator InnermostNormalCleanup;
/// The innermost EH scope on the stack.
stable_iterator InnermostEHScope;
/// The CGF this Stack belong to
CodeGenFunction* CGF;
/// The current set of branch fixups. A branch fixup is a jump to
/// an as-yet unemitted label, i.e. a label for which we don't yet
/// know the EH stack depth. Whenever we pop a cleanup, we have
/// to thread all the current branch fixups through it.
/// Fixups are recorded as the Use of the respective branch or
/// switch statement. The use points to the final destination.
/// When popping out of a cleanup, these uses are threaded through
/// the cleanup and adjusted to point to the new cleanup.
/// Note that branches are allowed to jump into protected scopes
/// in certain situations; e.g. the following code is legal:
/// struct A { ~A(); }; // trivial ctor, non-trivial dtor
/// goto foo;
/// A a;
/// foo:
/// bar();
SmallVector<BranchFixup, 8> BranchFixups;
char *allocate(size_t Size);
void deallocate(size_t Size);
void *pushCleanup(CleanupKind K, size_t DataSize);
: StartOfBuffer(nullptr), EndOfBuffer(nullptr), StartOfData(nullptr),
InnermostNormalCleanup(stable_end()), InnermostEHScope(stable_end()),
CGF(nullptr) {}
~EHScopeStack() { delete[] StartOfBuffer; }
/// Push a lazily-created cleanup on the stack.
template <class T, class... As> void pushCleanup(CleanupKind Kind, As... A) {
static_assert(alignof(T) <= ScopeStackAlignment,
"Cleanup's alignment is too large.");
void *Buffer = pushCleanup(Kind, sizeof(T));
Cleanup *Obj = new (Buffer) T(A...);
(void) Obj;
/// Push a lazily-created cleanup on the stack. Tuple version.
template <class T, class... As>
void pushCleanupTuple(CleanupKind Kind, std::tuple<As...> A) {
static_assert(alignof(T) <= ScopeStackAlignment,
"Cleanup's alignment is too large.");
void *Buffer = pushCleanup(Kind, sizeof(T));
Cleanup *Obj = new (Buffer) T(std::move(A));
(void) Obj;
// Feel free to add more variants of the following:
/// Push a cleanup with non-constant storage requirements on the
/// stack. The cleanup type must provide an additional static method:
/// static size_t getExtraSize(size_t);
/// The argument to this method will be the value N, which will also
/// be passed as the first argument to the constructor.
/// The data stored in the extra storage must obey the same
/// restrictions as normal cleanup member data.
/// The pointer returned from this method is valid until the cleanup
/// stack is modified.
template <class T, class... As>
T *pushCleanupWithExtra(CleanupKind Kind, size_t N, As... A) {
static_assert(alignof(T) <= ScopeStackAlignment,
"Cleanup's alignment is too large.");
void *Buffer = pushCleanup(Kind, sizeof(T) + T::getExtraSize(N));
return new (Buffer) T(N, A...);
void pushCopyOfCleanup(CleanupKind Kind, const void *Cleanup, size_t Size) {
void *Buffer = pushCleanup(Kind, Size);
std::memcpy(Buffer, Cleanup, Size);
void setCGF(CodeGenFunction *inCGF) { CGF = inCGF; }
/// Pops a cleanup scope off the stack. This is private to CGCleanup.cpp.
void popCleanup();
/// Push a set of catch handlers on the stack. The catch is
/// uninitialized and will need to have the given number of handlers
/// set on it.
class EHCatchScope *pushCatch(unsigned NumHandlers);
/// Pops a catch scope off the stack. This is private to CGException.cpp.
void popCatch();
/// Push an exceptions filter on the stack.
class EHFilterScope *pushFilter(unsigned NumFilters);
/// Pops an exceptions filter off the stack.
void popFilter();
/// Push a terminate handler on the stack.
void pushTerminate();
/// Pops a terminate handler off the stack.
void popTerminate();
// Returns true iff the current scope is either empty or contains only
// lifetime markers, i.e. no real cleanup code
bool containsOnlyLifetimeMarkers(stable_iterator Old) const;
/// Determines whether the exception-scopes stack is empty.
bool empty() const { return StartOfData == EndOfBuffer; }
bool requiresLandingPad() const;
/// Determines whether there are any normal cleanups on the stack.
bool hasNormalCleanups() const {
return InnermostNormalCleanup != stable_end();
/// Returns the innermost normal cleanup on the stack, or
/// stable_end() if there are no normal cleanups.
stable_iterator getInnermostNormalCleanup() const {
return InnermostNormalCleanup;
stable_iterator getInnermostActiveNormalCleanup() const;
stable_iterator getInnermostEHScope() const {
return InnermostEHScope;
/// An unstable reference to a scope-stack depth. Invalidated by
/// pushes but not pops.
class iterator;
/// Returns an iterator pointing to the innermost EH scope.
iterator begin() const;
/// Returns an iterator pointing to the outermost EH scope.
iterator end() const;
/// Create a stable reference to the top of the EH stack. The
/// returned reference is valid until that scope is popped off the
/// stack.
stable_iterator stable_begin() const {
return stable_iterator(EndOfBuffer - StartOfData);
/// Create a stable reference to the bottom of the EH stack.
static stable_iterator stable_end() {
return stable_iterator(0);
/// Translates an iterator into a stable_iterator.
stable_iterator stabilize(iterator it) const;
/// Turn a stable reference to a scope depth into a unstable pointer
/// to the EH stack.
iterator find(stable_iterator save) const;
/// Add a branch fixup to the current cleanup scope.
BranchFixup &addBranchFixup() {
assert(hasNormalCleanups() && "adding fixup in scope without cleanups");
return BranchFixups.back();
unsigned getNumBranchFixups() const { return BranchFixups.size(); }
BranchFixup &getBranchFixup(unsigned I) {
assert(I < getNumBranchFixups());
return BranchFixups[I];
/// Pops lazily-removed fixups from the end of the list. This
/// should only be called by procedures which have just popped a
/// cleanup or resolved one or more fixups.
void popNullFixups();
/// Clears the branch-fixups list. This should only be called by
/// ResolveAllBranchFixups.
void clearFixups() { BranchFixups.clear(); }
} // namespace CodeGen
} // namespace clang