[AST] CastExpr: BasePathSize is not large enough.

[lebedev.ri]

Summary

rC337815 / D49508 had to cannibalize one bit of CastExprBitfields::BasePathSize in order to squeeze PartOfExplicitCast boolean.
That reduced the maximal value of PartOfExplicitCast from 9 bits (~512) down to 8 bits (~256).
Apparently, that mattered. Too bad there weren't any tests.
It caused PR38356.

So we need to increase PartOfExplicitCast back at least to 9 bits, or a bit more.
For obvious reasons, we can't do that in CastExprBitfields - that would blow up the size of every Expr.
So we need to either just add a variable into the CastExpr (as done here),
or use llvm::TrailingObjects. The latter does not seem to be straight-forward.
Perhaps, that needs to be done not for the CastExpr itself, but for all of it's final children.

[erichkeane]

Its not that it needs to be 'at least 9 bits', IMO 9 bits was too small as well.

What I'd prefer to this solution is similar to what John McCall said on PR38356: Add the size to the trailing allocated space. Check out the 'create' methods for all of these, and make the first sizeof(void*) bytes be the size. I'd also suggest using 1 bit in CastExprBits to be a 'is base path empty', so that you don't have to allocate for it at all.

include/clang/AST/Expr.h

2801

The comment there is wrong. It was wrong at 9 bits as well, since it cannot handle our implimits. I'd like to see if you can find where the implimit for direct/indirect bases is stored, and write a static-assert for it here.

test/CodeGenCXX/castexpr-basepathsize-threshold.cpp

2

First, most of this test can be further simplified. Second, I'd like to see a test that actually tests the limit without hitting the recursive template limit.

21

Shouldn't we cover 16k?

[lebedev.ri]

• Store/prepend it into llvm::TrailingObjects<>
• Use unsigned short
• Assert that the storage is at least large enough to be compliant with implimits
• Don't allocate even that unsigned short unless the path is not empty (use one bit in CastExprBits)

test/CodeGenCXX/castexpr-basepathsize-threshold.cpp

2

First, most of this test can be further simplified.

OK? This is what creduce gave me. But yes, i know it has some issues with templates.

Second, I'd like to see a test that actually tests the limit without hitting the recursive template limit.

But that is true already. *this* test does not hit the recursive template limit.

21

No, for me 4096 already crashes elsewhere.

[erichkeane]

include/clang/AST/Expr.h

2791

I'll let @rjmccall comment here, but I think I'd be willing to give up 2 more bytes here and just go with unsigned int. It has the advantage of likely never being an issue again (since 4 billion bases is not an issue).

We might be paying for those 2 ANYWAY with alignment as well, right?

2800

Looking back up, I doubt the need for this either, since we have path_empty and path_size below.

2805

Why is this a reference? This seems odd... It seems that the const-cast magic above shouldn't be necessary either.

2814

This is a super weird way to do this. I really am not a fan of giving a reference to that spot instead of just setting it here.

2854

Is this necessary? Shouldn't path_empty just do this?

test/CodeGenCXX/castexpr-basepathsize-threshold.cpp

2

Looking again, it is probably reduced sufficiently. The cast and the recurse-depth is the only things that are important, so I guess I'm OK with this.

[erichkeane]

include/clang/AST/Expr.h

2805

Looking more into this, I think the reference here is an 'equal of two evils'. I can't catch myself really preferring one way over the other.

2815

This assert shouldn't be necessary. I think setBasePathSize could just take BasePathSizeTy.

[lebedev.ri]

Address some of @erichkeane review notes.

include/clang/AST/Expr.h

2791

Yes, i do expect that we are paying for 2 bytes in padding already.

2805

I'll make it a pointer and replace BasePathSizeTy BasePathSize() const with path_size().
I don't really think it would be good to get rid of this pointer/reference, since then
we'd have more than one function doing what CastExpr::BasePathSizeTy *CastExpr::BasePathSize() does now.

[rjmccall]

LGTM.

include/clang/AST/Expr.h

2791

Yeah, we could really use a size_t here without loss, but an unsigned is fine.

[lebedev.ri]

Drop pointless cast of zero.

[lebedev.ri]

In D50050#1183341, @rjmccall wrote:

LGTM.

Thank you for the review.
I'll land in ~+9 hours or so, since i did not hear from @erichkeane back yet..

[george.karpenkov]

@lebedev.ri @erichkeane The test fails for me on macOS whenever asan and ubsan are both enabled.
The failure is stack overflow at stack frame 943 (? maybe asan usage enforces lower stack size?)

[lebedev.ri]

In D50050#1287780, @george.karpenkov wrote:

@lebedev.ri @erichkeane The test fails for me on macOS whenever asan and ubsan are both enabled.
The failure is stack overflow at stack frame 943

(? maybe asan usage enforces lower stack size?)

Or higher stack usage.
But yes, sounds like it.

[george.karpenkov]

@lebedev.ri yeah ASAN is making stack frame size larger.

It seems @vsapsai is working on disabling this test under ASAN.

[vsapsai]

Disabling test in D54132.

include/clang/AST/Expr.h

public:
   }
 };
 
 /// CastExpr - Base class for type casts, including both implicit
 /// casts (ImplicitCastExpr) and explicit casts that have some
 /// representation in the source code (ExplicitCastExpr's derived
 /// classes).
 class CastExpr : public Expr {
+public:
+  using BasePathSizeTy = unsigned short;
+  static_assert(std::numeric_limits<BasePathSizeTy>::max() >= 16384,
+                "[implimits] Direct and indirect base classes [16384].");
+
 private:
   Stmt *Op;
 
   bool CastConsistency() const;
 
+  BasePathSizeTy BasePathSize() const {
+    if (BasePathIsEmpty())
+      return CastExpr::BasePathSizeTy(0);
+    return const_cast<CastExpr *>(this)->BasePathSize();
+  }
+  BasePathSizeTy &BasePathSize();
+
   const CXXBaseSpecifier * const *path_buffer() const {
     return const_cast<CastExpr*>(this)->path_buffer();
   }
   CXXBaseSpecifier **path_buffer();
 
   void setBasePathSize(unsigned basePathSize) {
-    CastExprBits.BasePathSize = basePathSize;
-    assert(CastExprBits.BasePathSize == basePathSize &&
-           "basePathSize doesn't fit in bits of CastExprBits.BasePathSize!");
+    assert(!BasePathIsEmpty() && basePathSize != 0);
+    BasePathSize() = basePathSize;
+    assert(BasePathSize() == basePathSize &&
+           "basePathSize doesn't fit in bits of BasePathSize!");
   }
 
 protected:
   CastExpr(StmtClass SC, QualType ty, ExprValueKind VK, const CastKind kind,
            Expr *op, unsigned BasePathSize)
       : Expr(SC, ty, VK, OK_Ordinary,
              // Cast expressions are type-dependent if the type is
              // dependent (C++ [temp.dep.expr]p3).
              ty->isDependentType(),
              // Cast expressions are value-dependent if the type is
              // dependent or if the subexpression is value-dependent.
              ty->isDependentType() || (op && op->isValueDependent()),
              (ty->isInstantiationDependentType() ||
               (op && op->isInstantiationDependent())),
              // An implicit cast expression doesn't (lexically) contain an
              // unexpanded pack, even if its target type does.
              ((SC != ImplicitCastExprClass &&
                ty->containsUnexpandedParameterPack()) ||
               (op && op->containsUnexpandedParameterPack()))),
         Op(op) {
     CastExprBits.Kind = kind;
     CastExprBits.PartOfExplicitCast = false;
+    CastExprBits.BasePathIsEmpty = BasePathSize == 0;
+    if (!BasePathIsEmpty())
       setBasePathSize(BasePathSize);
     assert(CastConsistency());
   }
 
   /// Construct an empty cast.
   CastExpr(StmtClass SC, EmptyShell Empty, unsigned BasePathSize)
     : Expr(SC, Empty) {
     CastExprBits.PartOfExplicitCast = false;
+    CastExprBits.BasePathIsEmpty = BasePathSize == 0;
+    if (!BasePathIsEmpty())
       setBasePathSize(BasePathSize);
   }
 
+  bool BasePathIsEmpty() const { return CastExprBits.BasePathIsEmpty; }
+
 public:
   CastKind getCastKind() const { return (CastKind) CastExprBits.Kind; }
   void setCastKind(CastKind K) { CastExprBits.Kind = K; }
 
   static const char *getCastKindName(CastKind CK);
   const char *getCastKindName() const { return getCastKindName(getCastKind()); }
 
   Expr *getSubExpr() { return cast<Expr>(Op); }
public:
   }
 
   /// If this cast applies a user-defined conversion, retrieve the conversion
   /// function that it invokes.
   NamedDecl *getConversionFunction() const;
 
   typedef CXXBaseSpecifier **path_iterator;
   typedef const CXXBaseSpecifier * const *path_const_iterator;
-  bool path_empty() const { return CastExprBits.BasePathSize == 0; }
-  unsigned path_size() const { return CastExprBits.BasePathSize; }
+  bool path_empty() const { return BasePathIsEmpty(); }
+  unsigned path_size() const { return BasePathSize(); }
   path_iterator path_begin() { return path_buffer(); }
   path_iterator path_end() { return path_buffer() + path_size(); }
   path_const_iterator path_begin() const { return path_buffer(); }
   path_const_iterator path_end() const { return path_buffer() + path_size(); }
 
   const FieldDecl *getTargetUnionField() const {
     assert(getCastKind() == CK_ToUnion);
     return getTargetFieldForToUnionCast(getType(), getSubExpr()->getType());

 ///   Base& b = d; // initializer is an ImplicitCastExpr
 ///                // to an lvalue of type Base
 ///   Base&& r = ref(); // initializer is an ImplicitCastExpr
 ///                     // to an xvalue of type Base
 /// }
 /// @endcode
 class ImplicitCastExpr final
     : public CastExpr,
-      private llvm::TrailingObjects<ImplicitCastExpr, CXXBaseSpecifier *> {
+      private llvm::TrailingObjects<ImplicitCastExpr, CastExpr::BasePathSizeTy,
+                                    CXXBaseSpecifier *> {
+  size_t numTrailingObjects(OverloadToken<CastExpr::BasePathSizeTy>) const {
+    return BasePathIsEmpty() ? 0 : 1;
+  }
+
 private:
   ImplicitCastExpr(QualType ty, CastKind kind, Expr *op,
                    unsigned BasePathLength, ExprValueKind VK)
     : CastExpr(ImplicitCastExprClass, ty, VK, kind, op, BasePathLength) {
   }
 
   /// Construct an empty implicit cast.
   explicit ImplicitCastExpr(EmptyShell Shell, unsigned PathSize)
public:
   }
 };
 
 /// CStyleCastExpr - An explicit cast in C (C99 6.5.4) or a C-style
 /// cast in C++ (C++ [expr.cast]), which uses the syntax
 /// (Type)expr. For example: @c (int)f.
 class CStyleCastExpr final
     : public ExplicitCastExpr,
-      private llvm::TrailingObjects<CStyleCastExpr, CXXBaseSpecifier *> {
+      private llvm::TrailingObjects<CStyleCastExpr, CastExpr::BasePathSizeTy,
+                                    CXXBaseSpecifier *> {
   SourceLocation LPLoc; // the location of the left paren
   SourceLocation RPLoc; // the location of the right paren
 
   CStyleCastExpr(QualType exprTy, ExprValueKind vk, CastKind kind, Expr *op,
                  unsigned PathSize, TypeSourceInfo *writtenTy,
                  SourceLocation l, SourceLocation r)
     : ExplicitCastExpr(CStyleCastExprClass, exprTy, vk, kind, op, PathSize,
                        writtenTy), LPLoc(l), RPLoc(r) {}
 
   /// Construct an empty C-style explicit cast.
   explicit CStyleCastExpr(EmptyShell Shell, unsigned PathSize)
     : ExplicitCastExpr(CStyleCastExprClass, Shell, PathSize) { }
 
+  size_t numTrailingObjects(OverloadToken<CastExpr::BasePathSizeTy>) const {
+    return BasePathIsEmpty() ? 0 : 1;
+  }
+
 public:
   static CStyleCastExpr *Create(const ASTContext &Context, QualType T,
                                 ExprValueKind VK, CastKind K,
                                 Expr *Op, const CXXCastPath *BasePath,
                                 TypeSourceInfo *WrittenTy, SourceLocation L,
                                 SourceLocation R);
 
   static CStyleCastExpr *CreateEmpty(const ASTContext &Context,

include/clang/AST/ExprCXX.h

 };
 
 /// A C++ \c static_cast expression (C++ [expr.static.cast]).
 ///
 /// This expression node represents a C++ static cast, e.g.,
 /// \c static_cast<int>(1.0).
 class CXXStaticCastExpr final
     : public CXXNamedCastExpr,
-      private llvm::TrailingObjects<CXXStaticCastExpr, CXXBaseSpecifier *> {
+      private llvm::TrailingObjects<CXXStaticCastExpr, CastExpr::BasePathSizeTy,
+                                    CXXBaseSpecifier *> {
   CXXStaticCastExpr(QualType ty, ExprValueKind vk, CastKind kind, Expr *op,
                     unsigned pathSize, TypeSourceInfo *writtenTy,
                     SourceLocation l, SourceLocation RParenLoc,
                     SourceRange AngleBrackets)
       : CXXNamedCastExpr(CXXStaticCastExprClass, ty, vk, kind, op, pathSize,
                          writtenTy, l, RParenLoc, AngleBrackets) {}
 
   explicit CXXStaticCastExpr(EmptyShell Empty, unsigned PathSize)
       : CXXNamedCastExpr(CXXStaticCastExprClass, Empty, PathSize) {}
 
+  size_t numTrailingObjects(OverloadToken<CastExpr::BasePathSizeTy>) const {
+    return BasePathIsEmpty() ? 0 : 1;
+  }
+
 public:
   friend class CastExpr;
   friend TrailingObjects;
 
   static CXXStaticCastExpr *Create(const ASTContext &Context, QualType T,
                                    ExprValueKind VK, CastKind K, Expr *Op,
                                    const CXXCastPath *Path,
                                    TypeSourceInfo *Written, SourceLocation L,

 
 /// A C++ @c dynamic_cast expression (C++ [expr.dynamic.cast]).
 ///
 /// This expression node represents a dynamic cast, e.g.,
 /// \c dynamic_cast<Derived*>(BasePtr). Such a cast may perform a run-time
 /// check to determine how to perform the type conversion.
 class CXXDynamicCastExpr final
     : public CXXNamedCastExpr,
-      private llvm::TrailingObjects<CXXDynamicCastExpr, CXXBaseSpecifier *> {
+      private llvm::TrailingObjects<
+          CXXDynamicCastExpr, CastExpr::BasePathSizeTy, CXXBaseSpecifier *> {
   CXXDynamicCastExpr(QualType ty, ExprValueKind VK, CastKind kind,
                      Expr *op, unsigned pathSize, TypeSourceInfo *writtenTy,
                      SourceLocation l, SourceLocation RParenLoc,
                      SourceRange AngleBrackets)
       : CXXNamedCastExpr(CXXDynamicCastExprClass, ty, VK, kind, op, pathSize,
                          writtenTy, l, RParenLoc, AngleBrackets) {}
 
   explicit CXXDynamicCastExpr(EmptyShell Empty, unsigned pathSize)
       : CXXNamedCastExpr(CXXDynamicCastExprClass, Empty, pathSize) {}
 
+  size_t numTrailingObjects(OverloadToken<CastExpr::BasePathSizeTy>) const {
+    return BasePathIsEmpty() ? 0 : 1;
+  }
+
 public:
   friend class CastExpr;
   friend TrailingObjects;
 
   static CXXDynamicCastExpr *Create(const ASTContext &Context, QualType T,
                                     ExprValueKind VK, CastKind Kind, Expr *Op,
                                     const CXXCastPath *Path,
                                     TypeSourceInfo *Written, SourceLocation L,

 /// @c reinterpret_cast<int>(VoidPtr).
 ///
 /// A reinterpret_cast provides a differently-typed view of a value but
 /// (in Clang, as in most C++ implementations) performs no actual work at
 /// run time.
 class CXXReinterpretCastExpr final
     : public CXXNamedCastExpr,
       private llvm::TrailingObjects<CXXReinterpretCastExpr,
+                                    CastExpr::BasePathSizeTy,
                                     CXXBaseSpecifier *> {
   CXXReinterpretCastExpr(QualType ty, ExprValueKind vk, CastKind kind,
                          Expr *op, unsigned pathSize,
                          TypeSourceInfo *writtenTy, SourceLocation l,
                          SourceLocation RParenLoc,
                          SourceRange AngleBrackets)
       : CXXNamedCastExpr(CXXReinterpretCastExprClass, ty, vk, kind, op,
                          pathSize, writtenTy, l, RParenLoc, AngleBrackets) {}
 
   CXXReinterpretCastExpr(EmptyShell Empty, unsigned pathSize)
       : CXXNamedCastExpr(CXXReinterpretCastExprClass, Empty, pathSize) {}
 
+  size_t numTrailingObjects(OverloadToken<CastExpr::BasePathSizeTy>) const {
+    return BasePathIsEmpty() ? 0 : 1;
+  }
+
 public:
   friend class CastExpr;
   friend TrailingObjects;
 
   static CXXReinterpretCastExpr *Create(const ASTContext &Context, QualType T,
                                         ExprValueKind VK, CastKind Kind,
                                         Expr *Op, const CXXCastPath *Path,
                                  TypeSourceInfo *WrittenTy, SourceLocation L,

 ///
 /// This expression node represents a const cast, e.g.,
 /// \c const_cast<char*>(PtrToConstChar).
 ///
 /// A const_cast can remove type qualifiers but does not change the underlying
 /// value.
 class CXXConstCastExpr final
     : public CXXNamedCastExpr,
-      private llvm::TrailingObjects<CXXConstCastExpr, CXXBaseSpecifier *> {
+      private llvm::TrailingObjects<CXXConstCastExpr, CastExpr::BasePathSizeTy,
+                                    CXXBaseSpecifier *> {
   CXXConstCastExpr(QualType ty, ExprValueKind VK, Expr *op,
                    TypeSourceInfo *writtenTy, SourceLocation l,
                    SourceLocation RParenLoc, SourceRange AngleBrackets)
       : CXXNamedCastExpr(CXXConstCastExprClass, ty, VK, CK_NoOp, op,
                          0, writtenTy, l, RParenLoc, AngleBrackets) {}
 
   explicit CXXConstCastExpr(EmptyShell Empty)
       : CXXNamedCastExpr(CXXConstCastExprClass, Empty, 0) {}
 
+  size_t numTrailingObjects(OverloadToken<CastExpr::BasePathSizeTy>) const {
+    return BasePathIsEmpty() ? 0 : 1;
+  }
+
 public:
   friend class CastExpr;
   friend TrailingObjects;
 
   static CXXConstCastExpr *Create(const ASTContext &Context, QualType T,
                                   ExprValueKind VK, Expr *Op,
                                   TypeSourceInfo *WrittenTy, SourceLocation L,
                                   SourceLocation RParenLoc,

 /// notation (C++ [expr.type.conv]).
 ///
 /// Example:
 /// \code
 ///   x = int(0.5);
 /// \endcode
 class CXXFunctionalCastExpr final
     : public ExplicitCastExpr,
-      private llvm::TrailingObjects<CXXFunctionalCastExpr, CXXBaseSpecifier *> {
+      private llvm::TrailingObjects<
+          CXXFunctionalCastExpr, CastExpr::BasePathSizeTy, CXXBaseSpecifier *> {
   SourceLocation LParenLoc;
   SourceLocation RParenLoc;
 
   CXXFunctionalCastExpr(QualType ty, ExprValueKind VK,
                         TypeSourceInfo *writtenTy,
                         CastKind kind, Expr *castExpr, unsigned pathSize,
                         SourceLocation lParenLoc, SourceLocation rParenLoc)
       : ExplicitCastExpr(CXXFunctionalCastExprClass, ty, VK, kind,
                          castExpr, pathSize, writtenTy),
         LParenLoc(lParenLoc), RParenLoc(rParenLoc) {}
 
   explicit CXXFunctionalCastExpr(EmptyShell Shell, unsigned PathSize)
       : ExplicitCastExpr(CXXFunctionalCastExprClass, Shell, PathSize) {}
 
+  size_t numTrailingObjects(OverloadToken<CastExpr::BasePathSizeTy>) const {
+    return BasePathIsEmpty() ? 0 : 1;
+  }
+
 public:
   friend class CastExpr;
   friend TrailingObjects;
 
   static CXXFunctionalCastExpr *Create(const ASTContext &Context, QualType T,
                                        ExprValueKind VK,
                                        TypeSourceInfo *Written,
                                        CastKind Kind, Expr *Op,

include/clang/AST/ExprObjC.h

 /// An Objective-C "bridged" cast expression, which casts between
 /// Objective-C pointers and C pointers, transferring ownership in the process.
 ///
 /// \code
 /// NSString *str = (__bridge_transfer NSString *)CFCreateString();
 /// \endcode
 class ObjCBridgedCastExpr final
     : public ExplicitCastExpr,
-      private llvm::TrailingObjects<ObjCBridgedCastExpr, CXXBaseSpecifier *> {
+      private llvm::TrailingObjects<
+          ObjCBridgedCastExpr, CastExpr::BasePathSizeTy, CXXBaseSpecifier *> {
   friend class ASTStmtReader;
   friend class ASTStmtWriter;
   friend class CastExpr;
   friend TrailingObjects;
 
   SourceLocation LParenLoc;
   SourceLocation BridgeKeywordLoc;
   unsigned Kind : 2;
 
+  size_t numTrailingObjects(OverloadToken<CastExpr::BasePathSizeTy>) const {
+    return BasePathIsEmpty() ? 0 : 1;
+  }
+
 public:
   ObjCBridgedCastExpr(SourceLocation LParenLoc, ObjCBridgeCastKind Kind,
                       CastKind CK, SourceLocation BridgeKeywordLoc,
                       TypeSourceInfo *TSInfo, Expr *Operand)
       : ExplicitCastExpr(ObjCBridgedCastExprClass, TSInfo->getType(), VK_RValue,
                          CK, Operand, 0, TSInfo),
         LParenLoc(LParenLoc), BridgeKeywordLoc(BridgeKeywordLoc), Kind(Kind) {}
 

include/clang/AST/Stmt.h

protected:
   class CastExprBitfields {
     friend class CastExpr;
     friend class ImplicitCastExpr;
 
     unsigned : NumExprBits;
 
     unsigned Kind : 6;
     unsigned PartOfExplicitCast : 1; // Only set for ImplicitCastExpr.
-    unsigned BasePathSize : 32 - 6 - 1 - NumExprBits;
+    unsigned BasePathIsEmpty : 1;
   };
 
   class CallExprBitfields {
     friend class CallExpr;
 
     unsigned : NumExprBits;
 
     unsigned NumPreArgs : 1;

lib/AST/Expr.cpp

if (E->getCastKind() == CK_UserDefinedConversion) {
       if (auto *MCE = dyn_cast<CXXMemberCallExpr>(SubExpr))
         return MCE->getMethodDecl();
     }
   }
 
   return nullptr;
 }
 
+CastExpr::BasePathSizeTy &CastExpr::BasePathSize() {
+  assert(!BasePathIsEmpty());
+  switch (getStmtClass()) {
+#define ABSTRACT_STMT(x)
+#define CASTEXPR(Type, Base)                                                   \
+  case Stmt::Type##Class:                                                      \
+    return *static_cast<Type *>(this)                                          \
+                ->getTrailingObjects<CastExpr::BasePathSizeTy>();
+#define STMT(Type, Base)
+#include "clang/AST/StmtNodes.inc"
+  default:
+    llvm_unreachable("non-cast expressions not possible here");
+  }
+}
+
 CXXBaseSpecifier **CastExpr::path_buffer() {
   switch (getStmtClass()) {
 #define ABSTRACT_STMT(x)
 #define CASTEXPR(Type, Base)                                                   \
   case Stmt::Type##Class:                                                      \
     return static_cast<Type *>(this)->getTrailingObjects<CXXBaseSpecifier *>();
 #define STMT(Type, Base)
 #include "clang/AST/StmtNodes.inc"
const FieldDecl *CastExpr::getTargetFieldForToUnionCast(const RecordDecl *RD,
   return nullptr;
 }
 
 ImplicitCastExpr *ImplicitCastExpr::Create(const ASTContext &C, QualType T,
                                            CastKind Kind, Expr *Operand,
                                            const CXXCastPath *BasePath,
                                            ExprValueKind VK) {
   unsigned PathSize = (BasePath ? BasePath->size() : 0);
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   ImplicitCastExpr *E =
     new (Buffer) ImplicitCastExpr(T, Kind, Operand, PathSize, VK);
   if (PathSize)
     std::uninitialized_copy_n(BasePath->data(), BasePath->size(),
                               E->getTrailingObjects<CXXBaseSpecifier *>());
   return E;
 }
 
 ImplicitCastExpr *ImplicitCastExpr::CreateEmpty(const ASTContext &C,
                                                 unsigned PathSize) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   return new (Buffer) ImplicitCastExpr(EmptyShell(), PathSize);
 }
 
 
 CStyleCastExpr *CStyleCastExpr::Create(const ASTContext &C, QualType T,
                                        ExprValueKind VK, CastKind K, Expr *Op,
                                        const CXXCastPath *BasePath,
                                        TypeSourceInfo *WrittenTy,
                                        SourceLocation L, SourceLocation R) {
   unsigned PathSize = (BasePath ? BasePath->size() : 0);
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   CStyleCastExpr *E =
     new (Buffer) CStyleCastExpr(T, VK, K, Op, PathSize, WrittenTy, L, R);
   if (PathSize)
     std::uninitialized_copy_n(BasePath->data(), BasePath->size(),
                               E->getTrailingObjects<CXXBaseSpecifier *>());
   return E;
 }
 
 CStyleCastExpr *CStyleCastExpr::CreateEmpty(const ASTContext &C,
                                             unsigned PathSize) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   return new (Buffer) CStyleCastExpr(EmptyShell(), PathSize);
 }
 
 /// getOpcodeStr - Turn an Opcode enum value into the punctuation char it
 /// corresponds to, e.g. "<<=".
 StringRef BinaryOperator::getOpcodeStr(Opcode Op) {
   switch (Op) {
 #define BINARY_OPERATION(Name, Spelling) case BO_##Name: return Spelling;

lib/AST/ExprCXX.cpp

CXXStaticCastExpr *CXXStaticCastExpr::Create(const ASTContext &C, QualType T,
                                              ExprValueKind VK,
                                              CastKind K, Expr *Op,
                                              const CXXCastPath *BasePath,
                                              TypeSourceInfo *WrittenTy,
                                              SourceLocation L,
                                              SourceLocation RParenLoc,
                                              SourceRange AngleBrackets) {
   unsigned PathSize = (BasePath ? BasePath->size() : 0);
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   auto *E =
       new (Buffer) CXXStaticCastExpr(T, VK, K, Op, PathSize, WrittenTy, L,
                                      RParenLoc, AngleBrackets);
   if (PathSize)
     std::uninitialized_copy_n(BasePath->data(), BasePath->size(),
                               E->getTrailingObjects<CXXBaseSpecifier *>());
   return E;
 }
 
 CXXStaticCastExpr *CXXStaticCastExpr::CreateEmpty(const ASTContext &C,
                                                   unsigned PathSize) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   return new (Buffer) CXXStaticCastExpr(EmptyShell(), PathSize);
 }
 
 CXXDynamicCastExpr *CXXDynamicCastExpr::Create(const ASTContext &C, QualType T,
                                                ExprValueKind VK,
                                                CastKind K, Expr *Op,
                                                const CXXCastPath *BasePath,
                                                TypeSourceInfo *WrittenTy,
                                                SourceLocation L,
                                                SourceLocation RParenLoc,
                                                SourceRange AngleBrackets) {
   unsigned PathSize = (BasePath ? BasePath->size() : 0);
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   auto *E =
       new (Buffer) CXXDynamicCastExpr(T, VK, K, Op, PathSize, WrittenTy, L,
                                       RParenLoc, AngleBrackets);
   if (PathSize)
     std::uninitialized_copy_n(BasePath->data(), BasePath->size(),
                               E->getTrailingObjects<CXXBaseSpecifier *>());
   return E;
 }
 
 CXXDynamicCastExpr *CXXDynamicCastExpr::CreateEmpty(const ASTContext &C,
                                                     unsigned PathSize) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   return new (Buffer) CXXDynamicCastExpr(EmptyShell(), PathSize);
 }
 
 /// isAlwaysNull - Return whether the result of the dynamic_cast is proven
 /// to always be null. For example:
 ///
 /// struct A { };
 /// struct B final : A { };

 CXXReinterpretCastExpr *
 CXXReinterpretCastExpr::Create(const ASTContext &C, QualType T,
                                ExprValueKind VK, CastKind K, Expr *Op,
                                const CXXCastPath *BasePath,
                                TypeSourceInfo *WrittenTy, SourceLocation L,
                                SourceLocation RParenLoc,
                                SourceRange AngleBrackets) {
   unsigned PathSize = (BasePath ? BasePath->size() : 0);
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   auto *E =
       new (Buffer) CXXReinterpretCastExpr(T, VK, K, Op, PathSize, WrittenTy, L,
                                           RParenLoc, AngleBrackets);
   if (PathSize)
     std::uninitialized_copy_n(BasePath->data(), BasePath->size(),
                               E->getTrailingObjects<CXXBaseSpecifier *>());
   return E;
 }
 
 CXXReinterpretCastExpr *
 CXXReinterpretCastExpr::CreateEmpty(const ASTContext &C, unsigned PathSize) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   return new (Buffer) CXXReinterpretCastExpr(EmptyShell(), PathSize);
 }
 
 CXXConstCastExpr *CXXConstCastExpr::Create(const ASTContext &C, QualType T,
                                            ExprValueKind VK, Expr *Op,
                                            TypeSourceInfo *WrittenTy,
                                            SourceLocation L,
                                            SourceLocation RParenLoc,
                                            SourceRange AngleBrackets) {
   return new (C) CXXConstCastExpr(T, VK, Op, WrittenTy, L, RParenLoc, AngleBrackets);
 }
 
 CXXConstCastExpr *CXXConstCastExpr::CreateEmpty(const ASTContext &C) {
   return new (C) CXXConstCastExpr(EmptyShell());
 }
 
 CXXFunctionalCastExpr *
 CXXFunctionalCastExpr::Create(const ASTContext &C, QualType T, ExprValueKind VK,
                               TypeSourceInfo *Written, CastKind K, Expr *Op,
                               const CXXCastPath *BasePath,
                               SourceLocation L, SourceLocation R) {
   unsigned PathSize = (BasePath ? BasePath->size() : 0);
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   auto *E =
       new (Buffer) CXXFunctionalCastExpr(T, VK, Written, K, Op, PathSize, L, R);
   if (PathSize)
     std::uninitialized_copy_n(BasePath->data(), BasePath->size(),
                               E->getTrailingObjects<CXXBaseSpecifier *>());
   return E;
 }
 
 CXXFunctionalCastExpr *
 CXXFunctionalCastExpr::CreateEmpty(const ASTContext &C, unsigned PathSize) {
-  void *Buffer = C.Allocate(totalSizeToAlloc<CXXBaseSpecifier *>(PathSize));
+  void *Buffer =
+      C.Allocate(totalSizeToAlloc<CastExpr::BasePathSizeTy, CXXBaseSpecifier *>(
+          PathSize ? 1 : 0, PathSize));
   return new (Buffer) CXXFunctionalCastExpr(EmptyShell(), PathSize);
 }
 
 SourceLocation CXXFunctionalCastExpr::getLocStart() const {
   return getTypeInfoAsWritten()->getTypeLoc().getLocStart();
 }
 
 SourceLocation CXXFunctionalCastExpr::getLocEnd() const {

test/CodeGenCXX/castexpr-basepathsize-threshold.cpp

+// RUN: %clang_cc1 %s -emit-llvm -o -
+
+// https://bugs.llvm.org/show_bug.cgi?id=38356
+// We only check that we do not crash.
+
+template <typename a, a b(unsigned), int c, unsigned...>
+struct d : d<a, b, c - 1> {};
+template <typename a, a b(unsigned), unsigned... e>
+struct d<a, b, 0, e...> {
+  a f[0];
+};
+struct g {
+  static g h(unsigned);
+};
+struct i {
+  void j() const;
+  // Current maximum depth of recursive template instantiation is 1024,
+  // thus, this \/ threshold value is used here. BasePathSize in CastExpr might
+  // not fit it, so we are testing that we do fit it.
+  // If -ftemplate-depth= is provided, larger values (4096 and up) cause crashes
+  // elsewhere.
+  d<g, g::h, (1U << 10U) - 2U> f;
+};
+void i::j() const {
+  const void *k{f.f};
+  (void)k;
+}