[cfe-dev] Alignment problems in Clang's internal data structures?

[Reid Kleckner]

On Tue, Feb 17, 2015 at 1:42 AM, Renato Golin <renato.golin at linaro.org>
wrote:

> On 17 February 2015 at 00:38, Ahmed Bougacha <ahmed.bougacha at gmail.com>
> wrote:
> >   std::size_t Size = sizeof(DeclRefExpr);
> >   if (...)
> >     Size += sizeof(...);
> >   ...
> >   void *Mem = Context.Allocate(Size, llvm::alignOf<DeclRefExpr>());
>
> This sounds wrong. You should have something like:
>
>    std::size_t Size = sizeof(DeclRefExpr);
>    std::size_t Align = llvm::alignOf<DeclRefExpr>();
>    if (...) {
>      Size += sizeof(...);
>      Align = std::max(Align, llvm::alignOf<...>());
>    }
>    ...
>    void *Mem = Context.Allocate(Size, Align);


This isn't enough either. You need to update the accessors to account for
alignment. They currently look like this:

  NamedDecl *&getInternalFoundDecl() {
    assert(hasFoundDecl());
    if (hasQualifier())
      return *reinterpret_cast<NamedDecl **>(&getInternalQualifierLoc() +
1);
    return *reinterpret_cast<NamedDecl **>(this + 1);
  }

I do *not* want to audit Clang for the 'reinterpret_cast<Foo*>(this + 1)'
pattern and add extra alignment code. We should just make sure that
anything allocated this way requires only pointer-size alignment.

I put this on http://llvm.org/pr22727.
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