r221933 - Objective-C. Fixes a regression caused by implementation

[Nico Weber]

On Thu, Nov 13, 2014 at 3:37 PM, Argyrios Kyrtzidis <kyrtzidis at apple.com>
wrote:

>
> On Nov 13, 2014, at 2:27 PM, Fariborz Jahanian <fjahanian at apple.com>
> wrote:
>
> /// ObjCMethodList - a linked list of methods with different signatures.
> struct ObjCMethodList {
>   ObjCMethodDecl *Method;
> +  /// \brief count of methods with same signature.
> +  unsigned Count;
>   /// \brief The next list object and 2 bits for extra info.
>   llvm::PointerIntPair<ObjCMethodList *, 2> NextAndExtraBits;
>
> -  ObjCMethodList() : Method(nullptr) { }
> -  ObjCMethodList(ObjCMethodDecl *M, ObjCMethodList *C)
> -    : Method(M), NextAndExtraBits(C, 0) { }
> +  ObjCMethodList() : Method(nullptr), Count(0) { }
> +  ObjCMethodList(ObjCMethodDecl *M, unsigned count, ObjCMethodList *C)
> +    : Method(M), Count(count), NextAndExtraBits(C, 0) { }
>
>   ObjCMethodList *getNext() const { return NextAndExtraBits.getPointer(); }
>   unsigned getBits() const { return NextAndExtraBits.getInt(); }
>
>
> Since we only care about whether there are multiple methods or a single
> method, and don’t care about the exact count, how about use a bit from
> ‘NextAndExtraBits' to indicate if it is a single method or not, and save
> space ?
>
> So something like:
>
>   llvm::PointerIntPair<ObjCMethodList *, 3> NextAndExtraBits;
>   bool isSingle() const { <bit stuff> }
>

I tried to look at this as part of PR21587. From what I understand, it
requires providing a specialization of PointerLikeTypeTraits
for ObjCMethodList that sets NumLowBitsAvailable to 3, and making sure
that ObjCMethodLists are always aligned (at least) to 8 byte boundaries.
ObjCMethodList are either allocated from a bump ptr allocator or by a
DenseMap storing pair<ObjCMethodList, ObjCMethodList> as values and
Selectors as keys. I can pass an explicit alignment to the bump ptr
allocator, but how can I convince the DenseMap to allocate ObjCMethodLists
at 8 byte boundaries? On 32-bit systems, Selector will be 4 bytes and I
think DenseMap stores keys and values in pairs, so I think the first
ObjCMethodList in the DenseMap's value will likely not be on an 8-byte
boundary in practice (since a table buckets is a 4-byte selector plus 2
8-byte ObjCMethodList and there's a bunch of them contiguously in memory).
So there'd have to be 4 bytes of padding after the key.

Is there some good way of doing this?

Thanks,
Nico
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20141225/cd6214d3/attachment.html>