[LLVMdev] alloc_size metadata

[John Criswell]

On 5/25/12 2:16 AM, Duncan Sands wrote:
> Hi John,
>
>>>> I'm implementing the alloc_size function attribute in clang.
>>> does anyone actually use this attribute? And if they do, can it 
>>> really buy
>>> them anything? How about "implementing" it by ignoring it!
>>
> ...
>>
>> Currently, SAFECode has a pass which just recognizes certain 
>> functions as
>> allocators and knows how to interpret the arguments to find the size. 
>> If we want
>> SAFECode to work with another allocator (like a program's custom 
>> allocator, the
>> Objective-C allocator, the Boehm garbage collector, etc), then that 
>> pass needs
>> to be modified to recognize it. Having to update this pass for every 
>> allocator
>> name and type is one of the few reasons why SAFECode only works with 
>> C/C++ and
>> not just any old language that is compiled down to LLVM IR.
>
>
>> Nuno's proposed feature would allow programmers to communicate the 
>> relevant
>> information about allocators to tools like SAFECode and ASan. I think 
>> it might
>> also make some of the optimizations in LLVM that require knowing about
>> allocators work on non-C/C++ code.
>
> these are good points.  The attribute and proposed implementation feel 
> pretty
> clunky though, which is my main gripe.

Hrm.  I haven't formed an opinion on what the attributes should look 
like.  I think supporting the ones established by GCC would be important 
for compatibility, and on the surface, they look reasonable.  Devising 
better ones for Clang is fine with me.  What about them feels klunky?

>
> Since LLVM already has utility functions for recognizing allocators 
> (i.e. that
> know about malloc, realloc and -fno-builtin etc) can't SAFECode just 
> make use
> of them?

It probably could.  It doesn't simply because SAFECode was written 
before these features existed within LLVM.
:)

> Then either (1) something like alloc_size is implemented, the LLVM
> utility learns about it, and SAFECode benefits automagically, or (2) 
> the LLVM
> utility is taught about other allocators like Ada's, and SAFECode 
> benefits
> automagically.

I'm not sure what you mean by "LLVM utility," but I think we're thinking 
along the same lines.  Clang/LLVM implement the alloc_size attributes, 
we change SAFECode to recognize it, and so when people use it, SAFECode 
benefits automagically.

Am I right that we're thinking the same thing, or did I completely 
misunderstand you?

-- John T.

>
> Ciao, Duncan.