[LLVMdev] alloc_size metadata

[Duncan Sands]

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.

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?  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.

Ciao, Duncan.