[LLVMdev] alloc_size metadata

Tue May 29 11:02:45 PDT 2012

On Tue, 29 May 2012 17:37:40 +0100
Nuno Lopes <nunoplopes at sapo.pt> wrote:

> >> On 25/05/12 17:22, John Criswell wrote:
> >> > 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?
> >>
> >> basically it feels like "I only know about C, here's something that
> >> pretends to be general but only handles C".  Consider a language
> >> with a string type that contains the string length as well as the
> >> characters.  It has a library function allocate_string(length).
> >> How much does it allocate?  length+4 bytes. That can't be
> >> represented by alloc_size.  What's more, it may well store the
> >> length at the start, and return a pointer to just after the
> >> length: a pointer to the first character.  alloc_size can't
> >> represent "the allocated memory starts 4 bytes before the return
> >> value" either.  In short, it feels like a hack for handling
> >> something that turns up in some particular C code that someone
> >> has, rather than a general solution to the general problem.
> >
> > I think this is a good point, here's a suggestion:
> >
> > Have the metadata name two functions, both assumed to have the same
> > signature as the tagged function, one which returns the offset of
> > the start of the allocated region and one which returns the length
> > of the allocated region. Alternatively, these functions could take
> > the same signature and additionally the returned pointer of the
> > tagged function, and then one function can return the start of the
> > region and the other the length.
> 
> Ok, so this seems to be the most general proposal, which can
> obviously handle all cases.
> Something like this would work:
> 
> define i8* @foo() {
>    %0 = tail call i32 @get_realloc_size(i8* null, i32 42)
>    %call = tail call i8* @my_recalloc(i8* null, i32 42) nounwind,  
> !alloc_size !{i32 %0}
>    ret i8* %call
> }
> 
> Basically I just added a function call as the metadata (it's not  
> currently possible to add the function itself to the metadata; the  
> function call is required instead).

I had in mind essentially what Duncan wrote in his response (just
adding the function names, and using some rule to construct the
function calls from the allocator calls).

> As long as the function is marked as readnone, I think it shouldn't  
> interfere with the optimizers, and we can have a later pass to drop  
> the metadata and remove the calls.  I still don't like having the  
> explicit calls there, though.  Any suggestions to remove the
> functions calls from there?
> 
> I feel that the offset function is probably not required. I've never  
> seen an allocation function that doesn't return a pointer to the  
> beginning of the allocated buffer. Also, I cannot remember of any  
> function in the C library that has that behavior.

Isn't this what posix_memalign does? Anything that needs to 'round up'
the alignment will return a pointer which may not point to the
beginning of the buffer. As Duncan noted, there are other uses as well.

> 
> We will also need a convenient syntax to export this feature in the  
> languages we support.
> I personally would like to see
> '__attribute__((alloc_size( strlen(x)+1 ))' in C, but the
> implementation seems to be non-trivial.

IMHO, this would be a very useful feature.

> 
> About Duncan's comment about having the memory builtin analysis  
> recognize this intrinsic, well I agree it should (and I'll take care  
> of that), but I'm not sure if we should be very aggressive in  
> optimizing based on this metadata.
> For example, do we really want to remove a call to a custom
> allocator whose return value is unused (like we do for malloc)?  If
> so, we'll also need a metadata node to mark de-allocation functions
> (so that sequences like my_free(my_malloc(xx)) are removed).

You might want to restrict that to custom allocators that have an
additional 'removable' attribute.

Out of curiosity, does any of this potentially effect the presence or
absence of 'inbounds' on GEP instructions?

Thanks again,
Hal

> 
> Any feedback on the issues described is highly appreciated!
> 
> Thanks,
> Nuno
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-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory