[llvm-dev] Function attributes for LibFunc and its impact on GlobalsAA

James Molloy via llvm-dev llvm-dev at lists.llvm.org
Thu Dec 3 02:41:46 PST 2015


I think that might be difficult to detect. If you wanted to force this
behaviour in your own toolchain, you could just use "-mllvm
-force-attribute=malloc:readnone" on the clang command line?


On Thu, 3 Dec 2015 at 10:21 Vaivaswatha Nagaraj <vn at compilertree.com> wrote:

> Hi James,
> Thank you for the response. I understand the concern about malloc/free
> hooks. Could we detect that a program has setup malloc hooks (assuming
> we're in a whole program compilation) and make assumptions (such as
> onlyAccessesArgMem()) when the program hasn't setup malloc hooks? Using a
> command line flag could be one option too.
> I'm currently working on a program where having these attributes could
> help GlobalsAA give significantly more precise results. Considering that
> this info is propagated to the caller, its caller and so on, this may have
> a wider impact than the program I'm currently looking at.
> Thanks,
>   - Vaivaswatha
> On Thu, Dec 3, 2015 at 2:57 PM, James Molloy <james at jamesmolloy.co.uk>
> wrote:
>> Hi Vaivaswatha,
>> I think not adding readnone/readonly to malloc/realloc is correct.
>> malloc/free hooks can be added to most implementations (for leak checking
>> and so on), so calling malloc could in fact call any other arbitrary code
>> that could write to memory.
>> Cheers,
>> James
>> On Wed, 2 Dec 2015 at 14:07 Vaivaswatha Nagaraj via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>> Hi,
>>> GlobalsAA, during propagation of mod-ref behavior in the call graph,
>>> looks at library functions (in GlobalsAAResult::AnalyzeCallGraph:
>>> F->isDeclaration() check), for attributes, and if the function does not
>>> have the onlyReadsMemory attribute set, forgets it.
>>> I noticed that library functions such as malloc/realloc do not have the
>>> attributes doesNotAccessMemory or onlyReadsMemory respectively set
>>> (FunctionAttrs.cpp). This leads to a loss of GlobalsAA information in the
>>> caller (and its caller and so on). Aren't these attributes stricter than
>>> necessary currently? I do not see why the presence of malloc/realloc in a
>>> function needs to invalidate all mod-ref info gathered for that function so
>>> far.
>>> Please let me know if the question is not clear. I'll try to extract out
>>> a simple test case from the program I'm looking at and post it, so as to
>>> have a concrete example.
>>> Thanks,
>>>   - Vaivaswatha
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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