[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 01:27:32 PST 2015

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.



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
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