[cfe-dev] [llvm-dev] AST of whole program

[Richard Smith via cfe-dev]

On Tue, Oct 4, 2016 at 6:57 PM, Anil Altinay via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi Vedant,
>
>
> *What kind of transformation are you interested in, and what kind of
> programsare you looking to transform?*
>
> I am trying to change the layout of fields(randomize) in a struct in a
> c program. I already figured out how to change the layout of fields in a
> struct but there are structs that I should not touch like structs defined
> in libraries. So if I randomize a struct in one compilation unit and then
> realize that actually, I shouldn't have randomized it when clang was
> working on another compilation unit, there is no way to go back and revert
> the layout of the struct that I already randomized in previous translation
> unit because it is already over. So what I am thinking is I should look all
> the translation units in AST level before they create llvm IR and decide
> which structs I should randomize, then randomize the structs I have decided
> to randomize, then let clang to create llvm IR using modified ASTs.
>
> I am trying to transform programs like apache.
>

This problem falls within a category of related problems, where you want
cross-source-file analysis in order to determine how to make
per-source-file changes. The way we usually solve those kinds of problems
is with a two-pass system: the first pass processes one translation unit at
a time, and performs a local analysis to determine what to do under what
circumstances. The local analysis results are then merged to determine what
transformations to perform. Then a second pass over all translation units
performs the actual code changes.

In your specific case, perhaps you could form a list for each TU of the
functions defined locally, and the functions to which each
potentially-reorderable type is passed. Then merge to form a complete list
of function definitions controlled by the program, and determine whether a
type is non-reorderable based on whether it's passed to a function that's
not in your list.

In addition to not requiring a whole-program AST, this approach also
parallelizes naturally (see http://www.hyrumwright.org/papers/icsm2013.pdf
for more on how we use this kind of approach at Google).


> *By 'AST of whole program', do you mean AST's for the source from all
> librarieslinked into the program?*
>
> I am not sure if I understand your question but I will try to explain what
> I meant. For each translation unit, AST gets created. The problem is I can
> only see AST of current translation unit. I cannot see AST of next
> translation unit because clang works on one translation unit at a time.
> Maybe I should dump AST of each translation unit to the disk, decide which
> structs can be randomized, change the AST on the disk and start compilation
> from modified ASTs. But this may be so slow and I do not really know how I
> can do this.
>
> Also I am not sure about one thing. Can I make sure that a struct is
> defined in a library or in the source code of the program by looking only
> one translation unit without any false flag? If I can, then there is no
> need for what I am asking for.
>
> I hope that I explained what I am trying to do clearly. If you have any
> suggestion how I should do this, I would really appreciate hearing your
> opinion. Thank you very much for your quick response.
>
> Anil
>
>
>
>
> On Tue, Oct 4, 2016 at 5:58 PM, Vedant Kumar <vsk at apple.com> wrote:
>
>> - llvm-dev, + cfe-dev
>>
>> Hi,
>>
>> What kind of transformation are you interested in, and what kind of
>> programs
>> are you looking to transform?
>>
>> By 'AST of whole program', do you mean AST's for the source from all
>> libraries
>> linked into the program?
>>
>> vedant
>>
>>
>> > On Oct 4, 2016, at 5:50 PM, Anil Altinay via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>> >
>> > Hello,
>> >
>> > I would like to do transformations on AST of a c program but I need to
>> have access to all ASTs created for the program to do right changes. LLVM
>> processes one translation unit at a time and because of it, I do not have
>> access to AST of all the translation units at the same time. Do you have
>> any suggestion how I can access all the ASTs created for a program, do
>> analysis on the ASTs and do modifications on the ASTs?
>> >
>> > As a summary:
>> >
>> >       • I need to have access to ASTs of the program at the same time.
>> >       • Do analysis on ASTs.
>> >       • Modify ASTs based on my analysis and create llvm IR from
>> modified ASTs.
>> > Thank you,
>> > Anil
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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