[llvm-dev] Rewriting LLVM IR intrinsic functions

[Dipanjan Das via llvm-dev]

Hi David,

On 14 September 2015 at 16:25, David Chisnall <David.Chisnall at cl.cam.ac.uk>
wrote:

> On 14 Sep 2015, at 06:58, Dipanjan Das via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> >
> > Hi David,
> >
> > I think you people can help me better if I explain what I am trying to
> achieve.
> >       • I have compiled gmp with LLVM 2.9
> >       • Linked the library statically with a stub program (gmp_demo.bc)
> >       • Transformed the bitcode to LLVM IR: llvm-dis gmp_demo.bc -o
> gmp_demo.ll
> >       • Trying to convert the IR to high-level C code: llc -march=c
> gmp_demo.ll
> > Though I know that llc C backend has many issues and has been removed
> from LLVM >=3.1, still I am desperate to make it work. The error I receive
> in the last step is the following: "LLVM ERROR: Code generator does not
> support intrinsic function 'llvm.uadd.with.overflow.i64'!"
> >
> > I was planning to rewrite LLVM intrinsics not supported by llc to
> equivalent LLVM IR supported by llc. Is this approach feasible?
>
> It would still help to understand exactly why you’re trying to do this.
> It sounds as if you’re using LLVM as a source-to-source translator, with C
> as the input and output language.  If this is the case, then the Clang
> rewriter infrastructure might be more useful to you.
>
>
I have started working on automatic program repair where I need to work
with a binary with source code unknown to us. May be using an in-house
tool, I can lift off the binary to LLVM IR. I was planning to transform
LLVM IR to equivalent high-level C code to try source code fixing
approaches on the same.



> The intrinsic that it is currently failing on is (as its name implies) an
> overflow-checked addition.  The reason that the C backed does not
> understand this builtin is simple: There is no way of (directly) expressing
> this in C, without extensions.
>
> You could potentially expand this in IR to a signed addition and a check
> that the sign bit had not changed, but it would be cleaner to simply modify
> the C backend to emit the Clang / GCC __builtin function that corresponds
> to the intrinsic.  You’re likely to find a number of other intrinsics that
> are not supported: the reason that the C backend was removed was that it
> never worked very well.  For example, anything involving exceptions can’t
> be implemented in C, so the C backend lowers it to setjmp / longjmp, which
> gives you a different ABI.
>


-- 

Thanks & Regards,
Dipanjan
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