[LLVMdev] [global-isel] Simplifying the simplifier

[James Courtier-Dutton]

On 10 August 2013 15:32, Nuno Lopes <nunoplopes at sapo.pt> wrote:
> Hi Jakob,
>
> Thanks for creating this interesting proposal.
> Let me just comment on this part:
>
>
>>> What might be better is to put some abstract interface between
>>> instcombine and the IR, so that instcombine can be run on these pseudo-MIs
>>> as well as on IR.
>>
>>
>> I like the idea of sharing code between IR and MI passes through an
>> abstract interface. I think that later stages in the IR pipeline also need
>> an instruction optimizer instead of a canonicalizer.
>>
>> An alternative approach would be to describe these transformations in a
>> DSL instead of C++.
>
>
>>  *Legalization*
>>  - What does 'more legal' mean? Can we restrict the possible legalization
>> transformations so the iterative process is guaranteed to make progress?
>
>
>
> I've been thinking for a while that we could express the instcombine
> transformations in a DSL, and then generate the C++ code from there.  The
> advantage is that if we formalize the LLVM IR, then we can check the
> correctness of all the instcombine transformations for "free".
> Moreover, instcombine has also suffered from infinite loops in the past
> (because canonicalization did not make progress for some inputs), which is
> also your concern with legalization of MI.  We have algorithms to prove
> termination of rewriting systems, so I believe we could also prove progress
> for both instcombine and MI legalization.
>
> I'm mixing MI legalization and instcombine, since I think that the
> correctness and progress checking technology that we would need is the
> exactly the same.
> I wouldn't mind working on this project.  But the time-frame on my side is
> academic, which may mean 1 or 2 years to be completed.
>

Just a comment:
If you believe you can represent all instcombine transformations in
DSL, you then transform them to C++.
Why not work out a method for checking correctness in the C++?
Then, you would not have to convert everything to DSL, and could run
the checks on the current C++.
The code in C++ should be able to be written in a static analysis
friendly way so that we could use static analysis to prove no infinite
loops.

James