[LLVMdev] Best intermediate form to...

[nonpoly]

Wow, thanks for the super fast reply!

I was asking about llvm-gcc because I want to be able to work with code that
compiles on GCC (since there is so much already out there), but in an
intermediate form.  So I take it that clang plans on being able to compile
code that compiles on GCC?  I'm assuming that's what they mean by "GCC
compatibility" on http://clang.llvm.org/ ;p

On a side note, is clang - at least for the most part - hand coded like g++
was for GCC?  Or is it making use of generators like lex/flex and
yacc/bison?

Thanks again!
-nonpoly


Nick Lewycky wrote:
> 
> nonpoly wrote:
>>
>> Hello Everyone!
>>     New to the forums, so hopefully I'm not a nuisance.  I just wanted to
>> know where to go (since I heard there were about 5 different intermediate
>> forms for llvm) to find the highest level intermediate form of llvm.
> 
> I assume you're referring to this comment:
> 
>    http://lwn.net/Articles/383707/
> 
> LLVM has one major IR that we mean when we say "the IR", and that's the 
> one described by the LangRef you linked to.
> 
> As for the having five different ones, it really comes down to what you 
> consider an IR. Many passes will to build up data structures on the side 
> in order to facilitate their optimizations. At what point do those cross 
> the line from being data to being another intermediate representation? I 
> would argue that a good test is whether the intention is whether you 
> lower from one to the next. For example, gcc lowers from GENERIC to 
> GIMPLE and never looks back, Open64 lower VHL to HL to ML to LL to VLL 
> the same way. The SCEV format at least isn't like that, but there is at 
> least one lowering step from the IR to the DAG/MachineInsts that 
> machine-specific CodeGen uses. MCInst may not be an IR on the grounds 
> that it's not intermediate, it's just a direct representation of 
> assembly instructions.
> 
>    I want
>> to be able to get as much information from a front-end as possible. 
>> Mainly
>> I need to find out dependencies and control flows, amongst a few other
>> things.  So far I've seen http://llvm.org/docs/LangRef.html which might
>> be
>> what I'm looking for, but I just wanted to make sure.
> 
> Yes.
> 
>>     Also I heard that there's something that can convert GCCs GIMPLE to
>> llvm
>> IR, which I looked up and I believe is llvm-gcc, but again, I want to
>> make
>> sure that's it.  Also, is there a plugin that converts llvm IR to GIMPLE,
>> or
>> is there a project currently working on it?
> 
> Yes, there's llvm-gcc which is a modified gcc 4.2. It's getting less 
> attention as more work is done on clang. There's also the plugin you 
> mentioned, dragonegg.llvm.org.
> 
> Nick
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> 
> 

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