[LLVMdev] CPP API User-level Question: Returning multiple values

[Dan Gohman]

Hi Tony,

I just checked LLVM 2.3 and ReturnInst has these:

   static ReturnInst* Create(Value * const* retVals, unsigned N,
                             Instruction *InsertBefore)

   static ReturnInst* Create(Value * const* retVals, unsigned N,
                             BasicBlock *InsertAtEnd)

which are what you're looking for.

In LLVM trunk, MRV-syntax LLVM assembly files and bitcode files
are auto-upgraded to first-class aggregates in the respective
readers. However at the C++ API level, clients must be adapted.

Dan

On Sep 12, 2008, at 1:43 PM, Tony Scudiero wrote:

> Dan,
>    Thanks for the info. Unfortunately for the time being we are using
> (for the most part) the 2.3 release (with a couple of patches that  
> Dave
> Greene has applied). The first-class aggregates is one of the things  
> we
> don't yet have in the LLVM we're working with. I'll look again to  
> see if
> there's a ReturnInst::Create( ) which I can pass an array of  
> llvm::Value
> *'s to, but I don't recall having seen one.
>    Unfortunately it's all going to change once we do bring our LLVM up
> to date with changes made to trunk since the LLVM2.3 release, but do  
> we
> need to get it working with the LLVM we have for the time being -  
> which
> means using the 2.3 MRV syntax for the time being.
>    Out of curiosity: does the MRV syntax will still work with
> first-class aggregates?
>    Thanks!
>
>    -Tony Scudiero
>
>
> Dan Gohman wrote:
>> On Sep 12, 2008, at 9:40 AM, Tony Scudiero wrote:
>>
>>
>>> Greetings,
>>>
>>
>> Hi Tony,
>>
>> This is an area that's undergone some changes recently. The LLVM 2.3
>> multiple-return-value (MRV) syntax has been replaced by the
>> first-class aggregates syntax in SVN trunk.
>>
>>
>>>   I'm working on getting our compiler's interface to LLVM to mimic
>>> the
>>> way the LLVM-GCC inserts instructions to generate AMD64 ABI  
>>> compliant
>>> code. I'm trying to create
>>>
>>>    ret i64 %mrv, double %double_mrv37
>>>
>>
>> This is LLVM 2.3 MRV syntax.
>>
>>
>>>   which is basically what LLVM-GCC puts out. However if I use lcc
>>> -march=cpp to get the API code I need it has the following line:
>>>
>>>   ReturnInst::Create(int64_t93, label_return);
>>>
>>>   with no reference to the double. I also can't find anything in the
>>> doxygen docs for a version of ReturnInst::Create( ) that takes two
>>> values for returning, nor could I find anything by generating an
>>> intentionally bad call and letting my gen-compiler list the possible
>>> ReturnInst which it knows about. Do I have to create the ReturnInst
>>> in a
>>> different way to do this? Any guidance for how to do this from  
>>> within
>>> the CPP API would be greatly appreciated. Thanks!!
>>>
>>
>>
>> I don't know the details about the LLVM 2.3 interface offhand.
>> I believe there's a form of ReturnInst::Create which you can
>> pass multiple values, probably an array of Value*.
>>
>> I can tell you about how to do this with the first-class
>> aggregates approach in svn trunk though.
>>
>> With first-class aggregates, it's necessary to build up the
>> aggregate return value one piece at a time. The LLVM syntax looks
>> like this:
>>
>> define { i64, double } @foo(i64 %x, double %y) nounwind {
>>   %a = insertvalue { i64, double } undef, i64 %x, 0
>>   %b = insertvalue { i64, double }    %a, double %y, 1
>>   ret { i64, double } %b
>> }
>>
>> See the LangRef.html for details on the insertvalue instruction.
>> The -march=cpp backend in SVN trunk supports this too.
>>
>> Also in svn trunk, if you're using the IRBuilder interface
>> you can use the CreateAggregateRet method, which takes care of
>> creating the InsertValueInsts for you.
>>
>> Dan
>>
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