[LLVMdev] struct returns

[Talin]

Kenneth Uildriks wrote:
> On Tue, Sep 15, 2009 at 11:55 AM, Dan Gohman <gohman at apple.com> wrote:
>   
>> On Sep 15, 2009, at 8:32 AM, Kenneth Uildriks wrote:
>>
>>     
>>> In the latest snapshot from SVN on X86, llc refuses to compile
>>> functions returning structs larger than two i32 members.
>>>
>>> According to the docs, such limitations can be expected to exist on
>>> other platforms.
>>>
>>> This leads to a number of questions and observations:
>>>
>>> 1. Is there a good way to retrieve the current target limitations on
>>> struct return sizes?
>>>       
>> No.  The information can be inferred from what's in the *CallingConv.td
>> files, though there are currently no utilities specifically for this.
>>
>>     
>>> 2. The sretpromotion pass does not take struct size limitations into
>>> account; it will happily convert an sret parameter with five members
>>> into a return value that llc chokes on.
>>>
>>> 3. There is no sretdemotion pass.
>>>
>>> 4. If the answer to #1 is "no", perhaps we need platform-specific
>>> sretpromotion and sretdemotion passes to allow small struct returns to
>>> happen efficiently while large struct returns can be successfully
>>> codegen'd, all without having to build such platform-specific
>>> knowledge into all front-ends.
>>>       
>> Sure. Alternatively, we could fix codegen to do this itself.  I'd be
>> happy to help anyone interested in working on this.
>>
>> I recently made a major reorganization of the calling-convention
>> lowering code which cleared away one of the major obstacles to
>> doing this within codegen.
>>
>> Dan
>>     
>
> That would be even better.  I suppose codegen would have to do
> something like the sret parameter demotion when the number of members
> exceeds the number of available registers?  Or did you have something
> else in mind?
>
> Fixing this would make my front-end noticeably simpler, and would
> probably benefit other front-ends as well, so I'd be willing to spend
> some time on it.
>
>   
It would definitely help my front end :)

-- Talin