[llvm-dev] first class types

[whitequark via llvm-dev]

On 2018-05-25 01:38, Alexandre Isoard wrote:
> Ah, that's why Clang is obsessively pushing them into return by
> reference?

Not only that. Even on targets that do correctly translate returning
structs by value, the resulting ABI is not defined (and it will, in
fact, not match the C ABI in practice), and clang usually needs to
follow the C or C++ ABI.

> 
> On Thu, May 24, 2018 at 6:29 PM, whitequark
> <whitequark at whitequark.org> wrote:
> 
>> On 2018-05-25 00:39, Alexandre Isoard via llvm-dev wrote:
>> 
>>> Hello,
>>> 
>>> I see here: https://llvm.org/docs/LangRef.html#ret-instruction [1]
>>> 
>>> That the return instruction must only return values of first class
>>> types, which would exclude struct and arrays. But some llvm
>>> instrinsics do return struct, and it does not seems to be enforced
>>> on
>>> any function.
>>> 
>>> Is that restriction lifted and the documentation not up to date?
>>> Can
>>> we return arrays?
>>> I see the same restriction for select. Can/should we lift it too?
>> 
>> In practice, you can return structs and arrays, and this while you
>> stay
>> within the optimizer, but how targets handle this is very variable.
>> X86 internally lowers it to sret or something very similar to sret
>> whereas lesser used targets (think MSP430) may outright assert.
>> 
>> My understanding is that calling an intrinsic that returns a struct
>> is defined (and never needs a ret instruction), but returning a
>> struct
>> from user code (which does need a ret instruction) is not, which is
>> why LangRef is written like that.
>> 
>> --
>> whitequark
> 
> --
> 
> ALEXANDRE ISOARD
> 
> 
> Links:
> ------
> [1] https://llvm.org/docs/LangRef.html#ret-instruction

-- 
whitequark