[llvm-dev] C returning struct by value

[James Y Knight via llvm-dev]

On Mon, Mar 28, 2016 at 5:14 PM, Michael Nicolella via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thanks for the explanation. It's good to hear the situation isn't felt to
> be ideal.
>
> The details here are going to be sensitive to the OS + target that I'm
> compiling for, right? So the effort here will be to understand and get
> right the calling convention details for each supported target, yes?
>

Yes, it is target specific. You should check
clang/lib/CodeGen/TargetInfo.cpp for details of how calls are lowered from
C for various targets. For some targets (e.g. x86-64), it's decidedly
non-trivial. You can also run clang -emit-llvm to see what actually gets
emitted for particular functions. If you stay away from passing
structs/unions by value, it becomes a *lot* simpler, though...

Some people have tried to make libraries for doing the ABI lowering
available in a way that's not tied to clang. Here's one: <
https://github.com/scross99/llvm-abi> (I have no idea how well, or if, it
works).

I think I've also seen mention of someone constructing the proper classes
to pass to clang to have it emit the C ABI calls from their non-C language,
although I'm not sure where I saw that.

Is there any current plan to change the way this works, or is it more of a
> dreamy cleanup item that maybe will get addressed some day?
>

I don't know of anybody working on changing the way this works. I'd
personally love to work on cleaning it up, someday...but that's a wish, not
a plan.


Appreciate the tip.
>
> On Mon, Mar 28, 2016 at 9:28 AM, Tim Northover <t.p.northover at gmail.com>
> wrote:
>
>> On 27 March 2016 at 21:48, Michael Nicolella via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > Can someone help me understand why this detail needs to be understood
>> by the frontend,
>>
>> Many of the backends can do automatic demotion to sret, but the
>> front-end still needs to be aware of the issues (particularly around
>> unions, since whether demotion is necessary can depend on more than
>> just the size of the type).
>>
>> I'd also expect marginally better code in some cases by using sret
>> explicitly: the demotion occurs pretty late on and a "%type *sret"
>> parameter better represents what will actually be happening later on.
>>
>> Cheers.
>>
>> Tim.
>>
>
>
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