[Issue 20752] __traits(isReturnOnStack, func) is incomplete and can't be trusted

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20752

Iain Buclaw <ibuclaw at gdcproject.org> changed:

           What    |Removed                     |Added
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                 CC|                            |ibuclaw at gdcproject.org

--- Comment #1 from Iain Buclaw <ibuclaw at gdcproject.org> ---
(In reply to Mathias LANG from comment #0)
> Since half of the logic for NRVO is in the backend, this traits simply
> cannot be relied on and is mostly useless.

For finding out if a struct is to be returned using (N)RVO, the right test to
use would be __traits(isPOD).

If a struct is non-POD, then it should _always_ be passed around by invisible
reference, no ifs, no buts, no excuses.

However, that's ignoring static arrays, which is a little more hairy to find
out, as they need to be big enough to be considered returning in memory.  But
just what is "big enough" is a very target-specific question.


> For example, the following compiles and triggers the assert with DMD:
>
[--snip--]
> 
> I would expect a compilation error, or the assert to pass.

Worse, some back-ends can't provide any useful information as there's no access
to "target".  This is from the gdc-ddmd branch with included explanation.

---
bool
Target::isReturnOnStack (TypeFunction *, bool)
{
  /* Need the back-end type to determine this, but this is called from the
     frontend before semantic processing is finished.  An accurate value
     is not currently needed anyway.  */
  return true;
}
---

A type could be synthesized, however. But it would be a best guess approach.

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