[llvm-dev] [EXTERNAL] Re: preferred way to return expected values

[George Rimar via llvm-dev]

FWIW, I've performed an experiment with the code below at godbolt.

(used -O2, https://godbolt.org/z/nY95nh)

```
#include <vector>
#include "llvm/Support/Error.h"

llvm::Expected<std::vector<int>> foo() {
 std::vector<int> V;
 V.push_back(0);
 return V;
}
```

If I understand the produced output correctly, then results are:

gcc 7.5: creates a copy.
gcc 8.1: uses move.

clang < 6.0: doesn't compile.
clang >= 6.0: uses move.

MSVS: was unable to compile, complains about "llvm/Support/Error.h" header.
I am using MSVS 2017 locally and it calls move constructor of Expected<> though,
so I think all MSVS >= 2017 (at least) should be fine.?



Best regards,
George | Developer | Access Softek, Inc
________________________________
От: Alexander Shaposhnikov <alexander.v.shaposhnikov at gmail.com>
Отправлено: 28 сентября 2020 г. 22:46
Кому: David Blaikie
Копия: Richard Smith; llvm-dev; Lang Hames; George Rimar; James Henderson; avl.lapshin at gmail.com
Тема: [EXTERNAL] Re: [llvm-dev] preferred way to return expected values

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Many thanks for the reply,
right, this is what the discussion is about.


On Mon, Sep 28, 2020 at 10:57 AM David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>> wrote:
To clarify, this is a discussion around whether given some move-only type X, implicitly convertible to Y and the code "Y func() { X x; return x; }" is that valid in LLVM? (and, as a corollary, if the type isn't move-only, is that code efficient (does it move rather than copying) - as in the vector example given)

I /believe/ the answer is that it is not valid. I think the set of compilers supported includes those that do not implement this rule. (either due to the language version we compile with, or due to it being a DR that some supported compiler versions do not implement). But that's just my rough guess.

On Sat, Sep 26, 2020 at 3:17 PM Alexander Shaposhnikov via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hello everyone!
It looks like in the LLVM codebase (including subprojects) there are some inconsistencies
in how values are returned from functions with the following (or similar) signature:
    Expected<std::vector<int>> createVector() {
        std::vector<int> V;
        ...
    }
It would be interesting to find out your opinion on this.
After some investigation I have found https://reviews.llvm.org/D70963 and https://reviews.llvm.org/D43322 which have some additional context regarding
the problem. The aforementioned diffs (and the comments on them) contain a lot of
details and the history of the problem (whether one should use the cast or not).
If I am not mistaken a part of the problem is that compilers' behaviors have changed over time, and e.g. the latest versions would use a move constructor while the older ones could use a copy constructor. So the question is where we stand at the moment / what is the recommended approach for the new code.

Many thanks in advance,
Alexander Shaposhnikov

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