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

Fri Oct 2 12:38:07 PDT 2020

> Is the code currently broken?

I don't know. Perhaps the code is fine, I did not try to compile LLVM with all that compilers

that we claim as supported though.  I just *guess* that we don't have a bot that builds LLVM with Clang 3.5,

for example. It sounds like a bit too old version to me to use (released in the middle of 2014, AFAIK).

Best regards,
George | Developer | Access Softek, Inc
________________________________
От: David Blaikie <dblaikie at gmail.com>
Отправлено: 2 октября 2020 г. 22:04
Кому: George Rimar
Копия: Alexander Shaposhnikov; Richard Smith; llvm-dev; Lang Hames; James Henderson; avl.lapshin at gmail.com
Тема: Re: [EXTERNAL] Re: [llvm-dev] preferred way to return expected values

On Fri, Oct 2, 2020 at 1:48 AM George Rimar <grimar at accesssoftek.com<mailto:grimar at accesssoftek.com>> wrote:

Thanks, David!

Few minor additions to the topic:

> I'm not sure which MSVC version on godbolt would be "MSVC 2017" that the LLVM docs refer to

I've found that the minimal available version of MSVC on godbolt is "WINE MSVC 2015: x64 msvc v19.0 (WINE)".

Your sample compiles fine with it: https://godbolt.org/z/hsPneK

Also, I've tried with "x64 msvc v19.10 (WINE)" (https://godbolt.org/z/vaqsPY)

wiki says that v19.10 corresponds to Visual Studio 2017 version 15.0 (https://en.wikipedia.org/wiki/Microsoft_Visual_C%2B%2B),

i.e. it is MSVS 2017 without updates. The sample also feels fine.

> Looks like we would need to bump the minimum Clang up from 3.5 to at least 3.9 to allow returns with implicit moves that include conversions.

Perhaps, we could have a bot to check that LLVMs codebase is compilable with compilers we claim to support.

Generally good to do, yes, but someone's got to pay for/setup the resources, etc.

I am not sure it is not a overkill though, but could help either to keep the documentation up to date or fix the code to match it.

Is the code currently broken?

- Dave

Best regards,
George | Developer | Access Softek, Inc
________________________________
От: David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>>
Отправлено: 1 октября 2020 г. 22:00
Кому: George Rimar
Копия: Alexander Shaposhnikov; Richard Smith; llvm-dev; Lang Hames; James Henderson; avl.lapshin at gmail.com<mailto:avl.lapshin at gmail.com>
Тема: Re: [EXTERNAL] Re: [llvm-dev] preferred way to return expected values

On Thu, Oct 1, 2020 at 2:08 AM George Rimar <grimar at accesssoftek.com<mailto:grimar at accesssoftek.com>> wrote:

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;
}
```

I think the easiest and portable way to test this functionality would be more like:

#include <memory>
struct base { virtual ~base(); };
struct derived : base { };
std::unique_ptr<base> f() {
  std::unique_ptr<derived> d;
  return d;
}

That shows whether the compiler's treating the return of a temporary as movable, even when the types aren't an exact match.

Clang 3.5 does not support this conversion: https://godbolt.org/z/5nsWM8
GCC 5.1 does support it: https://godbolt.org/z/cvd3d6
& I'm not sure which MSVC version on godbolt would be "MSVC 2017" that the LLVM docs refer to.

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.

That's interesting - I wonder if LLVM's documentation is out of date, which claims the minimum required Clang is 3.5: https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library

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.

May be something to do with which compiler the llvm library provided by godbolt is compiled with? which might make the above results not quite right (& why testing with the non-llvm-specific example might be clearer)

Looks like we would need to bump the minimum Clang up from 3.5 to at least 3.9 to allow returns with implicit moves that include conversions.
- Dave

Best regards,
George | Developer | Access Softek, Inc
________________________________
От: Alexander Shaposhnikov <alexander.v.shaposhnikov at gmail.com<mailto: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<mailto:avl.lapshin at gmail.com>
Тема: [EXTERNAL] Re: [llvm-dev] preferred way to return expected values

CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.  If you suspect potential phishing or spam email, report it to ReportSpam at accesssoftek.com<mailto:ReportSpam at accesssoftek.com>
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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