llvm dev - Sep 2016 - Adding [[nodiscard]] to Compiler.h

Sanjoy Das <sanjoy at playingwithpointers.com>
writes:
> Hi Justin,
>
> This SGTM generally, but please make the difference between
> LLVM_NODISCARD and LLVM_UNUSED_RESULT clear in the code. :)
Right, this is where it gets a little weird. LLVM_NODISCARD would be for
types, whereas LLVM_UNUSED_RESULT would be for functions. Depending on
your host compiler, using the wrong one might DTRT, but it won't across
all compilers.

Do you think documenting this is sufficient, or should we try to name
these to better represent where they should be used?
> However, we have to make a policy decision here:  if I have a
> LLVM_NODISCARD "Error" class should functions that return an
instance
> of Error _also_ be annotated with LLVM_UNUSED_RESULT?  IOW, how much
> repetition are we willing to live with to get good diagnostics on
> non-clang compilers?
>
> I personally would be happier with _not_ annotating functions return
> Error with LLVM_UNUSED_RESULT, since the clang bots would catch
> violations anyway, and the code will look slightly neater.
I'm also in favour of not annotating the functions - especially given a
case like Error (and it's friend, Expected<>). Annotating all of the
functions would add a lot of boilerplate for little gain.
> Thanks,
> -- Sanjoy
>
>
> On Fri, Sep 2, 2016 at 2:20 PM, Justin Bogner via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> I started to try to use llvm::Error recently. This has nice runtime
>> checks for if you didn't check the result, but I thought it would
be
>> really nice to get a compiler warning for the obvious cases of this
>> rather than having to wait for a runtime check.
>>
>> This, of course, is exactly what the C++17 [[nodiscard]] attribute is
>> for - with new enough compilers in C++17 mode we can just declare the
>> class like so:
>>
>>   class [[nodiscard]] Error { ... };
>>
>> So, I'd like to add an LLVM_NODISCARD macro to Compiler.h, and this
is
>> where it gets interesting. Pre-C++17, clang already allows
>> __attribute__((warn_unused_result)) and [[clang::warn_unused_result]]
to
>> be used on a class this way, with equivalent affects, so it'd be
nice to
>> use that if we aren't building in C++17 mode.
>>
>> We already have a LLVM_UNUSED_RESULT defined to this, but AFAICT gcc
>> only allows it on function declarations, and the MSVC equivalent
>> (_Check_return_) seems like it's not allowed on types either. I
guess we
>> want a new macro LLVM_NODISCARD, that's [[nodiscard]] if available,
else
>> [[clang::warn_unused_result]] if we're clang (maybe even clang 3.6
and
>> up, I'm not sure if it works before that), else empty.
>>
>> Thoughts?
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Hi Justin,

Justin Bogner wrote:
 > Sanjoy Das<sanjoy at playingwithpointers.com>  writes:
 >> Hi Justin,
 >>
 >> This SGTM generally, but please make the difference between
 >> LLVM_NODISCARD and LLVM_UNUSED_RESULT clear in the code. :)
 >
 > Right, this is where it gets a little weird. LLVM_NODISCARD would be for
 > types, whereas LLVM_UNUSED_RESULT would be for functions. Depending on
 > your host compiler, using the wrong one might DTRT, but it won't
across
 > all compilers.
 >
 > Do you think documenting this is sufficient, or should we try to name
 > these to better represent where they should be used?

Firstly, perhaps "LLVM_NODISCARD_TYPE" would be a better name?

Secondly, if we define the following when the host compiler is clang,

#define LLVM_UNUSED_RESULT __attribute__((warn_unused_result))
#define LLVM_NODISCARD_TYPE [[clang::warn_unused_result]]

via some shallow manual testing, it looks like using
LLVM_NODISCARD_TYPE on a function should break the build (though I'd
love it if someone more familiar with clang chimed in on this). So, is
the problem that we can accidentally use LLVM_UNUSED_RESULT on a type
and not know it?

I think there is a dirty trick here -- we could:

#define LLVM_UNUSED_RESULT __attribute__((warn_unused_result, enable_if(true,
"")))
#define LLVM_NODISCARD_TYPE [[clang::warn_unused_result]]

This breaks the (clang) build if LLVM_UNUSED_RESULT is used on a type.

It would still be possible to not do the right thing on GCC or MSVC
and be able to build a binary, but I think as long as the clang bots
catch bad uses we're okay.

-- Sanjoy`

My 2 cents: get rid of LLVM_UNUSED_RESULT, and move to LLVM_NODISCARD.

For compilers that support it, it should be a strict superset of features
and functionality. The standard feature was written directly based on the
clang warn_unused_result stuff.

I would just migrate us onto the spelling and usage pattern that got
standardized. All we have to lose are warnings from compilers other than
Clang until they implement this feature. That seems like not a huge loss to
me honestly.

-Chandler

On Sun, Sep 4, 2016 at 2:57 PM Sanjoy Das via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi Justin,
>
> Justin Bogner wrote:
>  > Sanjoy Das<sanjoy at playingwithpointers.com>  writes:
>  >> Hi Justin,
>  >>
>  >> This SGTM generally, but please make the difference between
>  >> LLVM_NODISCARD and LLVM_UNUSED_RESULT clear in the code. :)
>  >
>  > Right, this is where it gets a little weird. LLVM_NODISCARD would be
for
>  > types, whereas LLVM_UNUSED_RESULT would be for functions. Depending
on
>  > your host compiler, using the wrong one might DTRT, but it won't
across
>  > all compilers.
>  >
>  > Do you think documenting this is sufficient, or should we try to name
>  > these to better represent where they should be used?
>
> Firstly, perhaps "LLVM_NODISCARD_TYPE" would be a better name?
>
> Secondly, if we define the following when the host compiler is clang,
>
> #define LLVM_UNUSED_RESULT __attribute__((warn_unused_result))
> #define LLVM_NODISCARD_TYPE [[clang::warn_unused_result]]
>
> via some shallow manual testing, it looks like using
> LLVM_NODISCARD_TYPE on a function should break the build (though I'd
> love it if someone more familiar with clang chimed in on this). So, is
> the problem that we can accidentally use LLVM_UNUSED_RESULT on a type
> and not know it?
>
> I think there is a dirty trick here -- we could:
>
> #define LLVM_UNUSED_RESULT __attribute__((warn_unused_result,
> enable_if(true, "")))
> #define LLVM_NODISCARD_TYPE [[clang::warn_unused_result]]
>
> This breaks the (clang) build if LLVM_UNUSED_RESULT is used on a type.
>
> It would still be possible to not do the right thing on GCC or MSVC
> and be able to build a binary, but I think as long as the clang bots
> catch bad uses we're okay.
>
> -- Sanjoy`
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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