r203293 - [C++11] Revert uses of lambdas with array_pod_sort.

[Richard Smith]

On Fri, Mar 7, 2014 at 2:10 PM, Arthur O'Dwyer <arthur.j.odwyer at gmail.com>wrote:

> On Fri, Mar 7, 2014 at 1:51 PM, Benjamin Kramer
> <benny.kra at googlemail.com> wrote:
> >
> > [C++11] Revert uses of lambdas with array_pod_sort.
> >
> > Looks like GCC implements the lambda->function pointer conversion
> differently.
>
> > -    llvm::array_pod_sort(DeferredLocs.begin(), DeferredLocs.end(),
> > -                         [](const DeferredLocsTy::value_type *p1,
> > -                            const DeferredLocsTy::value_type *p2) {
> > -      if (p1->second->getLocStart() != p2->second->getLocStart())
> > -        return p1->second->getLocStart() < p2->second->getLocStart() ?
> -1 : 1;
> > -      return 0;
> > -    });
> > +    llvm::array_pod_sort(DeferredLocs.begin(), DeferredLocs.end(),
> SrcCmp);
>
> Incidentally, I like the bag-of-idioms
>
>     auto *byLocation =
>         +[](const DeferredLocsTy::value_type *p1, const
> DeferredLocsTy::value_type *p2) -> int {
>             return (p1->second->getLocStart() <
> p2->second->getLocStart() ? -1 : (p1->second->getLocStart() >
> p2->second->getLocStart());
>         };
>     llvm::array_pod_sort(DeferredLocs.begin(), DeferredLocs.end(),
> byLocation);
>
> That is,
> - Name the out-of-line lambda after the predicate it implements (in
> this case sorting "byLocation")
> - I bet you'll find compilers are happier with the explicitly
> function-pointer'ized "+[]" than with "[]" alone
>

I bet the opposite. MSVC (non-conformingly) gives lambdas three conversions
to function pointers with different calling conventions, and this won't
compile there.


> - The spaceship operator <=> is equivalent to (a < b) ? -1 : (a > b)
>

The more usual idiom of

  if (!=)
    return <

naturally extends to lexicographical comparisons of multiple things. I
don't see how to do that with (a<b?-1:a>b).


> However, FWIW and IMHO, I don't think lambdas are obviously desirable
> here. This looks a lot like "C++11isms for C++11isms' sake", not
> anything that would actually improve compiler speed or stability.
>
> my two cents,
> -Arthur
>
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