Re: [libstdc++/71500] make back reference work with icase

["Tim Shen via libstdc++" <libstdc++ at gcc dot gnu dot org>]

On Mon, Sep 18, 2017 at 4:01 PM, Jonathan Wakely <jwakely@redhat.com> wrote:
> On 18/09/17 10:58 -0700, Tim Shen via libstdc++ wrote:
>>
>> On Mon, Sep 18, 2017 at 10:26 AM, Jonathan Wakely <jwakely@redhat.com>
>> wrote:
>>>>
>>>> We need to rewrite this to check the lengths are equal first, and then
>>>> call the 3-argument version of std::equal.
>>>>
>>>> Alternatively, we could move the implementation of the C++14
>>>> std::equal overloads to __equal and make that available for C++11.
>>>> I'll try that.
>>>
>>>
>>>
>>> Here's a proof of concept patch for that. It's a bit ugly.
>>
>>
>> Instead of having iterator tags in the interface, we can probe the
>> random-access-ness inside __equal4/__equal4_p, can't we? It's similar
>> to the existing "if (_RAIters()) { ... }".
>>
>> I'd expect the patches to be renaming the current implementations and
>> adding wrappers, instead of adding new implementations.
>
>
> Well I decided to split the existing functions up and use tag
> dispatching, which is conceptually cleaner anyway. But as the
> RandomAccessIterator version doesn't need any operations that aren't
> valid for other categories, it's not strictly necessary. The tag
> dispatching version should generate slightly smaller code for
> unoptimized builds, but that's not very important.

Unoptimized builds don't inline small functions, therefore the first
patch generate two weak symbols, instead of one by the second patch.
It's unclear to me how would number of symbols penalize the
performance/binary size.

>
> Here's the patch doing it as you suggest. We can't call the new
> functions __equal because t hat name is already taken by a helper
> struct, hence __equal4.
>
> Do you prefer this version?

Yes, I prefer this version for readability reasons:
1) subjectively, less scattered code; and
2) ideally I want `if constexpr (...)`), the if version is closer.

I agree that it's not a big difference. I just wanted to point out the
small difference. I'm fine with either version.

Thanks for the prototyping!


-- 
Regards,
Tim Shen