Re: [RFC/PATCH] Use range-based for loops for traversing loops

["Kewen.Lin" <linkw@linux.ibm.com>]

on 2021/7/20 下午5:49, Jonathan Wakely wrote:
> On Tue, 20 Jul 2021 at 09:58, Kewen.Lin <linkw@linux.ibm.com> wrote:
>>
>> on 2021/7/19 下午11:59, Martin Sebor wrote:
>>> On 7/19/21 12:20 AM, Kewen.Lin wrote:
>>>> Hi,
>>>>
>>>> This patch follows Martin's suggestion here[1], to support
>>>> range-based for loops for traversing loops, analogously to
>>>> the patch for vec[2].
>>>>
>>>> Bootstrapped and regtested on powerpc64le-linux-gnu P9,
>>>> x86_64-redhat-linux and aarch64-linux-gnu, also
>>>> bootstrapped on ppc64le P9 with bootstrap-O3 config.
>>>>
>>>> Any comments are appreciated.
>>>
>>> Thanks for this nice cleanup!  Just a few suggestions:
>>>
>>> I would recommend against introducing new macros unless they
>>> offer a significant advantage over alternatives (for the two
>>> macros the patch adds I don't think they do).
>>>
>>> If improving const-correctness is one of our a goals
>>> the loops_list iterator type would need to a corresponding
>>> const_iterator type, and const overloads of the begin()
>>> and end() member functions.
>>>
>>> Rather than introducing more instances of the loop_p typedef
>>> I'd suggest to use loop *.  It has at least two advantages:
>>> it's clearer (it's obvious it refers to a pointer), and lends
>>> itself more readily to making code const-correct by declaring
>>> the control variable const: for (const class loop *loop: ...)
>>> while avoiding the mistake of using const loop_p loop to
>>> declare a pointer to a const loop.
>>>
>>
>> Thanks for the suggestions, Martin!  Will update them in V2.
>>
>> With some experiments, I noticed that even provided const_iterator
>> like:
>>
>>    iterator
>>    begin ()
>>    {
>>      return iterator (*this, 0);
>>    }
>>
>> +  const_iterator
>> +  begin () const
>> +  {
>> +    return const_iterator (*this, 0);
>> +  }
>>
>> for (const class loop *loop: ...) will still use iterator instead
>> of const_iterator pair.  We have to make the code look like:
>>
>>   const auto& const_loops = loops_list (...);
>>   for (const class loop *loop: const_loops)
>>
>> or
>>   template<typename T> constexpr const T &as_const(T &t) noexcept { return t; }
>>   for (const class loop *loop: as_const(loops_list...))
>>
>> Does it look good to add below as_const along with loops_list in cfgloop.h?
>>
>> +/* Provide the functionality of std::as_const to support range-based for
>> +   to use const iterator.  (We can't use std::as_const itself because it's
>> +   a C++17 feature.)  */
>> +template <typename T>
>> +constexpr const T &
>> +as_const (T &t) noexcept
> 
> The noexcept is not needed because GCC is built -fno-exceptions. For
> consistency with all the other code that doesn't use noexcept, it
> should probably not be there.
> 

Thanks for pointing out!   Fixed it in v2.

>> +{
>> +  return t;
>> +}
>> +
> 
> That's one option. Another option (which could coexist with as_const)
> is to add cbegin() and cend() members, which are not overloaded for
> const and non-const, and so always return a const_iterator:
> 
> const_iterator cbegin () const { return const_iterator (*this, 0); }
> iterator begin () const { return cbegin(); }
> 
> And similarly for `end () const` and `cend () const`.
> 

Thanks for the suggestion.  As you pointed out in the later reply, the
range-based for loop doesn't use cbegin and cend, so I didn't add them
in v2.

BR,
Kewen