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

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

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
+{
+  return t;
+}
+

BR,
Kewen