Re: [PATCH] gdb/linux-fork: simplify one_fork_p

[Pedro Alves <palves@redhat.com>]

On 1/19/20 4:56 PM, Christian Biesinger via gdb-patches wrote:
> On Sun, Jan 19, 2020 at 11:53 AM Simon Marchi <simon.marchi@polymtl.ca> wrote:
>>
>> On 2020-01-19 11:41 a.m., Christian Biesinger wrote:
>>> On Sun, Jan 19, 2020 at 11:11 AM Simon Marchi <simon.marchi@polymtl.ca> wrote:
>>>>
>>>> Unless I'm missing something, this function is a complicated way of
>>>> saying "fork_list.size () == 1".
>>>
>>> Before C++11, size() wasn't guaranteed to run in constant time, so I
>>> assume the code was written to handle that. But GDB uses C++11, so
>>> this change seems fine.
>>> https://en.cppreference.com/w/cpp/container/list/size
>>
>> Ahh, good point.  Although by the time that change was made, we were already
>> using C++11.  I don't remember if we had a C++ < 11 phase, but if we did it
>> was very short.

Yes, we had one.  It was short.  Everyone hated my unique_ptr emulation
so much that we moved quickly to C++11. :-D

>>
>> Thanks for looking at it, I'll push it now.
> 
> Ah. it's also possible that whoever wrote the code just assumed that
> size() would run in linear time, of course.

Note, I believe that size() isn't linear when compiled with gcc 4.8,
since the new C++11 ABI was only introduced in GCC 5:

 https://developers.redhat.com/blog/2015/02/05/gcc5-and-the-c11-abi/

But I think it's OK to ignore that.  Especially for non-hot code like
here.  I think it's reasonable to say that if you care about performance,
you'll want to compile with a newer compiler.

I was the one who wrote it (06974e6c05556e), but I don't remember why
I did it that way.  Might have been the non-O(1) issue, or it could have been
about blindly C++-fying code without realizing the potential simplification.  
I agree that size () == 1 works just as well, assuming C++11 std::list.

Thanks,
Pedro Alves