Unused GCC builtins

[Joel Sherrill]

On 1/22/2018 9:55 AM, David Brown wrote:
> On 22/01/18 16:46, Manuel Rigger wrote:
>> Hi everyone,
>>
>> As part of my research, we have been analyzing the usage of GCC builtins
>> in 5,000 C GitHub projects. One of our findings is that many of these
>> builtins are unused, even though they are described in the documentation
>> (see https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html#C-Extensions)
>> and obviously took time to develop and maintain. I’ve uploaded a CSV
>> file with the unused builtins to
>> http://ssw.jku.at/General/Staff/ManuelRigger/unused-builtins.csv.
>>
>> Details: We downloaded all C projects from GitHub that had more than 80
>> GitHub stars, which yielded almost 5,000 projects with a total of more
>> than one billion lines of C code. We filtered GCC, forks of GCC, and
>> other compilers as we did not want to incorporate internal usages of GCC
>> builtins or test cases. We extracted all builtin names from the GCC
>> docs, and also tried to find such names in the source code, which we
>> considered as builtin usages. We excluded subdirectories with GCC or
>> Clang, and removed other false positives. In total, we found 320k
>> builtin usages in these projects, and 3030 unused builtins out of a
>> total of 6039 builtins.
>>
>> What is your take on this? Do you believe that some of these unused
>> builtins could be removed from the GCC docs or deprecated? Or are they
>> used in special "niche" domains that we did not consider? If yes, do you
>> think it is worth to maintain them? Are some of them only used in C++
>> projects? Might it be possible to remove their implementations (which
>> has already happened for the Cilk Plus builtins)?
>>
>> We would be glad for any feedback.
>>
>> - Manuel
>>
> 
> Many of these are going to be used automatically by the compiler.  You
> write "strdup" in your code, and the compiler treats it as
> "__builtin_strdup".  I don't know that such functions need to be
> documented as extensions, but they are certainly in use.
> 
> You will also find that a large number of the builtins are for specific
> target processors, and projects using them are not going to turn up on
> GitHub.  They will be used in embedded software that is not open source.
> 
> I am sure there are builtins that are rarely or never used - but I doubt
> if it is anything like as many as you have identified from this survey.
> 

My first thought was that there is a lot of free and open source 
software that is not hosted at github. Larger projects are often 
self-hosted. Does this list cover all GNU, Savannah, sourceware.org, 
Apache, KDE, *BSD, Mozilla, etc projects?

You might get lucky and some like RTEMS and FreeBSD (I think) have
a github mirror. But github is not the entire universe of free and
open source software.

--joel sherrill
RTEMS