[RFC] GCC 8 Project proposal: Extensions supporting C Metaprogramming, pseudo-templates

Daniel Santos daniel.santos@pobox.com
Wed May 10 09:13:00 GMT 2017


Thanks for your feedback!

On 05/09/2017 08:29 AM, Allan Sandfeld Jensen wrote:
> On Tuesday 09 May 2017, Daniel Santos wrote:
>> The primary aim is to facilitate high-performance generic C
>> libraries for software where C++ is not suitable, but the cost of
>> run-time abstraction is unacceptable. A good example is the Linux
>> kernel, where the source tree is littered with more than 100 hand-coded
>> or boiler-plate (copy, paste and edit) search cores required to use the
>> red-black tree library.
> That is not a good excuse, they can just use a defined subset of C++. The cost
> of C++ abstractions is zero if you don't use them.

Put simply, there are many projects who will not likely be converting to 
C++ in the in our lifetimes.  As far as abstractions, I meant the 
abstraction penalty of static types which doesn't exist when using C++ 
templates (not at run-time anyway).  In C, generic libraries typically 
resolve abstract behavior with a callback function pointer.  This costs 
a function call in addition to the optimization barrier of the function 
call (which is the worst part of it).

>> To further the usefulness of such techniques, I propose the addition of
>> a c-family attribute to declare a parameter, variable (and possibly
>> other declarations) as "constprop" or some similar word. The purpose of
>> the attribute is to:
>>
>> 1.) Emit a warning or error when the value is not optimized away, and
>> 2.) Direct various optimization passes to prefer (or force) either
>> cloning or inlining of a function with such a parameter.
>>
> This I can get more behind, I have wanted a constexpr attribute for C for some
> time. If not for anything else to be able to use it in shared/system headers
> that can be used by both C and C++ and in C++ would be useful in constexpr
> expressions. If you can find a use for it in pure C as well, so much the
> better.
>
> `Allan

Maybe "constexpr" would be a better name, as it mirrors the C++11 
keyword.  When I first read about C++ getting constexpr, my first 
thought was, "Yeah, as if they needed yet another way to do 
metaprogramming!" :)  However, I hadn't gone so far as to investigate 
using this new attribute on functions since we already have 
__attribute__((const)).  I haven't used this before so maybe I'm not 
aware of something that makes it unusable for such cases? Which of 
course raises the question if __attribute__((const)) would work out 
since it's only currently used on function declarations (and pointers to 
function declarations, but I don't fully understand what that is doing 
in handle_const_attribute).

Thanks,
Daniel



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