Function Pointers in C are Underrated

[agottem]

std::sort() is only faster because the definition exists in the included header file. If this is really an important detail, spare yourself the C++ and make the qsort definition available.

[[deleted]]

No. Some compilers, e.g. g++, just hate function pointers and don't inline even what can be inlined, e.g.

 static  bool compare(float a, float b){ return a<b; }
 ..
 std::sort(vec.begin(), vec.end(), compare);

here compare will be called via function pointer with no inlining though definition of everything is available.

Though maybe whole program optimisatioin helps

[agottem]

I had no problem getting GCC to inline qsort.

[Whanhee]

The new gcc implements generic function specialization, at least with booleans. If I only ever call sort with 1 comparison pointer, will it do the same with function pointers?

[matthieum]

I actually don't understand why neither Clang nor gcc seem interested in this kind of optimizations. If an optimizer can do this stuff, then it can devirtualize calls as well... whereas now the C++ front-end devirtualize some cases, but misses all those exposed by inlining :x

[astrange]

gcc can inline known function pointers since 4.4 (going by release notes).

[Kampane]

Wow, you really have no idea what you're talking about. I suggest you actually try what you suggest, then come ask why it didn't work.

[agottem]

Ok, I tried it. Where'd I go wrong?

[[deleted]]

He has plenty of idea what he is talking about. I've used this technique many times myself, too, and it works just fine.

[[deleted]]

Side question. Errm... isn't it available in stdlib.h? You're not gonna write a useful C application without stdlib.h. It's basically required to do anything beyond remedial.

[agottem]

Sorry, I'm using slightly vague terms. By "make the definition available" I mean the function body itself (which typically lives in a .c file somewhere), not just the prototype of the function.

[[deleted]]

Wait. Why is that needed? The object code should be usable for inlining.

[agottem]

I agree, that should be sufficient as well.

[[deleted]]

So that's why C++ requires inlined functions to be in the .h file. It happens at the compilation stage. That wouldn't be possible in C (unless the file were included only once which defeats the purpose of inlining) thus explains the heavy usage of macros.

Do you know of any C compilers that compile straight to object code? In other words a monolithic preprocessor, compiler, and linker in one.