alt Hacker Newsqsort is obviously just an example, this situation applies to anything that takes a callback: in C++/Rust, that's almost always generic and the compiler will monomorphize the function and optimize around it, and in C it's almost always a function pointer and a userData argument for state passed on the stack. (and, of course, it applies not just to callbacks, but more broadly to anything templated).
I'm actually very curious about how good C compilers are at specializing situations like this, I don't actually know. In the vast majority cases, the C compiler will not have access to the code (either because of dynamic linking like in this example, or because the definition is in another translation unit), but what if it does? Either with static linking and LTO, or because the function is marked "inline" in a header? Will C compilers specialize as aggressively as Rust and C++ are forced to do?
If anyone has any resources that have looked into this, I would be curious to hear about it.
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My point is that the real issue is just whether or not the function call is compiled as part of the same unit as the function. If it is, then, certainly, modern C compilers can inline functions called via function pointers. The inlining itself is not made easier via the template magic.
Your C comparator function is already “monomirphized” - it’s just not type safe.
Dynamic linking will inhibit inlining entirely, and so yes qsort does not in practice get inlined if libc is dynamically linked. However, compilers can inline definitions across translation units without much of any issue if whole program optimization is enabled.
The use of function pointers doesn't have much of an impact on inlining. If the argument supplied as a parameter is known at compile time then the compiler has no issue performing the direct substitution whether it's a function pointer or otherwise.