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Comment by atlei | Hacker News Reader

My guess for most slow compiles (yes, pulled out of my ...hat):

1. Templates

2. No pre-compiled headers

3. Bad header-organization

We have a reasonable sized desktop application 5 mb (release) / 19 mb (debug) that fully rebuilds in 10 s / 12 s (incremental < 2 s usually).

Yes, we have optimized using a RAM disk etc, but the main reason for the speed is No Templates and combining precompiled headers with efficient header-organization.

Some interesting links:

- http://gamesfromwithin.com/physical-structure-and-c-part-2-b...

- http://developers.sun.com/solaris/articles/CC_perf/content.h...

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Except that templates give you runtime speed and our customers care about runtime speed, not compilation speed.

What kind of speed increase are you talking about? Your comment's logic seems to suggest that, for instance, C (a language without templates) is inherently slower than C++, which is not the case. Can you back your statements up with evidence?

I think that he is talking about the cases where you can use template to avoid function pointer in C.

Speed increases like those explained in http://www.tilander.org/aurora/2007/12/comparing-stdsort-and..., which compares qsort to std::sort.

Because templates are basically automated code copy-paste, the compiler can more easily inline at compile-time. Unfortunately, it also translates to lots more compiled code, which means slower compile times and fatter binaries. Still, writing something like the STL library in C with the same performance characteristics would be much more difficult, if not impossible.

I'm not a C++ expert, but that's my understanding of things, at least.

If you are worried about the performance hit caused by function calling through a function pointer you might want to give a look at http://sglib.sourceforge.net/.

Templates are much more powerful than C with macros.

Templates are more than copy/paste code, the compiler can make strong assumptions about the code path during the optimization phase because it's actual code.

It's not just a question of inlining calls but mainly a question of being able to remove branches altogether. Branches is the performance killer, remember?

Examples:

- with template specialization you can make "compile time" switches (horrible to reproduce with C + macros) - you can compute values at compile time (not quite possible in C + macros) - inlining permits copy elision (an optimization that doesn't exist in C since return by value is a non sequitur) - the CRTP is faster than an object with vtables because it avoids indirect calls - removing of dead branches, by inspecting the templates recursion, the compiler can determine which branches will never be taken and doesn't compile them - probably more!

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