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Re: Faster compilation speed


--- Stan Shebs <shebs@apple.com> wrote:
> Noel Yap wrote:
> 
> >Build speeds are most helped by minimizing the
> number
> >of files opened and closed during the build.
> >
> Is this assertion based on empirical measurement,
> and if so, for what
> source code and what system?  For instance, the
> longest source file
> in GCC is about 15K lines, and at -O2, only a small
> percentage of
> time is spent messing with files.  If I use
> -save-temps on cp/decl.c on
> one of my (Linux) machines, I get a total time of
> about 38 sec from
> source to asm.  If I just compile decl.i, it's about
> 37 sec, so that's
> 1 sec for *all* preprocessing, including all file
> opening/closing.

This is a good question.

John Lakos in _Large-Scale C++ Software Development_
has performed a rudimentary case study.  If the
conclusions are true, then your example indicates that
there wasn't much of a difference between the number
of files used when compiling decl.c and decl.i.

The study also indicates that having #include's within
header files is the largest contributor to the problem
(since nested #include's would increase the number of
file accesses combinatorially).

As another indication that the conclusion is true,
Lakos added guards around the #include lines
themselves and found compile times to dramatically
decrease.  For example:
#if header_h
#   include <header.h>
#endif

I can go on, but I doubt others on this list would
appreciate a reprint of the chapter.  If you don't
have the book, I suggest at least finding a copy and
reading this chapter.

> Obviously, other programs will have different
> characteristics, and if
> you have one for which file opening/closing
> dominates compile time,
> that will be very interesting.  But it's bad to try
> to optimize
> something before you have numerical evidence.

I agree.

Would you agree with Lakos's findings as evidence to
this fact?

Noel

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