Re: Faster compilation speed

[Mike Stump <mrs at apple dot com>]

On Saturday, August 10, 2002, at 05:49 PM, Nix wrote:
> > Within our system, builds on Windows are magnitudes
> > faster since we're able to take advantage of
> > precompiled headers.
Already solved problem for me.

> Example, with GCC-3.1, with a `hello world' iostreams-using program...
>
> The code:
>
> #include <iostream>
>
> int main (void)
>  {
>   std::cout << "Hello world";
>   return 0;
>  }
[ ... ]

> Complete run, with optimization:
>
> nix@loki 66 /tmp% c++ -O2 -ftime-report -o hello hello.C
>
> Execution times (seconds)
>  garbage collection    :   1.10 (11%) usr   0.11 ( 9%) sys   1.74 
> (11%) wall
>  cfg cleanup           :   0.01 ( 0%) usr   0.00 ( 0%) sys   0.01 ( 
> 0%) wall
>  life analysis         :   0.02 ( 0%) usr   0.00 ( 0%) sys   0.02 ( 
> 0%) wall
>  preprocessing         :   1.12 (11%) usr   0.22 (18%) sys   2.04 
> (13%) wall
>  lexical analysis      :   0.98 (10%) usr   0.22 (18%) sys   1.93 
> (12%) wall
>  parser                :   6.46 (65%) usr   0.63 (53%) sys   9.98 
> (62%) wall
>  expand                :   0.00 ( 0%) usr   0.00 ( 0%) sys   0.01 ( 
> 0%) wall
>  varconst              :   0.08 ( 1%) usr   0.00 ( 0%) sys   0.12 ( 
> 1%) wall
>  CSE                   :   0.02 ( 0%) usr   0.00 ( 0%) sys   0.03 ( 
> 0%) wall
>  CSE 2                 :   0.01 ( 0%) usr   0.00 ( 0%) sys   0.03 ( 
> 0%) wall
>  regmove               :   0.01 ( 0%) usr   0.00 ( 0%) sys   0.02 ( 
> 0%) wall
>  global alloc          :   0.02 ( 0%) usr   0.00 ( 0%) sys   0.04 ( 
> 0%) wall
>  flow 2                :   0.01 ( 0%) usr   0.00 ( 0%) sys   0.01 ( 
> 0%) wall
>  rename registers      :   0.02 ( 0%) usr   0.00 ( 0%) sys   0.03 ( 
> 0%) wall
>  scheduling 2          :   0.00 ( 0%) usr   0.01 ( 1%) sys   0.02 ( 
> 0%) wall
>  final                 :   0.01 ( 0%) usr   0.00 ( 0%) sys   0.01 ( 
> 0%) wall
>  TOTAL                 :   9.96             1.20            16.16
>
> Now obviously with a less toy example the time consumed optimizing 
> would
> rise; but that doesn't affect my point, that the lion's share of time
> spent in C++ header files is parsing time, and that speeding up the
> preprocessor will have limited effect now (thanks to Zack and Neil
> speeding it up so much already :) ).
With PFE:

bash-2.05a$ time g++ -O2 -ftime-report --load-pch foo t1.cc

Execution times (seconds)
 lexical analysis      :   0.00 ( 0%) usr   0.00 ( 0%) sys   0.03 (25%) 
wall
 parser                :   0.01 ( 8%) usr   0.00 ( 0%) sys   0.00 ( 0%) 
wall
 varconst              :   0.08 (62%) usr   0.00 ( 0%) sys   0.06 (50%) 
wall
 CSE                   :   0.01 ( 8%) usr   0.00 ( 0%) sys   0.00 ( 0%) 
wall
 TOTAL                 :   0.13             0.00             0.12

real    0m0.397s
user    0m0.070s
sys     0m0.260s


Without PFE:

bash-2.05a$ time g++ -O2 -ftime-report -include t1.h t1.cc

Execution times (seconds)
 preprocessing         :   0.39 (19%) usr   0.00 ( 0%) sys   0.19 ( 9%) 
wall
 lexical analysis      :   0.67 (32%) usr   0.00 ( 0%) sys   0.72 (34%) 
wall
 parser                :   0.90 (43%) usr   0.00 ( 0%) sys   1.09 (51%) 
wall
 varconst              :   0.04 ( 2%) usr   0.00 ( 0%) sys   0.03 ( 1%) 
wall
 CSE                   :   0.02 ( 1%) usr   0.00 ( 0%) sys   0.03 ( 1%) 
wall
 CSE 2                 :   0.00 ( 0%) usr   0.00 ( 0%) sys   0.03 ( 1%) 
wall
 regmove               :   0.01 ( 0%) usr   0.00 ( 0%) sys   0.00 ( 0%) 
wall
 symout                :   0.01 ( 0%) usr   0.00 ( 0%) sys   0.00 ( 0%) 
wall
 TOTAL                 :   2.07             0.00             2.12

real    0m2.378s
user    0m1.180s
sys     0m1.110s

Notice that preprocessing and lexical analysis both disappear.  Sure 
the parse takes time, but just 8%.  varconst is the cxx_finish_file 
pagefaulter which is more expensive than it really needs to be.  The 
speedup this shows is typical (6x) for larger applications.