Re: Faster compilation speed

[Nix <nix at esperi dot demon dot co dot uk>]

[Cc: list trimmed]
On Sat, 10 Aug 2002, Noel Yap spake:
> I would agree if you're talking about complete builds
> spanning only a few C/C++ files.  OTOH, when builds
> span many hundreds of these files, build-time (not
> just compile-time) starts getting bogged down on
> (mostly) reopening and repreprocessing the same files
> over and over.
> 
> Within our system, builds on Windows are magnitudes
> faster since we're able to take advantage of
> precompiled headers.

Are you sure that this isn't because GCC is having to parse the headers
over and over again, while the precompiled system can avoid that
overhead?

Especially for C++ header files (which tend to be large, complex,
interdependent, and include a lot of code), the parsing and compilation
time *vastly* dominates the preprocessing time.

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;
 }

Time spent preprocessing (distorted by the slowness of cpp's output
routines):

nix@loki 62 /tmp% time c++ -E -ftime-report hello.C >/dev/null

real    0m1.424s
user    0m0.710s
sys     0m0.100s

Time spent preprocessing and parsing (roughly; cpp's output routines are
still slow; on the trunk much less time will be spent preprocessing
because the integrated preprocessor doesn't have to do any output at all
there, instead feeding a token stream to the rest of the compiler):

nix@loki 60 /tmp% c++ -ftime-report -fsyntax-only hello.C 

Execution times (seconds)
 garbage collection    :   1.16 (12%) usr   0.08 ( 6%) sys   2.19 (13%) wall
 preprocessing         :   1.04 (11%) usr   0.29 (20%) sys   2.10 (12%) wall
 lexical analysis      :   0.99 (10%) usr   0.28 (20%) sys   1.87 (11%) wall
 parser                :   6.12 (65%) usr   0.75 (53%) sys  10.85 (63%) wall
 varconst              :   0.08 ( 1%) usr   0.00 ( 0%) sys   0.10 ( 1%) wall
 TOTAL                 :   9.44             1.42            17.21

(oddly, preprocessing took *longer* than it did using -E, which I'd not
 expected; but, still parsing vastly dominates preprocessing, and this isn't
 going near e.g. the STL headers)

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 :) ).

-- 
`There's something satisfying about killing JWZ over and over again.'
                                        -- 1i, personal communication