Re: g++ compile-time regression

[Carlo Wood <carlo at alinoe dot com>]

On Wed, Feb 28, 2001 at 12:36:59PM -0300, Alexandre Oliva wrote:
> On Feb 28, 2001, Carlo Wood <carlo@alinoe.com> wrote:
> 
> > 3.1 is ten times slower than 2.95.1 for a 'hello world' program :).
> 
> Bear in mind that 2.95.* doesn't have template (i.e.,
> Standard-compliant) iostreams.

I understand, which is why I posted this here and not to bugs *grin*
(and I put in that smiley).

Nevertheless, it is a concern to me.
A more realistic example is:

~/c++/libcw/src/libcwd/testsuite>time g++-2.95.1 -DHAVE_CONFIG_H -DCWDEBUG -Ilibcwd.tst -I- -I../include -I./../include  -g -pipe -Wall -Woverloaded-virtual -Wundef -Wpointer-arith -Winline -Wwrite-strings -Werror -c libcwd.tst/basic.cc -o tst_basic.o
 
 real    0m0.876s
 user    0m0.810s
 sys     0m0.060s

~/c++/libcw/src/libcwd/testsuite>time g++-3.1 -DHAVE_CONFIG_H -DCWDEBUG -Ilibcwd.tst -I- -I../include -I./../include  -g -pipe -Wall -Woverloaded-virtual -Wundef -Wpointer-arith -Winline -Wwrite-strings -Werror -c libcwd.tst/basic.cc -o tst_basic.o
 
 real    0m2.772s
 user    0m2.690s
 sys     0m0.080s

The work that the compiler does for the template heavy standard header files
is for templates with very frequently used template parameters and are in 99.9%
of the cases the same.  Nevertheless, the compiler does it again and again for
*every* compilation unit.  A large project with 100 seperate source files needs
to get this work done 100 times, resulting in 200 seconds of duplicated work.

[ Example: compilation of my libcwd project takes:

real    0m16.798s
user    0m14.000s
sys     0m2.750s

with g++-2.95.1 
and

real    0m37.400s
user    0m34.440s
sys     0m2.850s

with g++-3.0  (it doesn't compile with 3.1, I'll file a bug report later).
]

What is needed is a template database with frequently used templates that the
compiler can re-use, if only during the compilation run of one 'make' - but
there is no reason not to make this database more global imho (compare it with
the cache of a webbrowser).

-- 
Carlo Wood <carlo@alinoe.com>