GCC 3.3 compile speed regression - AN ANSWER

[Rob Taylor]

On Wed, 12 February 2003, at 02:29, Joe Buck wrote:
> On Tue, Feb 11, 2003 at 11:55:33AM -0000, Rob Taylor wrote:
> > g++ is so incredibly slow - observe:
> >
> > hellow.cpp:
> > #include <iostream>
> >
> > int main()
> > {
> >     std::cout << "Hello, world!\n";
> > }
> >
>
> Congrats, you just timed the compiler, the assembler, and the linker,
> not just the compiler.  The compiler is too slow, but if you want to time
> just the compiler you need to write
>
> time g++-3.1 -S hellow.cpp

ok, my bad, for the current purposes.
(NB - any chance of documenting -time? - it isn't in Option Summary)
(also, before i start, can anyone tell me whether pch works for c++ yet? if
so i'll repeat all these tests on mainline with pch on)

more detail:

robt@hal:~$ g++-3.1 -time  hellow.cpp
# cc1plus 1.40 0.06
# as 0.00 0.01
# collect2 0.19 0.02
robt@hal:~$ time g++-3.1 -S  hellow.cpp -o hellow.cpp.s

real    0m1.577s
user    0m1.490s
sys     0m0.090s
robt@hal:~$

I did some quick comparisons with the functionally equivalent

#include <stdio.h>

main()
{
   printf ("Hello World!\n");
}

as to what time gets spent in as and ld (just looking at user cpu secs as
are dominant factor)-
(avtime times 20 executions of command and avarages results)

robt@hal:~$ ./avtime.sh g++-3.1 -S hellow.cpp -o hellow.cpp.i
1.41667
robt@hal:~$ ./avtime.sh g++-3.1 hellow.cpp -o hellow.cpp.o
1.64
robt@hal:~$ ./avtime.sh g++-3.1 -S hellow.c -o hellow.c.i
0.06
robt@hal:~$ ./avtime.sh g++-3.1  hellow.c -o hellow.c.o
0.25

even though gcc generates nearly 10 times as much asm for the cpp case, the
total time spent in as+ld is pretty much constant.


the same timings on some production c++ using qt (not including ld)

~/avtime.sh
g++-3.1 -g -DfDEBUG -DfTRACES_ON -DCOMPILE_DEBUG -DLINUX -fsigned-char -D_IS
OC99_SOURCE -DUNICODE -fexceptions -fmessage-length=0 -Wall -W -Wno-reorder 
-Wno-unknown-pragmas -Wno-unused -Wno-non-template-friend -fPIC -DAUTOTEST_O
N -DLANGUAGE_SUPPORT_ON -DQT_DLL  -I$GUT_DIR/components -I$QTDIR/include -c
autopalette_window.cpp -o autopalette_window.cpp.o
21.5467

devel@hal:~/gut/components/gui_windows$ ~/avtime.sh
g++-3.1 -g -DfDEBUG -DfTRACES_ON -DCOMPILE_DEBUG -DLINUX -fsigned-char -D_IS
OC99_SOURCE -DUNICODE -fexceptions -fmessage-length=0 -Wall -W -Wno-reorder 
-Wno-unknown-pragmas -Wno-unused -Wno-non-template-friend -fPIC -DAUTOTEST_O
N -DLANGUAGE_SUPPORT_ON -DQT_DLL  -I$GUT_DIR/components -I$QTDIR/include -S
autopalette_window.cpp -o autopalette_window.cpp.s
19.7133


so thats a couple of seconds spent in gas for approx 4.6 meg of assembler.

I'm thinking that perhaps the best development solution would probably be a
speed branch, with a dedicated machine performing nightly timings on a
number of suitable test cases. I may be able to dedicate some
(machine+devel) time to such a thing.

Rob Taylor