Strange optimisation problem - gcc 4.3.2

[Mark Cave-Ayland <mark dot cave-ayland at siriusit dot co dot uk>]

Hi everyone,

I've been looking at adding some code to a performance-critical section 
of OpenBIOS, and I'm quite confused by how some of the changes I am 
making are affecting the overall performance.

For a benchmark, I am using a recursive fibonacci function to test the 
effect of any changes that I have made. The test machines is an Intel 
Core 2 x86 running under a 64-bit Debian Lenny installation.

Firstly, here are the benchmark results running from the unmodified SVN 
source tree:


build@zeno:~/src/openbios/openbios-devel$ time echo "28 fib-rec u. bye" 
| ./obj-x86/openbios-unix ./obj-x86/openbios-x86.dict
Welcome to OpenBIOS v1.0 built on Nov 9 2009 17:12
  Type 'help' for detailed information

[unix] Booting default not supported.

0 > 28 fib-rec u. bye 6197ecb
Farewell!

 ok

real    0m37.946s
user    0m37.178s
sys     0m0.020s


I then add a simple C function pointer below to the top of the forth 
kernel file (which is currently not referenced by any other code changes):


void (*debughook) (void);


If I then re-build and re-run the same benchmark, the results now look 
like this:


build@zeno:~/src/openbios/openbios-devel$ time echo "28 fib-rec u. bye" 
| ./obj-x86/openbios-unix ./obj-x86/openbios-x86.dict
Welcome to OpenBIOS v1.0 built on Nov 9 2009 17:17
  Type 'help' for detailed information

[unix] Booting default not supported.

0 > 28 fib-rec u. bye 6197ecb
Farewell!

 ok

real    0m52.564s
user    0m52.027s
sys     0m0.012s


So I'm really confused as to how adding a simply function pointer in the 
global declaration section (without even adding any code to reference 
it) suddenly incurs an extra 40% overhead? Can anyone explain why this 
is, and/or point me to any suitable gcc optimisation guides?

For reference, the gcc compiler is gcc 4.3.2 under Debian Lenny and the 
compile flags are:

-Os -g -Wall -Wredundant-decls -Wshadow -Wpointer-arith
-Wstrict-prototypes -Wmissing-declarations -Wundef -Wendif-labels 
-Wstrict-aliasing -Wwrite-strings -Wmissing-prototypes


Many thanks,

Mark.

--
Mark Cave-Ayland - Senior Technical Architect
PostgreSQL - PostGIS
Sirius Corporation plc - control through freedom
http://www.siriusit.co.uk
t: +44 870 608 0063

Sirius Labs: http://www.siriusit.co.uk/labs