In order to see what difference a different processor makes I also tried
the same code on a fairly old 32 bit "AMD Athlon(tm) XP 3000+" with the
current stable gcc (4.7.2). The difference is even more striking
(dereferencing is much faster). I see that the size of the code inside
the loop for the faster pointer access is exactly 8. No idea whether
that has any significance.
Here as well I performed several runs with similar results. Statistical
significance was established around n=2 ;-).
gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i486-linux-gnu/4.7/lto-wrapper
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.7.2-5'
--with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--enable-gnu-unique-object --enable-plugin --enable-objc-gc
--enable-targets=all --with-arch-32=i586 --with-tune=generic
--enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu
--target=i486-linux-gnu
Thread model: posix
gcc version 4.7.2 (Debian 4.7.2-5)
ppeterr@www:~/src/test/obj-vs-ptr$ cat t
#!/bin/bash
cat $1.c && gcc -std=c99 -O0 -g -o $1 $1.c && time ./$1
ppeterr@www:~/src/test/obj-vs-ptr$ ./t obj
int main()
{
int localInt;
for (int i = 0; i < 100000000; ++i)
localInt = i;
return 0;
}
real 0m0.418s
user 0m0.416s
sys 0m0.004s
ppeterr@www:~/src/test/obj-vs-ptr$ ./t ptr
int main()
{
int localInt;
int *localP = &localInt;
for (int i = 0; i < 100000000; ++i)
*localP = i;
return 0;
}
real 0m0.243s
user 0m0.240s
sys 0m0.000s
===============================================================
The disassembly is for the direct access (slower):
localInt = i;
80483eb: 8b 45 fc mov -0x4(%ebp),%eax
80483ee: 89 45 f8 mov %eax,-0x8(%ebp)
And for the pointer access (faster):
*localP = i;
80483f1: 8b 45 f8 mov -0x8(%ebp),%eax
80483f4: 8b 55 fc mov -0x4(%ebp),%edx
80483f7: 89 10 mov %edx,(%eax)
