plans for 64bit support (Joshua Root)

[Markus Weissmann]

On Dec 20, 2007, at 10:22 PM, Markus Weissmann wrote:

> On Dec 20, 2007, at 5:03 PM, Vincent Lefevre wrote:
>
>> On 2007-12-20 17:59:25 +1100, Joshua Root wrote:
>>> [1] <http://lixom.net/~olof/64bit-perf.pdf>
>>> [2] <http://www.geekpatrol.ca/2006/09/32-bit-vs-64-bit-performance/>
>>
>> I don't understand why they say that 5 instructions are needed for
>> constants in 64-bit binaries. Can't the PowerPC load the constant
>> from the memory with a single instruction? This is the solution
>> chosen on the ARM for complex constants (if they are in the cache,
>> this should be fast enough). But many constants are simple enough
>> to be loaded with a single instruction (on the ARM, these are 8-bit
>> values rotated by an even number of positions), in particular after
>> optimizing the code.
>>
>
> If I remember correctly, all powerpc instructions have a length of  
> 32 bit.
> Given that you need some bits for the opcode, a mere 16 bit remain  
> to stuff a constant value to it (for the load high/add intermediate  
> instructions).
> So, for a 64 bit value to load, you need to do a
> -2x loadhigh (2x high 16 bit)
> -2x add immediate (2x low 16 bit)
> -1x some combine statement (some shift operation or whatever)
>
> Keep in mind that these 64 bit constants only cost you for pointers.  
> If you want a 32 bit integer, you don't need to load 64 bit -- even  
> in 64 bit mode.
>


Oh, and don't forget that 64 bit Intel code is actually most often  
faster than 32 bit code, thanks to the double amount of registers and  
some other goodies;
I've compiled a 32bit/64bit universal `bzcat' and ran both version  
three times on my Core 2 Duo machine. For this randomly chosen (!)  
benchmark, I get an impressive edge of ~20% for 64 bit mode:

$ time ./bzcat-64 gcc-core-4.3-20071214.tar.bz2 >/dev/null
real	0m5.889s
user	0m5.168s
sys	0m0.096s

$ time ./bzcat-64 gcc-core-4.3-20071214.tar.bz2 >/dev/null
real	0m5.516s
user	0m5.120s
sys	0m0.088s

$ time ./bzcat-64 gcc-core-4.3-20071214.tar.bz2 >/dev/null
real	0m5.489s
user	0m5.137s
sys	0m0.085s

$ time ./bzcat-32 gcc-core-4.3-20071214.tar.bz2 >/dev/null
real	0m7.407s
user	0m6.707s
sys	0m0.107s

$ time ./bzcat-32 gcc43/gcc-core-4.3-20071214.tar.bz2 >/dev/null
real	0m6.966s
user	0m6.540s
sys	0m0.097s

$ time ./bzcat-32 gcc43/gcc-core-4.3-20071214.tar.bz2 >/dev/null
real	0m7.051s
user	0m6.583s
sys	0m0.103s


Regards,

-Markus


PS: To force the system to run `xy' in 32/64 bit mode (or even  
Rosetta), just make a copy of the executable with just the arch you  
want, e.g. `lipo -extract i386 bzcat -output bzcat-32' to extract 32  
bit intel code from bzcat and store it into bzcat-32.

--
Dipl. Inf. (FH) Markus W. Weissmann
http://www.macports.org/
http://www.mweissmann.de/