plans for 64bit support (Joshua Root)

[Markus Weissmann]

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.


Regards,

-Markus

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