Re: 64 bit runs slower than 32 bit?
Lester Caine wrote:
That 64bit runs slower than 32bit has always niggled me
Only having had replies to this off-list I though I'd follow up since I have been trying to do my homework. The last time I addressed this area it was moving code from 16bit to 32bit on 80386 processors. I've just found the books out ...
Optimizing code to run on 64bit hardware is not simply increasing the word sizes and checking that buffers are correctly aligned for 64 bit boundaries, it is also ensuring that processes can be optimised where appropriate. It may be that the current view is that this is the compilers problem? That seem to be the gist of what I am being told, but the compilers can only work with what they are supplied, and if there wrong process is selected they are not going to change it to a better one? Setting a buffer to 7 bytes and then using the 'spare' byte for something else for example? Using a process that has byte by byte requirements but does not actually then use the data generated.
The timing attack analysis is a good example of this and an area where a slight change to the process in general may give speed improvements even on the 32bit platform. memcpy/strcpy can be optimised by the compiler to use double word or quad word machine code instructions, but only if there is no byte/word restricted elements to the process. ( I have not invstigated IF compilers do achieve the best result ). The timing attack analysis has shown that this is the problem area but by not using memcpy so that the largest word base can be used for the comparison then the problem is managed better. Adding a conversion to provide a -1/0/1 result effectively blocks optimising even if only a 0/1 is finally used!
The windows platform should benefit from the same rationalisations, however it does seem as if the 64 bit builds are designed to optimise memcmp() as long as buffers are multiple of 8 bytes. 32 bit builds work the same as linux ...
Certainly some other platforms will have different requirements to optimise, but as part of the optimization of 64 bit support on Intel/Amd processor it would be useful to identify this base level functions which would benefit from closer inspection rather than the simple assumption that the time involved is not worth worrying about? If there is a difference in the 6th byte of a string, on a 64bit processor it will have to scan all 6 bytes, while optimised for 32bit on a 32bit processor, it would only scan 2. This sort of extra processing may account for a 64 bit setup being slower, and if the byte compare is not even needed, then both operations could be improved? Obviously we have differing pointer sizes which need moving around, but one is moving twice as much data per memory cycle ... if you have the correct memory setup.
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Lester Caine - G8HFL
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