On Wednesday 26 October 2005 10:06, Micha Nelissen wrote: > Vinzent Hoefler wrote: > >>Oh right! Something like a masked compare or so...probably not > >>implementable in an efficient way indeed I guess. > > > > Whatever you call efficient. A simple memory compare won't do, as > > others already pointed out, and everything else depends on the > > actual layout of the structure. > > Your structure is very short, code length will explode for larger > structures.
Yes, of course, especially when the structures do not contain large fields of contiguous data which can be compared with "repe cmpsd" or whatever equivalents other processors besides x86 might have. But that would - of course - also be the case, if you write your own comparison function in the higher level language (without specialized typecasts). > It's inefficient compared to "REPE CMPSD" with proper > context, this is only a few lines. Precisely. But that's just the way it has to be, there's no way around it unless you assume that unused padding bits/bytes always have the same value (which could be a wrong assumption in quite a lot of cases - try reading such values from an external hardware device with unused connections left floating). For these sort of things, it will be as inefficient as it can get either way, no matter if you let the compiler do the work or if you come up with your own comparison function. > > andl $56, %eax > > andl $56, %edx > > cmpb %al, %dl > > je .L49 > > .L42: > > movl %esi, %eax > > popl %edx > > popl %ebx > > popl %esi > > ret > > What does this part do btw? Compare lengths? No, it masks off the unused bits (Padded_Record.A is located in the middle of an octett like this XXX---XXX) and then does the comparison, I would suppose. It falls through directly to the exit code (restoring saved registers, and then returning the value) at .L42 if the comparison already fails. I really meant it, when I wrote, that the structure I told the compiler to build up is quite nasty. ;-) Vinzent. -- public key: http://www.t-domaingrabbing.ch/publickey.asc
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