[fpc-pascal] Records

Vinzent Hoefler JeLlyFish.software at gmx.net
Wed Oct 26 12:26:56 CEST 2005

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 

> 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. ;-)


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