Am 06.11.2013 18:52, schrieb Jim Leonard:
(Sorry if this isn't technical enough for the developer list, but it
requires understanding of the underlying architecture so I thought it
was appropriate.) I'm implementing something in FPC that will perform
very many comparisons across sets of data, and would like to know
whether or not FPC is more efficient with array indexing calculations
vs. doing the pointer math myself. For example, I have a choice of
doing either this:
for b:=0 to num-1 do
diff:=src^[b]-dst^[b];
Don't you mean "src[b]-dst[b]"? At least that would be the correct code
if you'd use FPC's implicit array indexing.
...or this:
for b:=0 to num-1 do begin
diff:=src^-dst^;
inc(src);
inc(dst);
end;
I read a few posts from 2010 and a post from 2013 that implies FPC's
array index calcs are just a MUL and suboptimial; is this still the
case? Which method produces faster code? A third way would be to
copy data into a dynamic array and no pointer use at all, ie.
"diff:=src[b]-dst[b];"... is this the fastest option?
The resulting instructions depend highly on the architecture. E.g. on
x86 and m68k for the first case a MOV/MOVE instruction with a multiplier
will be used while on other platforms this might involve an explicit
multiplication. Also it will depend on the optimization level you use
(e.g. on x86 the generated code for the first case is very different
between -O2 and -O3). In the end you'd need to calculate the instruction
timings to know whether one of the two is really faster than the other
(and then you can compare that later on with your own implementation to
check whether your code or the compiler's is faster ^^)
Regards,
Sven
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