[fpc-devel] Generated code of constant parameters of double type

[Florian Klaempfl]

Alexander Klenin schrieb:
> On Mon, Nov 17, 2008 at 05:39, Florian Klaempfl <florian at freepascal.org> wrote:
>> No, just look at the actual assignment:
>> i:=v;
>> By const/reference it generates 5 instructions, by value only 4. The rest is
>> entry code. If you've only one access, passing by reference might have an
>> advantage, the more accesses you have, the better direct passing is.
> 
> This is true only if the code is compiled without optimization. At -O2 level,
> the access code is indeed identical from performace POV.
> 
>> Just benchmark a loop ;)
> I did, see attachment. On the random old PC it gives me:

Yes, the compiler can hide in the this case the additional load because 
the procedure is very simple. Also alignment is part of the game, but 
the main point is that in the ref. case the compiler can use a stack 
pointer based stack while in the other case it needs a frame pointer 
based stack.

It simply depends on the procedure code (and the architecture!) what's 
faster

> By val = 1.47s
> By ref = 0.72s
> 
> By increasing the value of NR constant, you can diminish speed difference,
> but the by-ref case is always faster.
> 
> Anyway, for me the true reason for always using by-ref convention for
> const parameters
> is semantic consistency and Delphi compatibility.
> 
> 
> 
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