Slow if...then...else x86_64
lazarus / 64-FPC development

----------------------------------------------------------------------
lagprogramming · 2014-09-11 10:29

Hi! I have a couple of questions regarding the following functions:

Code: [Select]function q_const(const x,c:INTEGER):INTEGER;
begin
if c=x then  Result:=1 else  result:=0;
end;

function q_var(var x,c:INTEGER):INTEGER;
begin
if c=x then  Result:=1 else  result:=0;
end;


The first thing I'd like to know is:
Both "var" and "const" imply by reference. Should the produced
assembly code differ between the two functions?

The second thing I'd like to know is:
Code: [Select]
function q_var_jne(var x,c:integer):integer;assembler;register;
asm
movl    (%rsi),%eax
cmpl    (%rdi),%eax
jne     .Lj6
.Ll2:
movl    $1,%eax //RUNS SLOWER
jmp     .Lj9
.Lj6:
movl    $0,%eax
.Lj9:
end;



function q_var_je(var x,c:integer):integer;assembler;register;
asm
movl    (%rsi),%eax
cmpl    (%rdi),%eax
je      .Lj6
.Ll2:
movl    $0,%eax //RUNS SLOWER
jmp     .Lj9
.Lj6:
movl    $1,%eax
.Lj9:
end;

function q_const_jne(const x,c:integer):integer;assembler;register;
asm
        cmpl    %edi,%esi
        jne     .Lj6
.Ll2:
        movl    $1,%eax //RUNS SLOWER
        jmp     .Lj9
.Lj6:
        movl    $0,%eax
.Lj9:
end;

function q_const_je(const x,c:integer):integer;assembler;register;
asm
 cmpl   %edi,%esi
 je     .Lj6
.Ll2:
 movl   $0,%eax //RUNS SLOWER
 jmp    .Lj9
.Lj6:
 movl   $1,%eax
.Lj9:
end;


Is this the correct behavior when using je/jne? It appears like the
code starting the label je/jne points to starts to be executed faster.

----------------------------------------------------------------------
marcov · 2014-09-11 10:48

Afaik what CONST does is architecture dependent. On x86_64 it is not
by REF, afaik on x86 it is.

Guaranteed by ref is CONSTREF

See also http://bugs.freepascal.org/view.php?id=26089

----------------------------------------------------------------------
lagprogramming · 2014-09-11 11:07

I am the reporter of that. I've opened this discussion here because
that report will become harder to be read if I add too much useless
informations.
Thank you for the "CONSTREF" tip.