A simple way to optimise INC and DEC for the P4

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revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17889
Location: In your JS exploiting you and your system

revolution

Code:

macro inc v {add v,1}
macro dec v {sub v,1}

Of course, you need to make sure you do not need to preserve the carry flag!

07 Nov 2004, 03:16

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid

someone could create optimization macros for particular processors

07 Nov 2004, 19:31

Matrix

Joined: 04 Sep 2004
Posts: 1171
Location: Overflow

Matrix

revolution wrote:

Code:
macro inc v {add v,1}
macro dec v {sub v,1}
    
Of course, you need to make sure you do not need to preserve the carry flag!

Code:

macro inc v 
{
pushf
add v,1
popf
}
macro dec v 
{
pushf
sub v,1
popf
}

is it a speed optimization? its bigger in size.

07 Nov 2004, 19:42

beppe85

Joined: 23 Oct 2004
Posts: 181

beppe85

It's not optimization if it alter meaning of an instruction. In the first case(revolution), you need to preserve CF. In the second(Matrix), you need to not preserve any but CF. Also, touching the stack may slow down things and disrupt programmer intention.

I think if there's a better way to do in some circunstances, you do not do it by macros.

07 Nov 2004, 23:51

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17889
Location: In your JS exploiting you and your system

revolution

Beppe85: Yeah, I already mentioned about the carry, but I find very rarely do I actually need to preserve the CF.

Matrix: using add and sub are faster on the P4 because it modifies all the flags, but inc/dec preserve CF. So in the P4 inc/dec stall until the flags are updated.

08 Nov 2004, 01:51

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