add or inc

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DustWolf

Joined: 26 Jan 2006
Posts: 373
Location: Ljubljana, Slovenia

DustWolf 16 Aug 2006, 23:30

Hello,

What is faster (when done in a loop), this:

Code:

add esi,4

or this:

Code:

mov edx,4 ;outside the loop
...
add esi,edx ;in the loop

or this:

Code:

inc esi
inc esi
inc esi
inc esi

?

Thanks for the answer. Smile

16 Aug 2006, 23:30

UCM

Joined: 25 Feb 2005
Posts: 285
Location: Canada

UCM 16 Aug 2006, 23:53

The first is the fastest, winning the second by 1 clock (the mov instruction), and winning the 3rd by 3 clocks for each time it the loop is executed. All "add" and "inc" instructions take one cycle.

16 Aug 2006, 23:53

DustWolf

Joined: 26 Jan 2006
Posts: 373
Location: Ljubljana, Slovenia

DustWolf 16 Aug 2006, 23:56

UCM wrote:

The first is the fastest, winning the second by 1 clock (the mov instruction), and winning the 3rd by 3 clocks for each time it the loop is executed. All "add" and "inc" instructions take one cycle.

Yes but the Add eax,4 needs to wait for the system memory to pass on the 4, right?

16 Aug 2006, 23:56

UCM

Joined: 25 Feb 2005
Posts: 285
Location: Canada

UCM 17 Aug 2006, 01:28

No, since it is loaded with the rest of the instruction.

17 Aug 2006, 01:28

r22

Joined: 27 Dec 2004
Posts: 805

r22 17 Aug 2006, 03:16

If you want FASTER unroll the loop and do an add esi,8 or 12 or 16

In my experience you can get a 10-20% speed increase by unrolling a loop 2-5x.

17 Aug 2006, 03:16

MazeGen

Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia

MazeGen 17 Aug 2006, 11:06

UCM wrote:

All "add" and "inc" instructions take one cycle.

INC and DEC take 2 uops on P4.

17 Aug 2006, 11:06

UCM

Joined: 25 Feb 2005
Posts: 285
Location: Canada

UCM 17 Aug 2006, 12:49

I was looking at the AMD cycle charts Wink

17 Aug 2006, 12:49

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 18 Aug 2006, 14:27

Where can/do you get cycle charts from?

18 Aug 2006, 14:27

UCM

Joined: 25 Feb 2005
Posts: 285
Location: Canada

UCM 18 Aug 2006, 14:42

It is an appendix of the AMD Software Optimization Manual, publication number 22007 (revision K.) It has many optimization tips as well. In addition, for more recent charts, see the "Software Optimization Guide for AMD64 Processors", publication number 25112.

18 Aug 2006, 14:42

MazeGen

Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia

MazeGen 18 Aug 2006, 15:20

cod3b453 wrote:

Where can/do you get cycle charts from?

http://www.agner.org/optimize/#manuals

Get number 4. It contains details for P1, PMMX, PPro, PII, PIII, P4, PM, Core2, P4E, AMD64 Smile

18 Aug 2006, 15:20

cod3b453

Joined: 25 Aug 2004
Posts: 618

cod3b453 18 Aug 2006, 15:39

Thanks UCM & MazeGen! Cool

18 Aug 2006, 15:39

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