inc vs add

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b1528932

Joined: 21 May 2010
Posts: 287

b1528932 09 Jul 2010, 13:06

what is better? what should i use?

inc can be used with lock
add set more flags

is there a way to make fasm generate inc/dec using modrm version (in 32bit mode)?

09 Jul 2010, 13:06

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 09 Jul 2010, 13:22

b1528932 wrote:

what is better?

There is no such thing as "better". It is just different. But anyhow, what is your metric for determining "better"?

b1528932 wrote:

what should i use?

Probably doesn't matter at all. And there is no "should" either. Up to you what you want to use.

b1528932 wrote:

inc can be used with lock
add set more flags

Yes. inc is also less bytes.

b1528932 wrote:

is there a way to make fasm generate inc/dec using modrm version (in 32bit mode)?

Use db 0x??,0x?? if you want a particular encoding.

09 Jul 2010, 13:22

asmfan

Joined: 11 Aug 2006
Posts: 392
Location: Russian

asmfan 09 Jul 2010, 13:50

"add" can prevent partial flags stall and enhancing reordering

09 Jul 2010, 13:50

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 09 Jul 2010, 14:25

asmfan wrote:

"add" can prevent partial flags stall and enhancing reordering

... in some older variants of one company's CPUs.

09 Jul 2010, 14:25

sinsi

Joined: 10 Aug 2007
Posts: 789
Location: Adelaide

sinsi 09 Jul 2010, 14:39

With a lot of P4's inc or dec reg is slower then add or sub reg,1.
The difference between loop, dec and sub was interesting for code like

Code:

  loop label ;slowest on all intel cpu's

  dec ecx
  jnz label  ;dec is slow on most P4's

  sub ecx,1
  jnz label  ;this was the fastest on P4's

Fastest here means clocks, not time, but the differences were all over the place because of the different CPU's used - everything from a P3 to an i7.
Sometimes the P3 was faster, go figure.

09 Jul 2010, 14:39

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 09 Jul 2010, 16:00

revolution wrote:

b1528932 wrote:
inc can be used with lock
add set more flags
Yes. inc is also less bytes.

And actually ADD can also be used with LOCK.

09 Jul 2010, 16:00

roboman

Joined: 03 Dec 2006
Posts: 122
Location: USA

roboman 09 Jul 2010, 16:26

The general rule I try to follow is is when ever the reg inc or dec is getting used in the next command I'll use add because on some cpu's it has to wait for the inc/dec to clear through and you loose some of the optimization built into some of the cpu's. If it's not getting used right away the add/sub are bigger, so I tend to use inc/dec. But what the heck it's all faster then java, c and basic Smile

09 Jul 2010, 16:26

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