optimization

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FlashBurn

Joined: 06 Jan 2005
Posts: 87

FlashBurn 23 Jul 2005, 10:07

I like to know which code I should use!

Code:

mov esi,ADDR
mov eax,[esi+foo_t.data]

Code:

mov eax,[ADDR+foo_t.data]

23 Jul 2005, 10:07

Matrix

Joined: 04 Sep 2004
Posts: 1164
Location: Overflow

Matrix 23 Jul 2005, 14:34

hi FlashBurn

use which better suite your code,

if you need to use esi then use it...

but notice the difference

Code:

mov esi,ADDR 
mov eax,[esi+foo_t.data]

Code:

mov esi,[ADDR]
mov eax,[esi+foo_t.data]

Code:

dec [ADDR]
mov esi,[ADDR]
inc esi
mov eax,[esi+foo_t.data]

Code:

inc [ADDR]
mov esi,ADDR 
dec esi
mov eax,[esi+foo_t.data]

23 Jul 2005, 14:34

El Tangas

Joined: 11 Oct 2003
Posts: 120
Location: Sunset Empire

El Tangas 23 Jul 2005, 17:26

This kind of code, when you change a register just before using it as part of an adress, can generate a penalty called Address Generation Interlock (AGI), causing the processor to stall for 1 cycle.

Code:

mov esi,ADDR
mov eax,[esi+foo_t.data]

So, if you can choose, this is better.

Code:

mov eax,[ADDR+foo_t.data]

23 Jul 2005, 17:26

james

Joined: 07 Sep 2005
Posts: 45
Location: Australia

james 08 Sep 2005, 09:10

But are they really equivilant ?

08 Sep 2005, 09:10

Matrix

Joined: 04 Sep 2004
Posts: 1164
Location: Overflow

Matrix 08 Sep 2005, 14:11

james,
they are fully exchangeable, the difference is :

this uses register (dynamic)

Code:

mov esi,ADDR 
mov eax,[esi+foo_t.data]

this uses immediate value (fix)

Code:

mov eax,[ADDR+foo_t.data]

08 Sep 2005, 14:11

shoorick

Joined: 25 Feb 2005
Posts: 1614
Location: Ukraine

shoorick 08 Sep 2005, 14:18

but first alter esi

08 Sep 2005, 14:18

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7103
Location: Slovakia

vid 08 Sep 2005, 14:58

??? i don't understand, if ADDR is constant value (it has to be for "mov esi,ADDR" to work), and "foo_t.data" is constant value too, then "ADDR + foo_t.data" is also constant value, so you are asking on difference between

Code:

mov esi,const
mov eax,[esi + const]

and

Code:

mov eax,[const]

so difference is clear. sorry if i misunderstood something

08 Sep 2005, 14:58

Vasilev Vjacheslav

Joined: 11 Aug 2004
Posts: 392

Vasilev Vjacheslav 08 Sep 2005, 16:04

for dynamic value:

Code:

lea esi,[stack_structure]
mov eax,[esi+stack_structure.somevalue]

for immediate value:

Code:

mov eax,[constant_structure.somevalue]

ps. they are totally different

_________________
[not enough memory]

08 Sep 2005, 16:04

james

Joined: 07 Sep 2005
Posts: 45
Location: Australia

james 08 Sep 2005, 20:56

Hmmmn. I thought you had to dereference the addr first (ie: [ADDR])
then make an adjustment for the field you need.

Am I thinking that ADDR is a constant and not a pointer ?

08 Sep 2005, 20:56

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