Optimize code size

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Vitor_boss

Joined: 04 Jun 2012
Posts: 3
Location: Brazil

Vitor_boss 27 Jun 2012, 02:58

First of all I wish to tanks for this greatest compiler.
I got started on assembly language, I'm studying some codes, for a test I've copy a code to compare memory bytes and I saw size difference of compiled code, so I'll show the source and compiled one:

How you can see there is a bigger instruction size on memory manipulations. I think 60-80% of any application use memory R/W, this makes the output code 10-30% bigger.

I'm compiler with "format PE" on ASM code, there is something I'm doing wrong or forgotten?

27 Jun 2012, 02:58

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20445
Location: In your JS exploiting you and your system

revolution 27 Jun 2012, 05:16

When you access memory using the ESP register you have to use the SIB format for addressing. This is limitation of the x86 instruction design. Using EBP instead of ESP will make the code smaller.

27 Jun 2012, 05:16

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 29 Jun 2012, 07:59

I checked that its also true in 64-bit land:

Code:

use32
push ebp
mov  ebp,esp
cmp  dword[ebp+16],0
align 16
use64
push rbp
mov  ebp,esp
cmp  dword[rbp+16],0

These both result in exactly the same 7-byte sequence. esp/rsp adds 1 byte to both.

29 Jun 2012, 07:59

rugxulo

Joined: 09 Aug 2005
Posts: 2341
Location: Usono (aka, USA)

rugxulo 05 Jul 2012, 21:59

Quote:

push ebp
mov ebp,esp
cmp [esp+arg_8],0
mov ecx,[esp+arg_4]
mov eax,[esp+arg_0]
jnz short loc_40101F
xor eax,eax
inc eax
pop ebp
retn

Hopefully this is obvious, but just in case ... you didn't actually use EBP here at all! So why push/pop it?? (Hmmm, perhaps because you sometimes jump further on, but it seems mostly silly and useless. Please keep this in mind as a lot of compilers don't omit such stack frame setups even when not needed.)

05 Jul 2012, 21:59

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