CODEWITHSUNDEEP
assembly
user1917156
user1917156

Reputation: 3

I need help to understand these assembly instructions

I found this asm instructions who push a parameter into the stack before a call, but i think there is some useless instructions.

mov eax,esi
neg eax
sbb eax,eax
lea ecx,[esp+10h]
and eax,ecx
push eax

Can i just replace theses instructions with the following:

lea ecx,[esp+10h]
push ecx

Upvotes: 0

Views: 660

Answers (2)

Aki Suihkonen
Aki Suihkonen

Reputation: 20057

You can replace those instruction with

lea  eax, [esp+10]
test esi, esi
cmovz eax, esi    ;; push zero (i.e. esi) iff esi==0
push eax

or

       test esi, esi
       jz   skip
       lea  esi, [esp+10]
skip:  push esi

Neg will subtract eax from 0 and that produces carry flag unless eax==0; When carry flag is set, sbb a,a produces -1 (otherwise 0), that is used as a selection mask. It's not completely clear that cmov instruction will be faster than the branch.

Upvotes: 0

paxdiablo
paxdiablo

Reputation: 882586

No, I don't believe what you have is equivalent. The sbb instruction may leave eax as zero (all 0 bits) or negative one (all 1 bits), which will definitely affect what happens to the value pulled from ecx in the and instruction.

It looks like the original code will push either esp+10h or 0, depending on what was in esi to start with.

Upvotes: 1

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