Why my C compiler puts nop instructions?

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vivik

Joined: 29 Oct 2016
Posts: 671

vivik 24 Dec 2016, 09:02

I noticied this chain of instructions in my program:

Code:

0040117D  |.^EB AD          JMP SHORT 0040112C
0040117F  |> 90             NOP
00401180  |. 8B45 9C        MOV EAX,[EBP-64]

Why is it here? It's after jmp, and before the rest of the function, and there is one jump to this nop from above. It's either has something to do with alignment (doesn't look like it, overwise jump would happen to 401180, but now it happens to 40117f), or something with how processor works.

24 Dec 2016, 09:02

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20516
Location: In your JS exploiting you and your system

revolution 24 Dec 2016, 10:13

It is the assembler that does this, not the HLL compiler. Most probably the C compiler is using a two pass assembler as the back end.

24 Dec 2016, 10:13

AsmGuru62

Joined: 28 Jan 2004
Posts: 1693
Location: Toronto, Canada

AsmGuru62 24 Dec 2016, 15:49

Also notice the address 00401180 - it should be a branch to it from somewhere.
So, NOP is most likely for branch address alignment.

24 Dec 2016, 15:49

vivik

Joined: 29 Oct 2016
Posts: 671

vivik 24 Dec 2016, 19:34

AsmGuru62 wrote:

Also notice the address 00401180 - it should be a branch to it from somewhere.
So, NOP is most likely for branch address alignment.

Yes, that's the weird part: there is a jump to 401179, but not to 401180.

Maybe it's just a bug.

24 Dec 2016, 19:34

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20516
Location: In your JS exploiting you and your system

revolution 24 Dec 2016, 22:52

If you look at the intermediate .asm / .s files created by the compiler for the assembler then the reason for the NOP can be determined.

But make sure you enable Intel mode, not AT&T mode, else your eyes will bleed.

24 Dec 2016, 22:52

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