Fibonacci sequence in "2 instructions"

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moveax41h

Joined: 18 Feb 2018
Posts: 59

moveax41h 25 Aug 2018, 18:44

I saw a post which stated that the Fibonacci sequence can be computed using 2 asm instructions... However, when I implemented it, I realized that the registers must also be reset so is it really the case that the sequence can occur using only 2 instructions:

Code:

format PE console
entry main
main:
        xor eax, eax
        mov edx, 1
        .loop:
        xadd eax, edx
        loop .loop

Is there a more compact way to achieve this? The only way I can think of is if the OS hands off execution to the program with the guarantee that eax is 0 and edx is 1. But even still, we need to be concerned with ecx as well or the loop instruction may malfunction.[b/]

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25 Aug 2018, 18:44

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20308
Location: In your JS exploiting you and your system

revolution 25 Aug 2018, 20:55

If REP could be used with arbitrary instructions then we could do this:

Code:

rep xadd eax,edx

But it won't work.

25 Aug 2018, 20:55

rugxulo

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

rugxulo 26 Aug 2018, 02:05

revolution wrote:

If REP could be used with arbitrary instructions ...

It can, but it won't do what you think. Older cpus will ignore such prefixes. And it has been used to pair with or extend other instructions. "rep ret" (AMD optimization, replaced by "ret 0" in some later cases) or SSE2's "pause" (aka, "rep nop") or even SSE4's "lzcnt" (which IIRC is just "rep bsr").

(You probably were already aware of this. I don't claim to know all such corner cases.)

26 Aug 2018, 02:05

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20308
Location: In your JS exploiting you and your system

revolution 26 Aug 2018, 04:09

I think that in those cases Intel consider the entire byte sequence to be a single instruction. So it isn't being used as a REP prefix, but instead a reuse of the REP byte to make a new instruction.

It is also possible that in the future the REP XADD byte sequence might but redefined as some new instruction also.

26 Aug 2018, 04:09

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