flat assembler - [fasmg] polynomials - how to distinguish between A+A and A*2

Index > Macroinstructions > [fasmg] polynomials - how to distinguish between A+A and A*2

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zhak

Joined: 12 Apr 2005
Posts: 501
Location: Belarus

zhak 09 Mar 2017, 21:48

Tomasz, I applied some ideas you implemented in x86 instruction set macros for parsing memory operands. For example, using polynomials

Code:

repeat elementsof addr
  meta = addr metadata %
  if meta relativeto x86.reg32
    expr = expr + addr element % * addr scale %
  end if
end repeat

However, there's a problem. Let's take two 32-bit instructions:

Code:

inc byte [eax + eax]
inc byte [eax * 2]

Well, they do the same, but instruction encoding differ in modrm/sib part.

Code:

[eax + eax] => 0x04 0x00
[eax * 2]   => 0x04 0x45 0x00000000

I'm afraid there's no way to distinguish between A + A and A * 2 in expression, am I right? The way I see it can be achieved is by `match`ing, but I would like to avoid this for obvious performance reasons. The easiest way would, of course, be using always A + A form, but I'd like to keep macros as 'achieve what you're trying to'.

09 Mar 2017, 21:48

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8266
Location: Kraków, Poland

Tomasz Grysztar 09 Mar 2017, 22:18

Yes, there is no way to distinguish them, and you have to use explicit MATCH if you need to differentiate them.

On a side note: the (much simpler) implementation of register algebra in fasm 1 also did conflate these variants into the same result.

Depending on your constraints you could also try some other tricks. Let me think a moment...

09 Mar 2017, 22:18

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8266
Location: Kraków, Poland

Tomasz Grysztar 09 Mar 2017, 22:30

One peculiar trick that comes to my mind is to define registers as symbolic values to break the algebraic symmetry:

Code:

eax equ ea1+ea2

element ea1 : 'ea1'
element ea2 : 'ea2'

macro showpoly value
        local tmp
        tmp = value
        repeat 1, a : tmp scale 0
                display `a
        end repeat
        repeat elementsof tmp
                display '+',string tmp metadata %,'*'
                repeat 1, a: tmp scale %
                        display `a
                end repeat
        end repeat
        display 13,10
end macro

showpoly eax+eax        ; ea1+ea2+ea1+ea2
showpoly eax*2          ; ea1+ea2*2
showpoly 2*eax          ; ea1*2+ea2

Whether this is a viable solution depends on your macro framework.

09 Mar 2017, 22:30

zhak

Joined: 12 Apr 2005
Posts: 501
Location: Belarus

zhak 09 Mar 2017, 23:07

Thanks for the idea! Interestings, but pretty complex. What's more important, I was afraid of matching for nothing. Only one additional match (and one if were enough): If you know that expr scale 2 = 0 & expr scale 1 = 2 for expression Scale * Index + Base + Displacement (which I compute earlier), only match a*b, addr is enough to solve this case

09 Mar 2017, 23:07

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 19871
Location: In your JS exploiting you and your system

revolution 10 Mar 2017, 00:38

Matching is fraught with problems.

Code:

mov eax,[eax+eax+4*2]
mov eax,[4+4+eax*2]

There are so many ways to form valid expressions.

10 Mar 2017, 00:38

zhak

Joined: 12 Apr 2005
Posts: 501
Location: Belarus

zhak 10 Mar 2017, 08:13

Indeed, you're right. The first case I missed

10 Mar 2017, 08:13

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8266
Location: Kraków, Poland

Tomasz Grysztar 10 Mar 2017, 09:24

I'd think that you'd need to recognize just a few "special" expressions, since only then one could really expect to get a strictly defined result. That is: you could expect the address to be written exactly as "base+index*scale+displacement" or "index*scale+displacement" and only then enforce the strictly corresponding encoding (this is a bit similar to how the AT&T syntax in GAS handles it), while for any other free-form expression a good fail-safe would be to evaluate it algebraically and then optimize the instruction output just like fasm 1 does it.

If you like your match a*b, addr solution, then you could additionally check if "a" is a register. If we agree that only "eax*2" should be treated specially and variants "(eax+0)*2" should be evaluated and optimized, you'd need to detect if something is exactly a token corresponding to register. I have at least one trick that can do it:

Code:

__eax equ **

match a*b, addr
    match **, __#a

10 Mar 2017, 09:24

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