How many FLAGS can you juggle at once?

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baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 27 Nov 2008, 18:44

Given flags set by cmp al, 1, which values of al can we distinguish with ordinary jccs? Let's see…

Code:

   al     al-1  OSZAPC
   00      FF   010111
   01      00   001010
 02..7F  01..7E 000..0
   80      7F   100100
 81..FF  80..FE 010..0

So we can use something like

Code:

        cmp     al, 1
        jc      al_00
        jz      al_01
        jo      al_80
        js      al_81_FF
al_02_7F:

to identify three values and two ranges. But, there is more! Let alone AF (no cc to check it and aaa/aas/daa/das are invalid in 64-bit mode), FLAGS contains not-so-often-used PF. Now we try

Code:

       cmp     al, 2

   al     al-2  OSZAPC
   00      FE   010101
   01      FF   010111
   02      00   001010
 03..7F  01..7D 000..0
   80      7E   100110
   81      7F   100100
 82..FF  80..FD 010..0

Five values and two ranges with one cmp and several jccs? It's good.

Could this be improved? Your suggestions are welcome.

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27 Nov 2008, 18:44

bitRAKE

Joined: 21 Jul 2003
Posts: 4060
Location: vpcmpistri

bitRAKE 27 Nov 2008, 20:54

Very nice.

If we look at it from the perspective of X = ( (code + data) bytes / branch targets) then as branches -> inf, X -> 1. What is the smallest number of branches where X < 2?

(Using same initial assumption of variable in register. Also targets cannot be sequential or direct mapping because then we could just use JMP EAX, or silly stuff like NEG EAX/JMP EAX.)

For amd64, 17 branches minimum:

Code:

mov al,[tab+rax]
lea rax,[base+rax*8]
jmp rax

...16 branches if all targets in low memory 2^32.

27 Nov 2008, 20:54

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 27 Nov 2008, 21:46

bitRAKE,

Is your tab 4 GiB in size? Wink

xlat for sure. Wink

I'm talking about understanding of [RE]?FLAGS.

I have an essay about flags, around 10…15 lines, explaining them a bit. Would it be useful to post it?

27 Nov 2008, 21:46

bitRAKE

Joined: 21 Jul 2003
Posts: 4060
Location: vpcmpistri

bitRAKE 27 Nov 2008, 22:01

Something like:

Code:

sahf ; why cmp?
ja  _..0..0
.
.
.
(many more branches...)

_..0..0:
jge _OS0..0 ; =
jno _010..0
jnp _100.00 ; X
jmp _100.10 ; X

_OS0..0:
jno _000..0
jnp _110.00 ; X
jmp _110.10 ; X

_000..0:
jnp _000.00 ; X
jmp _000.10 ; X

_010..0:
jnp _010.00 ; X
jmp _010.10 ; X

...is not a solution?
(bytes greater than two per branch target)

No tab is only 15/16 bytes. XLAT is good, but requires (E/R)BX which is additional byte to load register. What to limit input values, huh?

Last edited by bitRAKE on 27 Nov 2008, 22:11; edited 1 time in total

27 Nov 2008, 22:01

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 27 Nov 2008, 22:10

bitRAKE,

With lahf you'll lose 3 bits of ah, definitely. Test for AF will be complicated too. Wink

Not to mention #UD in 64-bit mode occasionally…

Last edited by baldr on 27 Nov 2008, 22:24; edited 2 times in total

27 Nov 2008, 22:10

bitRAKE

Joined: 21 Jul 2003
Posts: 4060
Location: vpcmpistri

bitRAKE 27 Nov 2008, 22:13

bah, SAHF -- not LAHF.

27 Nov 2008, 22:13

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 27 Nov 2008, 22:16

bitRAKE,

Same mistake. Looked up sahf and wrote lahf…

27 Nov 2008, 22:16

bitRAKE

Joined: 21 Jul 2003
Posts: 4060
Location: vpcmpistri

bitRAKE 28 Nov 2008, 02:06

baldr wrote:

I have an essay about flags, around 10…15 lines, explaining them a bit. Would it be useful to post it?

Personally, I think this is the number one reason people use MASM like syntax - they don't know when to use what branches. Remembering what instructions effect which flags and why OF<>SF? A whole chapter on x86 program flow control would be a great asset to the community, imho. Could cover general purpose, FPU, and SIMD patterns.

_________________
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28 Nov 2008, 02:06

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