.IF Statement

[tda0626]

I want to test for a negative number so I can convert to an absolute value but would like to not use a conditional like below or jump. How would I do that?
edited for clarification...

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; Check for a negative number
.if ax & 08000h
     neg ax
.endif


Tim

[Vortex]

Hello,

You can check the most significant bit of ax :

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include    \masm32\include\masm32rt.inc

.data

msg1        db 'Postive',0
msg2        db 'Negative',0

.code

start:

    mov   ax,-9
    test  ax,8000h
    jnz   NegResult

    invoke  StdOut,ADDR msg1
    jmp    _exit

NegResult:

    invoke  StdOut,ADDR msg2
   
_exit:

    invoke  ExitProcess,0

END start


In the x86 assembly language, the TEST instruction performs a bitwise AND on two operands. The flags SF, ZF, PF are modified while the result of the AND is discarded. The OF and CF flags are set to 0, while AF flag is undefined.

https://en.wikipedia.org/wiki/TEST_(x86_instruction)

[NoCforMe]

Well, Vortex, the correct (and simplest) answer to the OP's question would have been "yes":

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.if ax & 08000h
    neg ax
.endif
does the same thing as TEST (which as you pointed out is a non-destructive AND).

[tda0626]

I want to do it without branching.

What about this?

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.if ax > 08000h
add ax, 1        ; add 1 since we get a 1s compliment from below
.endif
mov bx, ax
sar bx, 15
xor ax, bx


[NoCforMe]

I want to do it without branching.

What about this?

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.if ax > 08000h
        add ax, 1        ; add 1 since we get a 1s compliment from below
    .endif


Well, that's a branch right there.
(1s complement)

[jj2007]

The shortest (and usually fastest) version:

test ax, ax
.if Sign?
   ...

[daydreamer]

I want to do it without branching.

What about this?

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.if ax > 08000h
        add ax, 1        ; add 1 since we get a 1s compliment from below
    .endif
    mov bx, ax
    sar bx, 15
    xor ax, bx


Check replace j** with set** after cmp or test
Set** sets a register to 1 or 0 depending on cmp result instead  j**

[jj2007]

Set** sets a register to 1 or 0 depending on cmp result

That is correct, and I've used the "branchless" set* instruction sometimes. However, it's quite limited, as a) it sets the byte, not the DWORD, i.e. al, not eax, and b) you get only 0 or 1. If that is sufficient, perfect.

[sinsi]

I want to test for a negative number and can't seem to find any documentation on it but was wondering if this was ok.

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; Check for a negative number
.if ax & 08000h
     neg ax
.endif


Tim

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  mov dx,ax
  neg dx
  bt ax,15
  cmovc ax,dx


[HSE]

Hi all!

So far I understand, branchless means that you don't use any conditional instruction.

See code:

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include    \masm32\include\masm32rt.inc

.data
    msg1        db 'Postive',0
    msg2        db 'Negative',0
    table  dd offset msg1
           dd offset msg2
.code

start:
    mov eax,  0
    mov  ax, -9        ; value to test
    shr  ax, 15

    lea edx, table
    mov ecx, [edx+eax*4]

    print ecx, 13,10

    inkey

    invoke  ExitProcess,0

END start
In this case I used Vortex example. But usually table contains address to jump.

Regards, HSE.

[sinsi]

I figured branchless = no jumps

[NoCforMe]

I figured branchless = no jumps

Correctamundo.

I believe this is the canonical easiest way to implement a branch-less abs(), which is what the OP is after (16-bit version):

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  CWD               ;Sign-extend AX to DX:AX
  XOR  AX, DX
  SUB  AX, DX


[sinsi]

Couldn't see the forest for the trees, was blinded by the question and didn't follow the code.

Did you mean CWD?

[NoCforMe]

Whoops, yeah: should be

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  CWD              ;Sign-extend AX to DX:AX
  XOR  AX, DX
  SUB  AX, DX

[HSE]

I believe this is the canonical easiest way to implement a branch-less abs(), which is what the OP is after (16-bit version):

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  CBW              ;Sign-extend AX to DX:AX
  XOR  AX, DX
  SUB  AX, DX


More easy:

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and ax, 7FFFh