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NEON conditional execution

Laurent Ovaert
Note: This was originally posted on 17th February 2012 at http://forums.arm.com

Hi, I would like to perform the following operation simultaneously on 8x16bits registers:

Q0 = Q0 + Q1
if (Q0 >= Q2) Q0 = Q3

I am not clear if this is possible to do that ?
In normal mode, I know it is since the mov can be conditional, but in SIMD I don't know.
Parents
  • Note: This was originally posted on 17th February 2012 at http://forums.arm.com

    Thank you Marcus, so is this the common way to do "conditional" execution, using the VBIT instruction ?
    I can see all comparison instructions gives either all ones or all zeros, i was wondering what I could do with that, VBIT seems to make sense.

    If i wanted to do:

    [color=#222222][size=2]Q0 = Q0 + Q1[/size][/color]
    [color=#222222][size=2]if (Q0 >= Q2) Q0 = Q0 - Q3[/size][/color]
    [size=2]
    [/size]
    [size=2]Since there is no conditional instructions in SIMD mode, is the idea to always compute Q5=Q0-Q3 and then do the compare and use vbit to assign Q5 to Q0 via VBIT ?[/size]
    [size=2]It seems not as efficient as if it was a true conditional execution, but I don't see any other way.[/size]
    [size=2]
    [/size]
    [size=2]Is there some general guidelines or tricks on how to efficiently use the way the VCXX instructions work ?[/size]
    [size=2]
    [/size]
Reply
  • Note: This was originally posted on 17th February 2012 at http://forums.arm.com

    Thank you Marcus, so is this the common way to do "conditional" execution, using the VBIT instruction ?
    I can see all comparison instructions gives either all ones or all zeros, i was wondering what I could do with that, VBIT seems to make sense.

    If i wanted to do:

    [color=#222222][size=2]Q0 = Q0 + Q1[/size][/color]
    [color=#222222][size=2]if (Q0 >= Q2) Q0 = Q0 - Q3[/size][/color]
    [size=2]
    [/size]
    [size=2]Since there is no conditional instructions in SIMD mode, is the idea to always compute Q5=Q0-Q3 and then do the compare and use vbit to assign Q5 to Q0 via VBIT ?[/size]
    [size=2]It seems not as efficient as if it was a true conditional execution, but I don't see any other way.[/size]
    [size=2]
    [/size]
    [size=2]Is there some general guidelines or tricks on how to efficiently use the way the VCXX instructions work ?[/size]
    [size=2]
    [/size]
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