signum() aka sgn() function

jason wiseman

Intellectual 675 points

Is there an efficient way to implement the signum function efficiently using compiler intrinsics on the C2000 family?

Assuming x is a 16 bit signed int.

sgn(x) = 1 if x>0, -1 if x<0, and 0 if x=0

Using if statements results in slow code due to the branches/pipeline hits ect.

I'm assuing there is a very clever way to implement this using the sign bit or a right shift with sign extend by 15.

over 14 years ago

0 Archaeologist over 14 years ago

TI__Guru* 84285 points

If you've got a C28x device, try this:

__inline int sgn(int x) { return __max(-1, __min(1, x)); }

[Edit - fixed signs]

0 jason wiseman over 14 years ago in reply to Archaeologist

Intellectual 675 points

Intrinsic saturation to +-1. That is exactly what I need!

(should have been obvious to me)

Thanks!

0 jason wiseman over 14 years ago in reply to Archaeologist

Intellectual 675 points

I think the macro would have the opposite signs:

_min returns the smallest. so it would limit x to 1, or pass a negative value

_max returns the largest value so it would pass 1 or limit a negative x to -1

#define sgn(x) __max(-1, __min(1, x))

0 Tim Wentz over 14 years ago in reply to jason wiseman

TI__Intellectual 1270 points

Is that really more efficient than abusing 2's complement? Just curious to wonder what the power of intrinsics is

sgn = 0x8000 & x

Edit: I guess that would not return an exact -1/ +1 without some more manipulation, but the code it's being used in *might* not even need that (without knowing what the code is)

0 jason wiseman over 14 years ago in reply to Tim Wentz

Intellectual 675 points

Tim,

I was looking for sgn(x)='signum' functionality, which is 1,0,-1 for pos,zero,neg ints

The intrinsics are worth looking into if you need fast code. The _max(,) and _min(,) are very efficient at choosing the larger or smaller value for saturation ect due to the lack of a branch penalty. The compile to MAX and MIN assembly instructions on the C28xx.

Cheers,

Jason

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signum() aka sgn() function