# Fast abs/neg/sign for 32bit floats¶

• Author or source: ed.bew@raebybot
• Type: floating point functions
• Created: 2002-12-18 20:27:04
notes
```Haven't seen this elsewhere, probably because it is too obvious? Anyway, these functions
are intended for 32-bit floating point numbers only and should work a bit faster than the
regular ones.

fastabs() gives you the absolute value of a float
fastneg() gives you the negative number (faster than multiplying with -1)
fastsgn() gives back +1 for 0 or positive numbers, -1 for negative numbers

Cheers

Toby (www.tobybear.de)
```
code
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19``` ```// C/C++ code: float fastabs(float f) {int i=((*(int*)&f)&0x7fffffff);return (*(float*)&i);} float fastneg(float f) {int i=((*(int*)&f)^0x80000000);return (*(float*)&i);} int fastsgn(float f) {return 1+(((*(int*)&f)>>31)<<1);} //Delphi/Pascal code: function fastabs(f:single):single; begin i:=longint((@f)^) and \$7FFFFFFF;result:=single((@i)^) end; function fastneg(f:single):single; begin i:=longint((@f)^) xor \$80000000;result:=single((@i)^) end; function fastsgn(f:single):longint; begin result:=1+((longint((@f)^) shr 31)shl 1) end; ```

```Matthias (bekkah[AT]web[DOT]de) wrote me a mail with the following further improvements for the C++ parts of the code:

// C++ code:
inline float fastabs(const float f)
{int i=((*(int*)&f)&0x7fffffff);return (*(float*)&i);}

inline float fastneg(const float f)
{int i=((*(int*)&f)^0x80000000);return (*(float*)&i);}

inline int fastsgn(const float f)
{return 1+(((*(int*)&f)>>31)<<1);}

Thanks!
```
```Too bad these 'tricks' need two additional FWAITs to work in a raw FPU code. Maybe standard fabs and fneg are better? Although, that fastsgn() could be useful since there's no FPU equivalent for it.

Cheers,
Aleksey.
```
```I meant 'fchs' in place of 'fneg'.
```
```I don't know if this is any faster, but atleast you can avoid some typecasting.

function fastabs(f: Single): Single; var i: Integer absolute f;
begin i := i and \$7fffffff; Result := f; end;
```
```Note that a reasonable compiler should be able to perform these optimizations for you. I seem to recall that GCC in particular has the capability to replace calls to [f]abs() with instructions optimized for the platform.
```
```On MS compilers for x86, just do:
#pragma intrinsic(fabs)

...and then use fabs() for doubles, fabsf() for floats. The compiler will generate the FABS instruction, which is generally 1 cycle on modern x86 FPUs. (Internally, the FPU just masks the bit.)
```