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simd_20signum_20function

SIMD signum function

by Richard Russell, 30th December 2014 (based on a blog post by Raymond Chen)

The signum function returns -1 for all negative numbers, +1 for all positive non-zero numbers and zero for an input of zero; it corresponds to the SGN() function in BBC BASIC. There is no direct equivalent in assembly language, but the following code snippets illustrate how it may be calculated on multiple data values using MMX and SSE2 code. In all cases the input is in mm0 or xmm0 and the output is in mm1 or xmm1:

Signum of eight 8-bit values

```      pxor    mm1, mm1
pxor    mm2, mm2
pcmpgtb mm1, mm0 ; mm1 = pcmpgt(0, x)
pcmpgtb mm0, mm2 ; mm0 = pcmpgt(x, 0)
psubb   mm1, mm0 ; mm1 = signum```

Signum of four 16-bit values

```      pxor    mm1, mm1
pxor    mm2, mm2
pcmpgtw mm1, mm0 ; mm1 = pcmpgt(0, x)
pcmpgtw mm0, mm2 ; mm0 = pcmpgt(x, 0)
psubw   mm1, mm0 ; mm1 = signum```

Signum of two 32-bit values

```      pxor    mm1, mm1
pxor    mm2, mm2
pcmpgtd mm1, mm0 ; mm1 = pcmpgt(0, x)
pcmpgtd mm0, mm2 ; mm0 = pcmpgt(x, 0)
psubd   mm1, mm0 ; mm1 = signum```

Signum of sixteen 8-bit values (requires ASMLIB2 library)

```      pxor    xmm1, xmm1
pxor    xmm2, xmm2
pcmpgtb xmm1, xmm0 ; xmm1 = pcmpgt(0, x)
pcmpgtb xmm0, xmm2 ; xmm0 = pcmpgt(x, 0)
psubb   xmm1, xmm0 ; xmm1 = signum```

Signum of eight 16-bit values (requires ASMLIB2 library)

```      pxor    xmm1, xmm1
pxor    xmm2, xmm2
pcmpgtw xmm1, xmm0 ; xmm1 = pcmpgt(0, x)
pcmpgtw xmm0, xmm2 ; xmm0 = pcmpgt(x, 0)
psubw   xmm1, xmm0 ; xmm1 = signum```

Signum of four 32-bit values (requires ASMLIB2 library)

```      pxor    xmm1, xmm1
pxor    xmm2, xmm2
pcmpgtd xmm1, xmm0 ; xmm1 = pcmpgt(0, x)
pcmpgtd xmm0, xmm2 ; xmm0 = pcmpgt(x, 0)
psubd   xmm1, xmm0 ; xmm1 = signum```