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SIMD signum function
Post by rtr2 on Dec 29th, 2014, 11:06pm

Raymond Chen is continuing his series of Monday articles on SSE2 integer arithmetic. This week he is dealing with the signum function, i.e. SGN in BASIC. Here is a corrected version of his code for returning the signum of eight 16-bit integers in parallel:

Code:
      INSTALL @lib$+"ASMLIB2"
      DIM P% 100, L% -1, memory% 31
      memory% = (memory% + 15) AND -16

      ON ERROR LOCAL [OPT FN_asmext : ]
      [OPT 10
      .signum
      movdqu  xmm0, [memory%] ; input in xmm0
      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
      movdqu  [memory%], xmm1 ; output in xmm1
      ret
      ]
      RESTORE ERROR

This can be converted to perform 16 8-bit signums or 4 32-bit signums by changing pcmpgtw and psubw to pcmpgtb and psubb or pcmpgtd and psubd respectively.

You can also adapt it to use the 64-bit MMX registers (hence working on half the number of values); in that case no library is required of course:

Code:
      DIM P% 100, L% -1, memory% 15
      memory% = (memory% + 7) AND -8

      [OPT 10
      .signum
      movq    mm0, [memory%] ; input in mm0
      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
      movq    [memory%], mm1 ; output in mm1
      ret
      ]

Richard.