# Scalar OpenPOWER Audio and Video Opcodes

the fundamental principle of SV is a hardware for-loop. therefore the first (and in nearly 100% of cases only) place to put Vector operations is first and foremost in the *scalar* ISA.  However only by analysing those scalar opcodes *in* a SV Vectorisation context does it become clear why they are needed and how they may be designed.

This page therefore has acompanying discussion at <https://bugs.libre-soc.org/show_bug.cgi?id=230> for evolution of suitable opcodes.

# Audio

The fundamental principle for these instructions is:

* identify the scalar primitive
* assume that longer runs of scalars will have Simple-V vectorisatin applied
* assume that "swizzle" may be applied at the (vec2 - SUBVL=2) Vector level,
  in order to perform the necessary HI/LO selection normally hard-coded
  into SIMD ISAs.

Thus for example, where OpenPOWER VSX has vpkswss, this would be achieved in SV with simply:

* addition of a scalar ext/clamp instruction
* 1st op, swizzle-selection vec2 "select X only" from source to dest:
  dest.X = extclamp(src.X)
* 2nd op, swizzle-select vec2 "select Y only" from source to dest
  dest.Y = extclamp(src.Y)

Macro-op fusion may be used to detect that these two interleave cleanly, overlapping the vec2.X with vec2.Y to produce a single vec2.XY operation.

## Scalar element operations

* clamping / saturation for signed and unsigned.  best done similar to FP rounding modes, i.e. with an SPR.
* average-add.  result = (src1 + src2 + 1) >> 1
* abs-diff: result = (src1 > src2) ? (src1-src2) : (src2-src1)
* signed min/max

# Video

TODO

# VSX SIMD

## vpkpx

vpkpx is a 32-bit to 16-bit 8888 into 1555 conversion

SV notes:

a single 32-bit to 16-bit operation should suffice, fitting cleanly into one single scalar op:

    dest[0]     = src[7]
    dest[1 : 5] = src[8 :12]
    dest[6 :10] = src[16:20]
    dest[11:15] = src[24:28]

## vpks[\*][\*]s

signed and unsigned, these are N-to-M (N=64/32/16, M=32/16/8) chop/clamp/sign/zero-extend operations
 
## vupkhpx / vupklpx

these are 16-bit to 32-bit 1555 to 8888 conversion

## vavgs\*

signed and unsigned, 8/16/32: these are all of the form:

    result = truncate((a + b + 1) >> 1))

## vabsdu\*

unsigned 8/16/32: these are all of the form:

    result = (src1 > src2) ? truncate(src1-src2) :
                             truncate(src2-src1)

## vmaxs\* / vmaxu\* (and min)

signed and unsigned, 8/16/32: these are all of the form:

    result = (src1 > src2) ? src1 : src2 # max
    result = (src1 < src2) ? src1 : src2 # min