[Mesa-dev] RFC: tgsi opcodes for 32x32 muls with 64bit results

[Roland Scheidegger]

Currently, there's no way to get the high bits of a 32x32
signed/unsigned integer multiplication with tgsi.
However, all of d3d10, OpenGL, and OpenCL support that, so we need it as
well.
There's essentially two ways how it could be done:
- a 2-destination instruction returning both high and low bits (this is
how it looks like in d3d10 and glsl)
- use the existing umul for the low bits and have another instruction
for the high bits (this is how it looks like in opencl)

Well there's other possibilities but these looked like they'd match both
APIs and HW reasonably (well with the exception of things like sse2
which would prefer 2x2 32bit inputs and return 2x64bit as one reg...).

Actually it's two new instructions because unlike for the low bits it
matters for the high bits if the source operands are signed or unsigned.

Personally I'm favoring two separate instructions for low and high bits
to not have to deal with multi-destination instructions, but if someone
makes a strong case for one returning both low and high bits I could be
convinced otherwise. I think though two instructions matches most hw
very well (with the exception of software renderers and possibly intel
graphics but then a good backend could certainly recognize this).

So here's what the docs would say about these instructions:


.. opcode:: IMUL_HI - Signed Integer Multiply High Bits

   The high 32bits of the multiplication of 2 signed integers is returned.

.. math::

  dst.x = src0.x \times src1.x >> 32

  dst.y = src0.y \times src1.y >> 32

  dst.z = src0.z \times src1.z >> 32

  dst.w = src0.w \times src1.w >> 32


.. opcode:: UMUL_HI - Unsigned Integer Multiply High Bits

   The high 32bits of the multiplication of 2 unsigned integers is returned.

.. math::

  dst.x = src0.x \times src1.x >> 32

  dst.y = src0.y \times src1.y >> 32

  dst.z = src0.z \times src1.z >> 32

  dst.w = src0.w \times src1.w >> 32


Roland