[LLVMdev] adding new instructions to support "swizzle" and "writemask"

Tzu-Chien Chiu tzuchien.chiu at gmail.com
Wed Apr 20 01:03:43 PDT 2005

Hello, everyone:

I am writing a compiler for a programmable graphics hardware. Each
registers of the hardware has four channels, namely 'r', 'b', 'g',
'a', and each channel is a 32-bit floating point. It's similar to the
high and low 8-bit of an x86 16-bit general purpose register "AX" can
be individually referenced as "AH" and "AL". What's different is the
hardware further "source register swizzle" and "writemask". For

    # The following two instructions are equivalent.
    # They cost the same instruction slot, and have same
    # execution time. Four channels are added in parallel.
    add r0,      r1,      r2
    add r0.xyzw, r1.xyzw, r2.xyzw 
    # equivalent to:
    #   r0.x = r1.yy + r2.w
    #   r0.z = r1.yy + r2.x
    # r0.y and r0.w remains unchanged
    add r0.xz, r1.y, r2.wx
Note that the channel y of r1 is replicated in the third instruction.

Detailed documentation:

The code must be be transformed in SSA (.ll file). The problem is that
no existing LLVM instruction or intrinsic function supports swizzle
and writemask.

I have a few solutions:

(1) Treat each channel of a register as a individual SSA variable.

This could generate inefficient machine code. 

For example, the instruction:

  add r0.xz, r1.y, r2.wx
is translated to two LLVM instructions:

  r0_x = add float r1_y, r2_w
  r0_z = add float r1_y, r2_x
Subsequent optimization passes could insert other instructions between
these two instruction (for example, in instruction scheduling pass). I
don't know how they could be easily merged back into one instruction.
It could lead to inefficient machine code (though correct).

(2) Add new LLVM instructions, "swizzle" and "merge".

  # A swizzle instruction acts like a channel "selector",
  # selecting channels from the temporary registers r1 and r2.
  temp_0 = swizzle.yy r1 
  temp_1 = swizzle.wx r2
  temp_3 = add float temp_0, temp_1
  temp_4 = merge.xz float temp_3, r0

But implementing swizzle and merge instructions like this seems non-trivial.

I'd know if anyone knows if there is possible and ealier alternatives?
Thank you.

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