[libre-riscv-dev] [isa-dev] FP reciprocal sqrt extension proposal

[Jacob Lifshay]

On Sat, Jul 13, 2019 at 10:30 AM Aneesh Raveendran <aneeshr2020 at gmail.com>
wrote:
>
> Hi all,
>     Myself Aneesh Raveendran. I worked on RISC-V floating point
co-processor. I have few doubts regarding floating point reciprocal
square-root.
>
> 1. In which application/bench marking suites will infer floating point
reciprocal square-root operations?
reciprocal sqrt is used a lot in 3D graphics for normalizing vectors -- the
pseudocode for normalizing 3D a vector is:
fn normalize(x: float, y: float, z: float) -> (float, float, float) {
    let sum_of_squares = x * x + y * y + z * z;
    let factor = rsqrt(sum_of_squares);
    return (factor * x, factor * y, factor * z);
}

It can also be used in machine learning to normalize 1-hot output vectors,
though would not be particularly performance critical for that particular
usecase.
> 2. If this instruction is proposing, what could be the possible
instruction formats? (opcodes, f7, f5 field values )
The proposed instructions are:
+----------+---------+-------+-----+--------+----+---------+
| Mnemonic | funct7  | rs2   | rs1 | funct3 | rd | opcode  |
+==========+=========+=======+=====+========+====+=========+
| fsqrt.s  | 0111100 | 00000 | rs1 | rm     | rd | 1010011 |
+----------+---------+-------+-----+--------+----+---------+
| fsqrt.d  | 0111101 | 00000 | rs1 | rm     | rd | 1010011 |
+----------+---------+-------+-----+--------+----+---------+
| fsqrt.q  | 0111110 | 00000 | rs1 | rm     | rd | 1010011 |
+----------+---------+-------+-----+--------+----+---------+
| fsqrt.h  | 0111111 | 00000 | rs1 | rm     | rd | 1010011 |
+----------+---------+-------+-----+--------+----+---------+

> 3. Any testsuites are available to verify the functional correctness of
the module?
mpfr implements reciprocal sqrt, however it doesn't support all of RISC-V's
rounding modes and may be missing support for other features needed for
testing.
Softfloat doesn't currently implement rsqrt.
I have not researched other softfloat libraries yet.

Jacob Lifshay