# Zftrans - transcendental operations

See:

* <http://bugs.libre-riscv.org/show_bug.cgi?id=127>
* <https://www.khronos.org/registry/spir-v/specs/unified1/OpenCL.ExtendedInstructionSet.100.html>
* Discussion: <http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2019-August/002342.html>
* [[rv_major_opcode_1010011]] for opcode listing.
* [[zfpacc_proposal]] for accuracy settings proposal

Extension subsets:

* **Zftrans**: standard transcendentals (best suited to 3D)
* **ZftransExt**: extra functions (useful, not generally needed for 3D,
  can be synthesised using Ztrans)
* **Ztrigpi**: trig. xxx-pi sinpi cospi tanpi
* **Ztrignpi**: trig non-xxx-pi sin cos tan
* **Zarctrigpi**: arc-trig. a-xxx-pi: atan2pi asinpi acospi
* **Zarctrignpi**: arc-trig. non-a-xxx-pi: atan2, asin, acos
* **Zfhyp**: hyperbolic/inverse-hyperbolic.  sinh, cosh, tanh, asinh,
  acosh, atanh (can be synthesised - see below)
* **ZftransAdv**: much more complex to implement in hardware
* **Zfrsqrt**: Reciprocal square-root.

Minimum recommended requirements for 3D: Zftrans, Ztrigpi, Zarctrigpi,
Zarctrignpi

[[!toc levels=2]]

# TODO:

* Decision on accuracy, moved to [[zfpacc_proposal]]
<http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2019-August/002355.html>
* Errors **MUST** be repeatable.
* How about four Platform Specifications? 3DUNIX, UNIX, 3DEmbedded and Embedded?
<http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2019-August/002361.html>
  Accuracy requirements for dual (triple) purpose implementations must
  meet the higher standard.
* Reciprocal Square-root is in its own separate extension (Zfrsqrt) as
  it is desirable on its own by other implementors.  This to be evaluated.

# Proposed Opcodes vs Khronos OpenCL Opcodes <a name="khronos_equiv"></a>

This list shows the (direct) equivalence between proposed opcodes and
their Khronos OpenCL equivalents.

See
<https://www.khronos.org/registry/spir-v/specs/unified1/OpenCL.ExtendedInstructionSet.100.html>

Special FP16 opcodes are *not* being proposed, except by indirect / inherent
use of the "fmt" field that is already present in the RISC-V Specification.

"Native" opcodes are *not* being proposed: implementors will be expected
to use the (equivalent) proposed opcode covering the same function.

"Fast" opcodes are *not* being proposed, because the Khronos Specification
fast\_length, fast\_normalise and fast\_distance OpenCL opcodes require
vectors (or can be done as scalar operations using other RISC-V instructions).

The OpenCL FP32 opcodes are **direct** equivalents to the proposed opcodes.
Deviation from conformance with the Khronos Specification - including the
Khronos Specification accuracy requirements - is not an option.

[[!table data="""
Proposed opcode | OpenCL FP32 | OpenCL FP16 | OpenCL native | OpenCL fast |
FSIN            | sin         | half\_sin   | native\_sin   | NONE        |
FCOS            | cos         | half\_cos   | native\_cos   | NONE        |
FTAN            | tan         | half\_tan   | native\_tan   | NONE        |
NONE (1)        | sincos      | NONE        | NONE          | NONE        |
FASIN           | asin        | NONE        | NONE          | NONE        |
FACOS           | acos        | NONE        | NONE          | NONE        |
FATAN           | atan        | NONE        | NONE          | NONE        |
FSINPI          | sinpi       | NONE        | NONE          | NONE        |
FCOSPI          | cospi       | NONE        | NONE          | NONE        |
FTANPI          | tanpi       | NONE        | NONE          | NONE        |
FASINPI         | asinpi      | NONE        | NONE          | NONE        |
FACOSPI         | acospi      | NONE        | NONE          | NONE        |
FATANPI         | atanpi      | NONE        | NONE          | NONE        |
FSINH           | sinh        | NONE        | NONE          | NONE        |
FCOSH           | cosh        | NONE        | NONE          | NONE        |
FTANH           | tanh        | NONE        | NONE          | NONE        |
FASINH          | asinh       | NONE        | NONE          | NONE        |
FACOSH          | acosh       | NONE        | NONE          | NONE        |
FATANH          | atanh       | NONE        | NONE          | NONE        |
FRSQRT          | rsqrt       | half\_rsqrt | native\_rsqrt | NONE        |
FCBRT           | cbrt        | NONE        | NONE          | NONE        |
FEXP2           | exp2        | half\_exp2  | native\_exp2  | NONE        |
FLOG2           | log2        | half\_log2  | native\_log2  | NONE        |
FEXPM1          | expm1       | NONE        | NONE          | NONE        |
FLOG1P          | log1p       | NONE        | NONE          | NONE        |
FEXP            | exp         | half\_exp   | native\_exp   | NONE        |
FLOG            | log         | half\_log   | native\_log   | NONE        |
FEXP10          | exp10       | half\_exp10 | native\_exp10 | NONE        |
FLOG10          | log10       | half\_log10 | native\_log10 | NONE        |
FATAN2          | atan2       | NONE        | NONE          | NONE        |
FATAN2PI        | atan2pi     | NONE        | NONE          | NONE        |
FPOW            | pow         | NONE        | NONE          | NONE        |
FROOT           | rootn       | NONE        | NONE          | NONE        |
FHYPOT          | hypot       | NONE        | NONE          | NONE        |
FRECIP          | NONE        | half\_recip | native\_recip | NONE        |
"""]]

Note (1) FSINCOS is macro-op fused (see below).

# List of 2-arg opcodes

[[!table  data="""
opcode    | Description            | pseudo-code                | Extension   |
FATAN2    | atan2 arc tangent      | rd = atan2(rs2, rs1)       | Zarctrignpi |
FATAN2PI  | atan2 arc tangent / pi | rd = atan2(rs2, rs1) / pi  | Zarctrigpi  |
FPOW      | x power of y           | rd = pow(rs1, rs2)         | ZftransAdv  |
FROOT     | x power 1/y            | rd = pow(rs1, 1/rs2)       | ZftransAdv  |
FHYPOT    | hypotenuse             | rd = sqrt(rs1^2 + rs2^2)   | Zftrans     |
"""]]

# List of 1-arg transcendental opcodes

[[!table  data="""
opcode   | Description              | pseudo-code             | Extension  |
FRSQRT   | Reciprocal Square-root   | rd = sqrt(rs1)          | Zfrsqrt    |
FCBRT    | Cube Root                | rd = pow(rs1, 3)        | Zftrans    |
FRECIP   | Reciprocal               | rd = 1.0 /   rs1        | Zftrans    |
FEXP2    | power-of-2               | rd = pow(2, rs1)        | Zftrans    |
FLOG2    | log2                     | rd = log2(rs1)          | Zftrans    |
FEXPM1   | exponential minus 1      | rd = pow(e, rs1) - 1.0  | Zftrans    |
FLOG1P   | log plus 1               | rd = log(e, 1 + rs1)    | Zftrans    |
FEXP     | exponential              | rd = pow(e, rs1)        | ZftransExt |
FLOG     | natural log (base e)     | rd = log(e, rs1)        | ZftransExt |
FEXP10   | power-of-10              | rd = pow(10, rs1)       | ZftransExt |
FLOG10   | log base 10              | rd = log10(rs1)         | ZftransExt |
"""]]

# List of 1-arg trigonometric opcodes

[[!table  data="""
opcode   | Description              | pseudo-code             | Extension |
FSIN     | sin (radians)            | rd = sin(rs1)           | Ztrignpi    |
FCOS     | cos (radians)            | rd = cos(rs1)           | Ztrignpi    |
FTAN     | tan (radians)            | rd = tan(rs1)           | Ztrignpi    |
FASIN    | arcsin (radians)         | rd = asin(rs1)          | Zarctrignpi |
FACOS    | arccos (radians)         | rd = acos(rs1)          | Zarctrignpi |
FATAN    | arctan (radians)         | rd = atan(rs1)          | Zarctrignpi |
FSINPI   | sin times pi             | rd = sin(pi * rs1)      | Ztrigpi |
FCOSPI   | cos times pi             | rd = cos(pi * rs1)      | Ztrigpi |
FTANPI   | tan times pi             | rd = tan(pi * rs1)      | Ztrigpi |
FASINPI  | arcsin / pi              | rd = asin(rs1) / pi     | Zarctrigpi |
FACOSPI  | arccos / pi              | rd = acos(rs1) / pi     | Zarctrigpi |
FATANPI  | arctan / pi              | rd = atan(rs1) / pi     | Zarctrigpi |
FSINH    | hyperbolic sin (radians) | rd = sinh(rs1)          | Zfhyp |
FCOSH    | hyperbolic cos (radians) | rd = cosh(rs1)          | Zfhyp |
FTANH    | hyperbolic tan (radians) | rd = tanh(rs1)          | Zfhyp |
FASINH   | inverse hyperbolic sin   | rd = asinh(rs1)         | Zfhyp |
FACOSH   | inverse hyperbolic cos   | rd = acosh(rs1)         | Zfhyp |
FATANH   | inverse hyperbolic tan   | rd = atanh(rs1)         | Zfhyp |
"""]]

# Synthesis, Pseudo-code ops and macro-ops

The pseudo-ops are best left up to the compiler rather than being actual
pseudo-ops, by allocating one scalar FP register for use as a constant
(loop invariant) set to "1.0" at the beginning of a function or other
suitable code block.

* FSINCOS - fused macro-op between FSIN and FCOS (issued in that order).
* FSINCOSPI - fused macro-op between FSINPI and FCOSPI (issued in that order).

FATANPI example pseudo-code:

    lui t0, 0x3F800 // upper bits of f32 1.0
    fmv.x.s ft0, t0
    fatan2pi.s rd, rs1, ft0

Hyperbolic function example (obviates need for Zfhyp except for
high-performance or correctly-rounding):

    ASINH( x ) = ln( x + SQRT(x**2+1))

# Reciprocal

Used to be an alias. Some imolementors may wish to implement divide as y times recip(x)

# To evaluate: should LOG be replaced with LOG1P (and EXP with EXPM1)?

RISC principle says "exclude LOG because it's covered by LOGP1 plus an ADD".
Research needed to ensure that implementors are not compromised by such
a decision
<http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2019-August/002358.html>

> > correctly-rounded LOG will return different results than LOGP1 and ADD.
> > Likewise for EXP and EXPM1

> ok, they stay in as real opcodes, then.

# ATAN / ATAN2 commentary

Discussion starts here:
<http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2019-August/002470.html>

from Mitch Alsup:

would like to point out that the general implementations of ATAN2 do a
bunch of special case checks and then simply call ATAN.

    double ATAN2( double y, double x )
    {   // IEEE 754-2008 quality ATAN2

        // deal with NANs
        if( ISNAN( x )             ) return x;
        if( ISNAN( y )             ) return y;

        // deal with infinities
        if( x == +∞    && |y|== +∞  ) return copysign(  π/4, y );
        if( x == +∞                 ) return copysign(  0.0, y );
        if( x == -∞    && |y|== +∞  ) return copysign( 3π/4, y );
        if( x == -∞                 ) return copysign(    π, y );
        if(               |y|== +∞  ) return copysign(  π/2, y );

        // deal with signed zeros
        if( x == 0.0  &&  y != 0.0 ) return copysign(  π/2, y );
        if( x >=+0.0  &&  y == 0.0 ) return copysign(  0.0, y );
        if( x <=-0.0  &&  y == 0.0 ) return copysign(    π, y );

        // calculate ATAN2 textbook style
        if( x  > 0.0               ) return     ATAN( |y / x| );
        if( x  < 0.0               ) return π - ATAN( |y / x| );
    }


Yet the proposed encoding makes ATAN2 the primitive and has ATAN invent
a constant and then call/use ATAN2.

When one considers an implementation of ATAN, one must consider several
ranges of evaluation::

     x  [  -∞, -1.0]:: ATAN( x ) = -π/2 + ATAN( 1/x );
     x  (-1.0, +1.0]:: ATAN( x ) =      + ATAN(   x );
     x  [ 1.0,   +∞]:: ATAN( x ) = +π/2 - ATAN( 1/x );

I should point out that the add/sub of π/2 can not lose significance
since the result of ATAN(1/x) is bounded 0..π/2

The bottom line is that I think you are choosing to make too many of
these into OpCodes, making the hardware function/calculation unit (and
sequencer) more complicated that necessary.

--------------------------------------------------------

I might suggest that if there were a way for a calculation to be performed
and the result of that calculation

chained to a subsequent calculation such that the precision of the
result-becomes-operand is wider than

what will fit in a register, then you can dramatically reduce the count
of instructions in this category while retaining

acceptable accuracy:

     z = x / y

can be calculated as::

     z = x * (1/y)

Where 1/y has about 26-to-32 bits of fraction. No, it's not IEEE 754-2008
accurate, but GPUs want speed and

1/y is fully pipelined (F32) while x/y cannot be (at reasonable area). It
is also not "that inaccurate" displaying 0.625-to-0.52 ULP.

Given that one has the ability to carry (and process) more fraction bits,
one can then do high precision multiplies of  π or other transcendental
radixes.

And GPUs have been doing this almost since the dawn of 3D.

    // calculate ATAN2 high performance style
    // Note: at this point x != y
    //
    if( x  > 0.0             )
    {
        if( y < 0.0 && |y| < |x| ) return - π/2 - ATAN( x / y );
        if( y < 0.0 && |y| > |x| ) return       + ATAN( y / x );
        if( y > 0.0 && |y| < |x| ) return       + ATAN( y / x );
        if( y > 0.0 && |y| > |x| ) return + π/2 - ATAN( x / y );
    }
    if( x  < 0.0             )
    {
        if( y < 0.0 && |y| < |x| ) return + π/2 + ATAN( x / y );
        if( y < 0.0 && |y| > |x| ) return + π   - ATAN( y / x );
        if( y > 0.0 && |y| < |x| ) return + π   - ATAN( y / x );
        if( y > 0.0 && |y| > |x| ) return +3π/2 + ATAN( x / y );
    }

This way the adds and subtracts from the constant are not in a precision
precarious position.