DRAFT Scalar Transcendentals

Summary:

This proposal extends Power ISA scalar floating point operations to add IEEE754 transcendental functions (pow, log etc) and trigonometric functions (sin, cos etc). These functions are also 98% shared with the Khronos Group OpenCL Extended Instruction Set.

Authors/Contributors:

  • Luke Kenneth Casson Leighton
  • Jacob Lifshay
  • Dan Petroski
  • Mitch Alsup
  • Allen Baum
  • Andrew Waterman
  • Luis Vitorio Cargnini

See:

Extension subsets:

TODO: rename extension subsets -- we're not on RISC-V anymore.

  • 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, Ztrignpi, Zarctrignpi, with Ztrigpi and Zarctrigpi as augmentations.

Minimum recommended requirements for Mobile-Embedded 3D: Ztrignpi, Zftrans, with Ztrigpi as an augmentation.

The Platform Requirements for 3D are driven by cost competitive factors and it is the Trademarked Vulkan Specification that provides clear direction for 3D GPU markets, but nothing else (IEEE754). Implementors must note that minimum Compliance with the Third Party Vulkan Specification (for power-area competitive reasons with other 3D GPU manufacturers) will not qualify for strict IEEE754 accuracy Compliance or vice-versa.

Implementors must make it clear which accuracy level is implemented and provide a switching mechanism and throw Illegal Instruction traps if fully compliant accuracy cannot be achieved. It is also the Implementor's responsibility to comply with all Third Party Certification Marks and Trademarks (Vulkan, OpenCL). Nothing in this specification in any way implies that any Third Party Certification Mark Compliance is granted, nullified, altered or overridden by this document.

TODO:

Requirements

This proposal is designed to meet a wide range of extremely diverse needs, allowing implementors from all of them to benefit from the tools and hardware cost reductions associated with common standards adoption in Power ISA (primarily IEEE754 and Vulkan).

The use-cases are:

  • 3D GPUs
  • Numerical Computation
  • (Potentially) A.I. / Machine-learning (1)

(1) although approximations suffice in this field, making it more likely to use a custom extension. High-end ML would inherently definitely be excluded.

The power and die-area requirements vary from:

  • Ultra-low-power (smartwatches where GPU power budgets are in milliwatts)
  • Mobile-Embedded (good performance with high efficiency for battery life)
  • Desktop Computing
  • Server / HPC / Supercomputing

The software requirements are:

  • Full public integration into GNU math libraries (libm)
  • Full public integration into well-known Numerical Computation systems (numpy)
  • Full public integration into upstream GNU and LLVM Compiler toolchains
  • Full public integration into Khronos OpenCL SPIR-V compatible Compilers seeking public Certification and Endorsement from the Khronos Group under their Trademarked Certification Programme.

Proposed Opcodes vs Khronos OpenCL vs IEEE754-2019

This list shows the (direct) equivalence between proposed opcodes, their Khronos OpenCL equivalents, and their IEEE754-2019 equivalents. 98% of the opcodes in this proposal that are in the IEEE754-2019 standard are present in the Khronos Extended Instruction Set.

See https://www.khronos.org/registry/spir-v/specs/unified1/OpenCL.ExtendedInstructionSet.100.html and https://ieeexplore.ieee.org/document/8766229

  • Special FP16 opcodes are not being proposed, except by indirect / inherent use of elwidth overrides that is already present in the SVP64 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 Power ISA 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, as it results in non-compliance, and the vendor may not use the Trademarked words "Vulkan" etc. in conjunction with their product.

IEEE754-2019 Table 9.1 lists "additional mathematical operations". Interestingly the only functions missing when compared to OpenCL are compound, exp2m1, exp10m1, log2p1, log10p1, pown (integer power) and powr.

opcode OpenCL FP32 OpenCL FP16 OpenCL native IEEE754 Power ISA
fsin sin half_sin native_sin sin NONE
fcos cos half_cos native_cos cos NONE
ftan tan half_tan native_tan tan NONE
NONE (1) sincos NONE NONE NONE NONE
fasin asin NONE NONE asin NONE
facos acos NONE NONE acos NONE
fatan atan NONE NONE atan NONE
fsinpi sinpi NONE NONE sinPi NONE
fcospi cospi NONE NONE cosPi NONE
ftanpi tanpi NONE NONE tanPi NONE
fasinpi asinpi NONE NONE asinPi NONE
facospi acospi NONE NONE acosPi NONE
fatanpi atanpi NONE NONE atanPi NONE
fsinh sinh NONE NONE sinh NONE
fcosh cosh NONE NONE cosh NONE
ftanh tanh NONE NONE tanh NONE
fasinh asinh NONE NONE asinh NONE
facosh acosh NONE NONE acosh NONE
fatanh atanh NONE NONE atanh NONE
fatan2 atan2 NONE NONE atan2 NONE
fatan2pi atan2pi NONE NONE atan2pi NONE
frsqrt rsqrt half_rsqrt native_rsqrt rSqrt fsqrte, fsqrtes (4)
fcbrt cbrt NONE NONE NONE (2) NONE
fexp2 exp2 half_exp2 native_exp2 exp2 NONE
flog2 log2 half_log2 native_log2 log2 NONE
fexpm1 expm1 NONE NONE expm1 NONE
flog1p log1p NONE NONE logp1 NONE
fexp exp half_exp native_exp exp NONE
flog log half_log native_log log NONE
fexp10 exp10 half_exp10 native_exp10 exp10 NONE
flog10 log10 half_log10 native_log10 log10 NONE
fpow pow NONE NONE pow NONE
fpown pown NONE NONE pown NONE
fpowr powr half_powr native_powr powr NONE
frootn rootn NONE NONE rootn NONE
fhypot hypot NONE NONE hypot NONE
frecip NONE half_recip native_recip NONE (3) fre, fres (4)
NONE NONE NONE NONE compound NONE
fexp2m1 NONE NONE NONE exp2m1 NONE
fexp10m1 NONE NONE NONE exp10m1 NONE
flog2p1 NONE NONE NONE log2p1 NONE
flog10p1 NONE NONE NONE log10p1 NONE
fminnum08 fmin fmin NONE minNum xsmindp (5)
fmaxnum08 fmax fmax NONE maxNum xsmaxdp (5)
fmin19 fmin fmin NONE minimum NONE
fmax19 fmax fmax NONE maximum NONE
fminnum19 fmin fmin NONE minimumNumber vminfp (6), xsminjdp (5)
fmaxnum19 fmax fmax NONE maximumNumber vmaxfp (6), xsmaxjdp (5)
fminc fmin fmin NONE NONE xsmincdp (5)
fmaxc fmax fmax NONE NONE xsmaxcdp (5)
fminmagnum08 minmag minmag NONE minNumMag NONE
fmaxmagnum08 maxmag maxmag NONE maxNumMag NONE
fminmag19 minmag minmag NONE minimumMagnitude NONE
fmaxmag19 maxmag maxmag NONE maximumMagnitude NONE
fminmagnum19 minmag minmag NONE minimumMagnitudeNumber NONE
fmaxmagnum19 maxmag maxmag NONE maximumMagnitudeNumber NONE
fminmagc minmag minmag NONE NONE NONE
fmaxmagc maxmag maxmag NONE NONE NONE
fmod fmod fmod NONE NONE
fremainder remainder remainder remainder NONE

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

Note (2) synthesised in IEEE754-2019 as "rootn(x, 3)"

Note (3) synthesised in IEEE754-2019 using "1.0 / x"

Note (4) these are estimate opcodes that help accelerate software emulation

Note (5) f64-only (though can be used on f32 stored in f64 format), requires VSX.

Note (6) 4xf32-only, requires VMX.

List of 2-arg opcodes

opcode Description pseudocode Extension
fatan2 atan2 arc tangent FRT = atan2(FRB, FRA) Zarctrignpi
fatan2pi atan2 arc tangent / pi FRT = atan2(FRB, FRA) / pi Zarctrigpi
fpow x power of y FRT = pow(FRA, FRB) ZftransAdv
fpown x power of n (n int) FRT = pow(FRA, RB) ZftransAdv
fpowr x power of y (x +ve) FRT = exp(FRA log(FRB)) ZftransAdv
frootn x power 1/n (n integer) FRT = pow(FRA, 1/RB) ZftransAdv
fhypot hypotenuse FRT = sqrt(FRA2 + FRB2) ZftransAdv
fminnum08 IEEE 754-2008 minNum FRT = minNum(FRA, FRB) (1) TBD
fmaxnum08 IEEE 754-2008 maxNum FRT = maxNum(FRA, FRB) (1) TBD
fmin19 IEEE 754-2019 minimum FRT = minimum(FRA, FRB) TBD
fmax19 IEEE 754-2019 maximum FRT = maximum(FRA, FRB) TBD
fminnum19 IEEE 754-2019 minimumNumber FRT = minimumNumber(FRA, FRB) TBD
fmaxnum19 IEEE 754-2019 maximumNumber FRT = maximumNumber(FRA, FRB) TBD
fminc C ternary-op minimum FRT = FRA < FRB ? FRA : FRB TBD
fmaxc C ternary-op maximum FRT = FRA > FRB ? FRA : FRB TBD
fminmagnum08 IEEE 754-2008 minNumMag FRT = minmaxmag(FRA, FRB, False, fminnum08) (2) TBD
fmaxmagnum08 IEEE 754-2008 maxNumMag FRT = minmaxmag(FRA, FRB, True, fmaxnum08) (2) TBD
fminmag19 IEEE 754-2019 minimumMagnitude FRT = minmaxmag(FRA, FRB, False, fmin19) (2) TBD
fmaxmag19 IEEE 754-2019 maximumMagnitude FRT = minmaxmag(FRA, FRB, True, fmax19) (2) TBD
fminmagnum19 IEEE 754-2019 minimumMagnitudeNumber FRT = minmaxmag(FRA, FRB, False, fminnum19) (2) TBD
fmaxmagnum19 IEEE 754-2019 maximumMagnitudeNumber FRT = minmaxmag(FRA, FRB, True, fmaxnum19) (2) TBD
fminmagc C ternary-op minimum magnitude FRT = minmaxmag(FRA, FRB, False, fminc) (2) TBD
fmaxmagc C ternary-op maximum magnitude FRT = minmaxmag(FRA, FRB, True, fmaxc) (2) TBD
fmod modulus FRT = fmod(FRA, FRB) TBD
fremainder IEEE 754 remainder FRT = remainder(FRA, FRB) TBD

Note (1): for the purposes of minNum/maxNum, -0.0 is defined to be less than +0.0. This is left unspecified in IEEE 754-2008.

Note (2): minmaxmag(x, y, cmp, fallback) is defined as:

def minmaxmag(x, y, is_max, fallback):
    a = abs(x) < abs(y)
    b = abs(x) > abs(y)
    if is_max:
        a, b = b, a  # swap
    if a:
        return x
    if b:
        return y
    # equal magnitudes, or NaN input(s)
    return fallback(x, y)

List of 1-arg transcendental opcodes

opcode Description pseudocode Extension
frsqrt Reciprocal Square-root FRT = sqrt(FRA) Zfrsqrt
fcbrt Cube Root FRT = pow(FRA, 1.0 / 3) ZftransAdv
frecip Reciprocal FRT = 1.0 / FRA Zftrans
fexp2m1 power-2 minus 1 FRT = pow(2, FRA) - 1.0 ZftransExt
flog2p1 log2 plus 1 FRT = log(2, 1 + FRA) ZftransExt
fexp2 power-of-2 FRT = pow(2, FRA) Zftrans
flog2 log2 FRT = log(2. FRA) Zftrans
fexpm1 exponential minus 1 FRT = pow(e, FRA) - 1.0 ZftransExt
flog1p log plus 1 FRT = log(e, 1 + FRA) ZftransExt
fexp exponential FRT = pow(e, FRA) ZftransExt
flog natural log (base e) FRT = log(e, FRA) ZftransExt
fexp10m1 power-10 minus 1 FRT = pow(10, FRA) - 1.0 ZftransExt
flog10p1 log10 plus 1 FRT = log(10, 1 + FRA) ZftransExt
fexp10 power-of-10 FRT = pow(10, FRA) ZftransExt
flog10 log base 10 FRT = log(10, FRA) ZftransExt

List of 1-arg trigonometric opcodes

opcode Description pseudocode Extension
fsin sin (radians) FRT = sin(FRA) Ztrignpi
fcos cos (radians) FRT = cos(FRA) Ztrignpi
ftan tan (radians) FRT = tan(FRA) Ztrignpi
fasin arcsin (radians) FRT = asin(FRA) Zarctrignpi
facos arccos (radians) FRT = acos(FRA) Zarctrignpi
fatan arctan (radians) FRT = atan(FRA) Zarctrignpi
fsinpi sin times pi FRT = sin(pi * FRA) Ztrigpi
fcospi cos times pi FRT = cos(pi * FRA) Ztrigpi
ftanpi tan times pi FRT = tan(pi * FRA) Ztrigpi
fasinpi arcsin / pi FRT = asin(FRA) / pi Zarctrigpi
facospi arccos / pi FRT = acos(FRA) / pi Zarctrigpi
fatanpi arctan / pi FRT = atan(FRA) / pi Zarctrigpi
fsinh hyperbolic sin (radians) FRT = sinh(FRA) Zfhyp
fcosh hyperbolic cos (radians) FRT = cosh(FRA) Zfhyp
ftanh hyperbolic tan (radians) FRT = tanh(FRA) Zfhyp
fasinh inverse hyperbolic sin FRT = asinh(FRA) Zfhyp
facosh inverse hyperbolic cos FRT = acosh(FRA) Zfhyp
fatanh inverse hyperbolic tan FRT = atanh(FRA) Zfhyp

Opcode Tables for PO=59/63 XO=1---011--

Power ISA v3.1B opcodes extracted from:

  • Power ISA v3.1B Appendix D Table 23 sheet 2 of 4 page 1391
  • Power ISA v3.1B Appendix D Table 25 sheet 2 of 4 page 1399

Parenthesized entries are not part of fptrans.

  • Entries whose mnemonic ends in s are only in PO=59.
  • Entries whose mnemonic does not end in s are only in PO=63.
  • Entries whose mnemonic ends in (s) are in both PO=59 and PO=63.
XO LSB half →
XO MSB half ↓
01100 01101 01110 01111
10000 10000 01100
fcbrt(s) (draft)
10000 01101
fsinpi(s) (draft)
10000 01110
fatan2pi(s) (draft)
10000 01111
fasinpi(s) (draft)
10001 10001 01100
fcospi(s) (draft)
10001 01101
ftanpi(s) (draft)
10001 01110
facospi(s) (draft)
10001 01111
fatanpi(s) (draft)
10010 10010 01100
frsqrt(s) (draft)
10010 01101
fsin(s) (draft)
10010 01110
fatan2(s) (draft)
10010 01111
fasin(s) (draft)
10011 10011 01100
fcos(s) (draft)
10011 01101
ftan(s) (draft)
10011 01110
facos(s) (draft)
10011 01111
fatan(s) (draft)
10100 10100 01100
frecip(s) (draft)
10100 01101
fsinh(s) (draft)
10100 01110
fhypot(s) (draft)
10100 01111
fasinh(s) (draft)
10101 10101 01100
fcosh(s) (draft)
10101 01101
ftanh(s) (draft)
10101 01110
facosh(s) (draft)
10101 01111
fatanh(s) (draft)
10110 10110 01100
fminnum08(s) (draft)
10110 01101
fmin19(s) (draft)
10110 01110
fminnum19(s) (draft)
10110 01111
fminc(s) (draft)
10111 10111 01100
fmaxnum08(s) (draft)
10111 01101
fmax19(s) (draft)
10111 01110
fmaxnum19(s) (draft)
10111 01111
fmaxc(s) (draft)
XO LSB half →
XO MSB half ↓
01100 01101 01110 01111
11000 11000 01100
fexp2m1(s) (draft)
11000 01101
flog2p1(s) (draft)
11000 01110
fminmagnum08(s) (draft)
11000 01111
fmaxmagnum08(s) (draft)
11001 11001 01100
fexpm1(s) (draft)
11001 01101
flogp1(s) (draft)
11001 01110
(fctid)
11001 01111
(fctidz)
11010 11010 01100
fexp10m1(s) (draft)
11010 01101
flog10p1(s) (draft)
11010 01110
(fcfid(s))
11010 01111
fmod(s) (draft)
11011 11011 01100
fpown(s) (draft)
11011 01101
frootn(s) (draft)
11011 01110
fminmag19(s) (draft)
11011 01111
fmaxmag19(s) (draft)
11100 11100 01100
fexp2(s) (draft)
11100 01101
flog2(s) (draft)
11100 01110
fminmagnum19(s) (draft)
11100 01111
fmaxmagnum19(s) (draft)
11101 11101 01100
fexp(s) (draft)
11101 01101
flog(s) (draft)
11101 01110
(fctidu)
11101 01111
(fctiduz)
11110 11110 01100
fexp10(s) (draft)
11110 01101
flog10(s) (draft)
11110 01110
(fcfidu(s))
11110 01111
fremainder(s) (draft)
11111 11111 01100
fpowr(s) (draft)
11111 01101
fpow(s) (draft)
11111 01110
fminmagc(s) (draft)
11111 01111
fmaxmagc(s) (draft)

DRAFT List of 2-arg opcodes

These are X-Form, recommended Major Opcode 63 for full-width and 59 for half-width (ending in s).

0.5 6.10 11.15 16.20 21..30 31 name Form
NN FRT FRA FRB 1xxxx011xx Rc transcendental X-Form
NN FRT FRA RB 1xxxx011xx Rc transcendental X-Form

Recommended 10-bit XO assignments:

opcode Description Major 59 and 63 bits 16..20
fatan2(s) atan2 arc tangent 10010 01110 FRB
fatan2pi(s) atan2 arc tangent / π 10000 01110 FRB
fpow(s) xy 11111 01101 FRB
fpown(s) xn (n ∈ ℤ) 11011 01100 RB
fpowr(s) xy (x >= 0) 11111 01100 FRB
frootn(s) n√x (n ∈ ℤ) 11011 01101 RB
fhypot(s) √(x2 + y2) 10100 01110 FRB
fminnum08(s) IEEE 754-2008 minNum 10110 01100 FRB
fmaxnum08(s) IEEE 754-2008 maxNum 10111 01100 FRB
fmin19(s) IEEE 754-2019 minimum 10110 01101 FRB
fmax19(s) IEEE 754-2019 maximum 10111 01101 FRB
fminnum19(s) IEEE 754-2019 minimumNumber 10110 01110 FRB
fmaxnum19(s) IEEE 754-2019 maximumNumber 10111 01110 FRB
fminc(s) C ternary-op minimum 10110 01111 FRB
fmaxc(s) C ternary-op maximum 10111 01111 FRB
fminmagnum08(s) IEEE 754-2008 minNumMag 11000 01110 FRB
fmaxmagnum08(s) IEEE 754-2008 maxNumMag 11000 01111 FRB
fminmag19(s) IEEE 754-2019 minimumMagnitude 11011 01110 FRB
fmaxmag19(s) IEEE 754-2019 maximumMagnitude 11011 01111 FRB
fminmagnum19(s) IEEE 754-2019 minimumMagnitudeNumber 11100 01110 FRB
fmaxmagnum19(s) IEEE 754-2019 maximumMagnitudeNumber 11100 01111 FRB
fminmagc(s) C ternary-op minimum magnitude 11111 01110 FRB
fmaxmagc(s) C ternary-op maximum magnitude 11111 01111 FRB
fmod(s) modulus 11010 01111 FRB
fremainder(s) IEEE 754 remainder 11110 01111 FRB

DRAFT List of 1-arg transcendental opcodes

These are X-Form, and are mostly identical in Special Registers Altered to fsqrt (the exact fp exceptions they can produce differ). Recommended Major Opcode 63 for full-width and 59 for half-width (ending in s).

Special Registers Altered (FIXME: come up with correct list):

FPRF FR FI FX OX UX XX
VXSNAN VXIMZ VXZDZ
CR1                    (if Rc=1)
0.5 6.10 11.15 16.20 21..30 31 name Form
NN FRT /// FRB 1xxxx011xx Rc transcendental X-Form

Recommended 10-bit XO assignments:

opcode Description Major 59 and 63
frsqrt(s) 1 / √x 10010 01100
fcbrt(s) ∛x 10000 01100
frecip(s) 1 / x 10100 01100
fexp2m1(s) 2x - 1 11000 01100
flog2p1(s) log2 (x + 1) 11000 01101
fexp2(s) 2x 11100 01100
flog2(s) log2 x 11100 01101
fexpm1(s) ex - 1 11001 01100
flogp1(s) loge (x + 1) 11001 01101
fexp(s) ex 11101 01100
flog(s) loge x 11101 01101
fexp10m1(s) 10x - 1 11010 01100
flog10p1(s) log10 (x + 1) 11010 01101
fexp10(s) 10x 11110 01100
flog10(s) log10 x 11110 01101

DRAFT List of 1-arg trigonometric opcodes

These are X-Form, and are mostly identical in Special Registers Altered to fsqrt (the exact fp exceptions they can produce differ). Recommended Major Opcode 63 for full-width and 59 for half-width (ending in s)

Special Registers Altered:

FPRF FR FI FX OX UX XX
VXSNAN VXIMZ VXZDZ
CR1                    (if Rc=1)
0.5 6.10 11.15 16.20 21..30 31 name Form
NN FRT /// FRB 1xxxx011xx Rc trigonometric X-Form

Recommended 10-bit XO assignments:

opcode Description Major 59 and 63
fsin(s) sin (radians) 10010 01101
fcos(s) cos (radians) 10011 01100
ftan(s) tan (radians) 10011 01101
fasin(s) arcsin (radians) 10010 01111
facos(s) arccos (radians) 10011 01110
fatan(s) arctan (radians) 10011 01111
fsinpi(s) sin(π * x) 10000 01101
fcospi(s) cos(π * x) 10001 01100
ftanpi(s) tan(π * x) 10001 01101
fasinpi(s) arcsin(x) / π 10000 01111
facospi(s) arccos(x) / π 10001 01110
fatanpi(s) arctan(x) / π 10001 01111
fsinh(s) hyperbolic sin 10100 01101
fcosh(s) hyperbolic cos 10101 01100
ftanh(s) hyperbolic tan 10101 01101
fasinh(s) inverse hyperbolic sin 10100 01111
facosh(s) inverse hyperbolic cos 10101 01110
fatanh(s) inverse hyperbolic tan 10101 01111

Subsets

The full set is based on the Khronos OpenCL opcodes. If implemented entirely it would be too much for both Embedded and also 3D.

The subsets are organised by hardware complexity, need (3D, HPC), however due to synthesis producing inaccurate results at the range limits, the less common subsets are still required for IEEE754 HPC.

MALI Midgard, an embedded / mobile 3D GPU, for example only has the following opcodes:

E8 - fatan_pt2
F0 - frcp (reciprocal)
F2 - frsqrt (inverse square root, 1/sqrt(x))
F3 - fsqrt (square root)
F4 - fexp2 (2^x)
F5 - flog2
F6 - fsin1pi
F7 - fcos1pi
F9 - fatan_pt1

These in FP32 and FP16 only: no FP64 hardware, at all.

Vivante Embedded/Mobile 3D (etnaviv https://github.com/laanwj/etna_viv/blob/master/rnndb/isa.xml) only has the following:

sin, cos2pi
cos, sin2pi
log2, exp
sqrt and rsqrt
recip.

It also has fast variants of some of these, as a CSR Mode.

AMD's R600 GPU (R600_Instruction_Set_Architecture.pdf) and the RDNA ISA (RDNA_Shader_ISA_5August2019.pdf, Table 22, Section 6.3) have:

COS2PI (appx)
EXP2
LOG (IEEE754)
RECIP
RSQRT
SQRT
SIN2PI (appx)

AMD RDNA has F16 and F32 variants of all the above, and also has F64 variants of SQRT, RSQRT and RECIP. It is interesting that even the modern high-end AMD GPU does not have TAN or ATAN, where MALI Midgard does.

Also a general point, that customised optimised hardware targetting FP32 3D with less accuracy simply can neither be used for IEEE754 nor for FP64 (except as a starting point for hardware or software driven Newton Raphson or other iterative method).

Also in cost/area sensitive applications even the extra ROM lookup tables for certain algorithms may be too costly.

These wildly differing and incompatible driving factors lead to the subset subdivisions, below.

Transcendental Subsets

Zftrans

LOG2 EXP2 RECIP RSQRT

Zftrans contains the minimum standard transcendentals best suited to 3D. They are also the minimum subset for synthesising log10, exp10, exp1m, log1p, the hyperbolic trigonometric functions sinh and so on.

They are therefore considered "base" (essential) transcendentals.

ZftransExt

LOG, EXP, EXP10, LOG10, LOGP1, EXP1M

These are extra transcendental functions that are useful, not generally needed for 3D, however for Numerical Computation they may be useful.

Although they can be synthesised using Ztrans (LOG2 multiplied by a constant), there is both a performance penalty as well as an accuracy penalty towards the limits, which for IEEE754 compliance is unacceptable. In particular, LOG(1+FRA) in hardware may give much better accuracy at the lower end (very small FRA) than LOG(FRA).

Their forced inclusion would be inappropriate as it would penalise embedded systems with tight power and area budgets. However if they were completely excluded the HPC applications would be penalised on performance and accuracy.

Therefore they are their own subset extension.

Zfhyp

SINH, COSH, TANH, ASINH, ACOSH, ATANH

These are the hyperbolic/inverse-hyperbolic functions. Their use in 3D is limited.

They can all be synthesised using LOG, SQRT and so on, so depend on Zftrans. However, once again, at the limits of the range, IEEE754 compliance becomes impossible, and thus a hardware implementation may be required.

HPC and high-end GPUs are likely markets for these.

ZftransAdv

CBRT, POW, POWN, POWR, ROOTN

These are simply much more complex to implement in hardware, and typically will only be put into HPC applications.

  • Zfrsqrt: Reciprocal square-root.

Trigonometric subsets

Ztrigpi vs Ztrignpi

  • Ztrigpi: SINPI COSPI TANPI
  • Ztrignpi: SIN COS TAN

Ztrignpi are the basic trigonometric functions through which all others could be synthesised, and they are typically the base trigonometrics provided by GPUs for 3D, warranting their own subset.

(programmerjake: actually, all other GPU ISAs mentioned in this document have sinpi/cospi or equivalent, and often not sin/cos, because sinpi/cospi are actually waay easier to implement because range reduction is simply a bitwise mask, whereas for sin/cos range reduction is a full division by pi)

In the case of the Ztrigpi subset, these are commonly used in for loops with a power of two number of subdivisions, and the cost of multiplying by PI inside each loop (or cumulative addition, resulting in cumulative errors) is not acceptable.

In for example CORDIC the multiplication by PI may be moved outside of the hardware algorithm as a loop invariant, with no power or area penalty.

Again, therefore, if SINPI (etc.) were excluded, programmers would be penalised by being forced to divide by PI in some circumstances. Likewise if SIN were excluded, programmers would be penaslised by being forced to multiply by PI in some circumstances.

Thus again, a slightly different application of the same general argument applies to give Ztrignpi and Ztrigpi as subsets. 3D GPUs will almost certainly provide both.

Zarctrigpi and Zarctrignpi

  • Zarctrigpi: ATAN2PI ASINPI ACOSPI
  • Zarctrignpi: ATAN2 ACOS ASIN

These are extra trigonometric functions that are useful in some applications, but even for 3D GPUs, particularly embedded and mobile class GPUs, they are not so common and so are typically synthesised, there.

Although they can be synthesised using Ztrigpi and Ztrignpi, there is, once again, both a performance penalty as well as an accuracy penalty towards the limits, which for IEEE754 compliance is unacceptable, yet is acceptable for 3D.

Therefore they are their own subset extensions.

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:

fmvis ft0, 0x3F800 // upper bits of f32 1.0 (BF16)
fatan2pis FRT, FRA, ft0

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

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

Evaluation and commentary

Moved to discussion