OBSOLETE, superceded by transcendentals

Zftrans - transcendental operations

Summary:

This proposal extends RISC-V 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:

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

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

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.

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 RISC-V (primarily IEEE754 and Vulkan).

There are four different, disparate platform's needs (two new):

• 3D Embedded Platform (new)
• Embedded Platform
• 3D UNIX Platform (new)
• UNIX Platform

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 (2)

(2) Supercomputing is left out of the requirements as it is traditionally covered by Supercomputer Vectorisation Standards (such as RVV).

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.

The "contra"-requirements are:

• NOT for use with RVV (RISC-V Vector Extension). These are scalar opcodes. Ultra Low Power Embedded platforms (smart watches) are sufficiently resource constrained that Vectorisation (of any kind) is likely to be unnecessary and inappropriate.
• The requirements are not for the purposes of developing a full custom proprietary GPU with proprietary firmware driven by hardware centric optimised design decisions as a priority over collaboration.
• A full custom proprietary GPU ASIC Manufacturer may benefit from this proposal however the fact that they typically develop proprietary software that is not shared with the rest of the community likely to use this proposal means that they have completely different needs.
• This proposal is for sharing of effort in reducing development costs

Requirements Analysis

Platforms:

3D Embedded will require significantly less accuracy and will need to make power budget and die area compromises that other platforms (including Embedded) will not need to make.

3D UNIX Platform has to be performance-price-competitive: subtly-reduced accuracy in FP32 is acceptable where, conversely, in the UNIX Platform, IEEE754 compliance is a hard requirement that would compromise power and efficiency on a 3D UNIX Platform.

Even in the Embedded platform, IEEE754 interoperability is beneficial, where if it was a hard requirement the 3D Embedded platform would be severely compromised in its ability to meet the demanding power budgets of that market.

Thus, learning from the lessons of SIMD considered harmful this proposal works in conjunction with the zfpacc proposal, so as not to overburden the OP32 ISA space with extra "reduced-accuracy" opcodes.

Use-cases:

There really is little else in the way of suitable markets. 3D GPUs have extremely competitive power-efficiency and power-budget requirements that are completely at odds with the other market at the other end of the spectrum: Numerical Computation.

Interoperability in Numerical Computation is absolutely critical: it implies (correlates directly with) IEEE754 compliance. However full IEEE754 compliance automatically and inherently penalises a GPU on performance and die area, where accuracy is simply just not necessary.

To meet the needs of both markets, the two new platforms have to be created, and zfpacc proposal is a critical dependency. Runtime selection of FP accuracy allows an implementation to be "Hybrid" - cover UNIX IEEE754 compliance and 3D performance in a single ASIC.

Power and die-area requirements:

This is where the conflicts really start to hit home.

A "Numerical High performance only" proposal (suitable for Server / HPC only) would customise and target the Extension based on a quantitative analysis of the value of certain opcodes for HPC only. It would conclude, reasonably and rationally, that it is worthwhile adding opcodes to RVV as parallel Vector operations, and that further discussion of the matter is pointless.

A "Proprietary GPU effort" (even one that was intended for publication of its API through, for example, a public libre-licensed Vulkan SPIR-V Compiler) would conclude, reasonably and rationally, that, likewise, the opcodes were best suited to be added to RVV, and, further, that their requirements conflict with the HPC world, due to the reduced accuracy. This on the basis that the silicon die area required for IEEE754 is far greater than that needed for reduced-accuracy, and thus their product would be completely unacceptable in the market if it had to meet IEEE754, unnecessarily.

An "Embedded 3D" GPU has radically different performance, power and die-area requirements (and may even target SoftCores in FPGA). Sharing of the silicon to cover multi-function uses (CORDIC for example) is absolutely essential in order to keep cost and power down, and high performance simply is not. Multi-cycle FSMs instead of pipelines may be considered acceptable, and so on. Subsets of functionality are also essential.

An "Embedded Numerical" platform has requirements that are separate and distinct from all of the above!

Mobile Computing needs (tablets, smartphones) again pull in a different direction: high performance, reasonable accuracy, but efficiency is critical. Screen sizes are not at the 4K range: they are within the 800x600 range at the low end (320x240 at the extreme budget end), and only the high-performance smartphones and tablets provide 1080p (1920x1080). With lower resolution, accuracy compromises are possible which the Desktop market (4k and soon to be above) would find unacceptable.

Meeting these disparate markets may be achieved, again, through zfpacc proposal, by subdividing into four platforms, yet, in addition to that, subdividing the extension into subsets that best suit the different market areas.

Software requirements:

A "custom" extension is developed in near-complete isolation from the rest of the RISC-V Community. Cost savings to the Corporation are large, with no direct beneficial feedback to (or impact on) the rest of the RISC-V ecosystem.

However given that 3D revolves around Standards - DirectX, Vulkan, OpenGL, OpenCL - users have much more influence than first appears. Compliance with these standards is critical as the userbase (Games writers, scientific applications) expects not to have to rewrite extremely large and costly codebases to conform with non-standards-compliant hardware.

Therefore, compliance with public APIs (Vulkan, OpenCL, OpenGL, DirectX) is paramount, and compliance with Trademarked Standards is critical. Any deviation from Trademarked Standards means that an implementation may not be sold and also make a claim of being, for example, "Vulkan compatible".

For 3D, this in turn reinforces and makes a hard requirement a need for public compliance with such standards, over-and-above what would otherwise be set by a RISC-V Standards Development Process, including both the software compliance and the knock-on implications that has for hardware.

For libraries such as libm and numpy, accuracy is paramount, for software interoperability across multiple platforms. Some algorithms critically rely on correct IEEE754, for example. The conflicting accuracy requirements can be met through the zfpacc extension.

Collaboration:

The case for collaboration on any Extension is already well-known. In this particular case, the precedent for inclusion of Transcendentals in other ISAs, both from Graphics and High-performance Computing, has these primitives well-established in high-profile software libraries and compilers in both GPU and HPC Computer Science divisions. Collaboration and shared public compliance with those standards brooks no argument.

The combined requirements of collaboration and multi accuracy requirements mean that overall this proposal is categorically and wholly unsuited to relegation of "custom" status.

Quantitative Analysis

This is extremely challenging. Normally, an Extension would require full, comprehensive and detailed analysis of every single instruction, for every single possible use-case, in every single market. The amount of silicon area required would be balanced against the benefits of introducing extra opcodes, as well as a full market analysis performed to see which divisions of Computer Science benefit from the introduction of the instruction, in each and every case.

With 34 instructions, four possible Platforms, and sub-categories of implementations even within each Platform, over 136 separate and distinct analyses is not a practical proposition.

A little more intelligence has to be applied to the problem space, to reduce it down to manageable levels.

Fortunately, the subdivision by Platform, in combination with the identification of only two primary markets (Numerical Computation and 3D), means that the logical reasoning applies uniformly and broadly across groups of instructions rather than individually, making it a primarily hardware-centric and accuracy-centric decision-making process.

In addition, hardware algorithms such as CORDIC can cover such a wide range of operations (simply by changing the input parameters) that the normal argument of compromising and excluding certain opcodes because they would significantly increase the silicon area is knocked down.

However, CORDIC, whilst space-efficient, and thus well-suited to Embedded, is an old iterative algorithm not well-suited to High-Performance Computing or Mid to High-end GPUs, where commercially-competitive FP32 pipeline lengths are only around 5 stages.

Not only that, but some operations such as LOG1P, which would normally be excluded from one market (due to there being an alternative macro-op fused sequence replacing it) are required for other markets due to the higher accuracy obtainable at the lower range of input values when compared to LOG(1+P).

(Thus we start to see why "proprietary" markets are excluded from this proposal, because "proprietary" markets would make hardware-driven optimisation decisions that would be completely inappropriate for a common standard).

ATAN and ATAN2 is another example area in which one market's needs conflict directly with another: the only viable solution, without compromising one market to the detriment of the other, is to provide both opcodes and let implementors make the call as to which (or both) to optimise, at the hardware level.

Likewise it is well-known that loops involving "0 to 2 times pi", often done in subdivisions of powers of two, are costly to do because they involve floating-point multiplication by PI in each and every loop. 3D GPUs solved this by providing SINPI variants which range from 0 to 1 and perform the multiply inside the hardware itself. In the case of CORDIC, it turns out that the multiply by PI is not even needed (is a loop invariant magic constant).

However, some markets may not wish to use CORDIC, for reasons mentioned above, and, again, one market would be penalised if SINPI was prioritised over SIN, or vice-versa.

In essence, then, even when only the two primary markets (3D and Numerical Computation) have been identified, this still leaves two (three) diametrically-opposed accuracy sub-markets as the prime conflict drivers:

• Embedded Ultra Low Power
• IEEE754 compliance
• Khronos Vulkan compliance

Thus the best that can be done is to use Quantitative Analysis to work out which "subsets" - sub-Extensions - to include, provide an additional "accuracy" extension, be as "inclusive" as possible, and thus allow implementors to decide what to add to their implementation, and how best to optimise them.

This approach only works due to the uniformity of the function space, and is not an appropriate methodology for use in other Extensions with huge (non-uniform) market diversity even with similarly large numbers of potential opcodes. BitManip is the perfect counter-example.

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.

For RISCV opcode encodings see ?rv major opcode 1010011

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 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, 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.

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

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

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

List of 2-arg opcodes

List of 1-arg transcendental opcodes

List of 1-arg trigonometric opcodes

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 FP32 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+rs1) in hardware may give much better accuracy at the lower end (very small rs1) than LOG(rs1).

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.

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:

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))

Evaluation and commentary

This section will move later to discussion.

Reciprocal

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

Others may have shared hardware for recip and divide, others may not.

To avoid penalising one implementor over another, recip stays.

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.

We therefore I think have a case for bringing back ATAN and including ATAN2.

The reason is that whilst a microcode-like GPU-centric platform would do ATAN2 in terms of ATAN, a UNIX-centric platform would do it the other way round.

(that is the hypothesis, to be evaluated for correctness. feedback requested).

This because we cannot compromise or prioritise one platfrom's speed/accuracy over another. That is not reasonable or desirable, to penalise one implementor over another.

Thus, all implementors, to keep interoperability, must both have both opcodes and may choose, at the architectural and routing level, which one to implement in terms of the other.

Allowing implementors to choose to add either opcode and let traps sort it out leaves an uncertainty in the software developer's mind: they cannot trust the hardware, available from many vendors, to be performant right across the board.

Standards are a pig.

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.