[libre-riscv-dev] [isa-dev] Re: FP transcendentals (trigonometry, root/exp/log) proposal

Jacob Lifshay programmerjake at gmail.com
Sat Sep 14 03:06:48 BST 2019

On Fri, Sep 13, 2019, 18:47 Mitchalsup <mitchalsup at aol.com> wrote:

> Mitch Alsup
> MitchAlsup at aol.com
> -----Original Message-----
> From: Jacob Lifshay <programmerjake at gmail.com>
> To: Luke Kenneth Casson Leighton <luke.leighton at gmail.com>
> Cc: RISC-V ISA Dev <isa-dev at groups.riscv.org>; Mitchalsup <
> mitchalsup at aol.com>; allen.baum <allen.baum at esperantotech.com>;
> libre-riscv-dev <libre-riscv-dev at lists.libre-riscv.org>
> Sent: Fri, Sep 13, 2019 8:33 pm
> Subject: Re: [isa-dev] Re: FP transcendentals (trigonometry, root/exp/log)
> proposal
> On Fri, Sep 13, 2019, 18:06 lkcl <luke.leighton at gmail.com> wrote:
> On Saturday, September 14, 2019 at 4:56:07 AM UTC+8, Jacob Lifshay wrote:
> > Some notes:
> >
> >
> > I think it may be worthwhile to have separate Ztrans extension names to
> indicate the levels of accuracy that are implemented, allowing a
> low-precision implementation of all instructions outside of F and D while
> having full-precision implementations of F and D for code compatibility.
> Hum, hum, don't know. My concern: that would be an NxM table of extension
> names. There are around 8 Ztrans ectensions, times four (so far, just found
> that OpenCL is different from Vulkan so that's 5) which would be 40
> potential different extension names, rather than N+M which would be 12-13.
> Assuming higher-precision operations are allowed to be used to implement
> lower-precision modes, we could have the higher-precision extensions just
> imply the lower precision extensions.
> alternatively, we could introduce the concept of extension parameters
> (like C++ template parameters).
> While I have no vote in the matter--I would implore you not to shoot
> yourself in the foot this way.

please do elaborate on why you think it's a bad idea -- the riscv spec
already seems to have parameterized extensions: XLEN (which is the pointer
size in bits) as well as FLEN (size of fp register in bits)

parameterizing the Zftrans subextensions would be similar -- parameterizing
each based on the maximum precision implemented. so, the ISA string could
be something like: RV64GC_Zfacc_Zftrigpi.vlk_Zfrsqrt.full_Zfhyp.ml which
would indicate that fsinpi and friends support the Vulkan/OpenCL precision
mode and lower, frsqrt supports all precision modes, and ftanh and friends
supports the machine learning precision mode and lower (though there isn't
anything lower than ML mode in the current specs).


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