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

Jacob Lifshay programmerjake at gmail.com
Thu Aug 8 08:10:46 BST 2019

On Wed, Aug 7, 2019, 23:30 Andrew Waterman <waterman at eecs.berkeley.edu>

> Hi folks,
> We would seem to be putting the cart before the horse.  ISA-level support
> for correctly rounded transcendentals is speciously attractive, but its
> utility is not clearly evident and is possibly negative.  It does not make
> sense to allocate opcode space under these circumstances.

Since there are ways to implement transcendental functions in HW that are
faster than anything possible in SW (I think Mitch mentioned a 5-cycle sin
implementation), I would argue that having instructions for them is
beneficial, and, since they would be useful on a large number of different
implementations (GPUs, HPC, bigger desktop/server processors), it's worth
standardizing the instructions, since otherwise the custom opcodes used for
them would become effectively standardized (as mentioned by Luke) and no
longer useful as custom opcodes on implementations that need fast
transcendental functions.

I have no problems ending up with different encodings and/or semantics than
currently chosen, as long as that's done early enough and in a public
manner so that we can implement without undue delay the chosen opcodes
without being incompatible with the final spec.

Jacob Lifshay


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