[libre-riscv-dev] [Bug 44] IEEE754 FPU inverse-sqrt

bugzilla-daemon at libre-riscv.org bugzilla-daemon at libre-riscv.org
Mon Apr 29 08:55:02 BST 2019


Jacob Lifshay <programmerjake at gmail.com> changed:

           What    |Removed                     |Added
                 CC|                            |programmerjake at gmail.com

--- Comment #1 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #0)
> for 32-bit, to use "wtf" algorithm:
> https://en.wikipedia.org/wiki/Fast_inverse_square_root#Overview_of_the_code
> 16-bit and 64-bit, requires thought.

The "wtf" algorithm is not well suited for our needs, since Vulkan requires a
much more accurate inv-sqrt operation than that algorithm provides, it requires
several floating-point mul-adds, and adding more iterations of newton's method
to fix the accuracy problems will increase the latency well beyond what a
radix-4 digit-recurrence algorithm needs due to needing several passes through
a multiplier.

One paper I found that may be useful:
A Hardware Algorithm for Computing Reciprocal Square Root:

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