Tuesday, 2020-02-18

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st-gourichon-fidHi !13:17
st-gourichon-fidQuestion about the FOMU.14:01
st-gourichon-fidOur application needs relatively fast non-integer computation.14:02
st-gourichon-fidFixed point would be okay, maybe even fine.  Floats would be ideal.14:02
st-gourichon-fidIIRC RiscV spec mentions fixed point but I could not find an implementation of RiscV that would fit float or even fixed point inside an Ice40.14:03
st-gourichon-fidWe are considering putting another softcore, like Arm or Mips inside the Ice40, if it can fit fast calculus (float or fixed point).14:04
xobsYou could definitely replace the core, but I don't know that it'll be fast.14:04
st-gourichon-fidBut maybe that would be too big whatever the CPU architecture we choose?14:04
xobsYou could certainly try.14:15
xobsYour options are to make a CSR-mapped (memory-mapped) accelerator, or add an extension to the CPU.14:15
xobsThere is a fixed-point working group, but I don't think they've published anything yet.14:16
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ovfst-gourichon-fid: what kind of hardware support do you envision you will require for fixed point computations?15:13
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st-gourichon-fidovf, not sure I understand your question. Do you mean, like 16.16 bits?15:53
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st-gourichon-fidxobs, thansk for the suggestion of a memory-mapped accelerator.  Indeed it would be simple to integrate.16:18
st-gourichon-fidBut non-standard, have to use a modified toolchain.16:19
tntwell, is there a standard extension for fixed point ?16:20
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ovfst-gourichon-fid: i meant, what specific non-standard functionality/instructions do you require? it is true that one can benefit from some specific types of saturation arithmetic, but generally the biggest draw of fixed point is its not needing any hardware support outside of conventional integer operations.16:28
st-gourichon-fidovf, thanks for the extended question!16:29
st-gourichon-fidI have not mentioned saturation arithmetic yet it may be necessary.  It remains one often overlooked benefit of usual floats, that they don't wrap.16:33
st-gourichon-fidAll the details are not yet defined, yet it is desirable that users of our platform can just write code (ideally Python, else C or similar), without thinking much more than "real number with limited precision".16:37
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ovfit looks to me more of a tooling than hardware capacity problem, albeit far from a trivial one. there is a proposal for fixed-point types in c, N1169, which both gcc and clang purport to support to various degrees, but from what i know less than well. python implementations don't seem to have an option of built-in fixed point support.16:57
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st-gourichon-fidovf, thanks for the reference.  You confirm my intuitions.17:04
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