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fcvt.bf16.s encoding collides with fround.h from zfa · Issue #33 · riscv/riscv-b...

 2 years ago
source link: https://github.com/riscv/riscv-bfloat16/issues/33
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fcvt.bf16.s encoding collides with fround.h from zfa #33

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asb opened this issue Mar 27, 2023 · 12 comments

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Contributor

The Zfa extension describes fround.h as "encoded like FCVT.H.S, but with rs2=4". This collides with the proposed fcvt.bf16.s encoding which also uses 4 in the rs2 position:

field bits<32> Inst = { 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, rs1{4}, rs1{3}, rs1{2}, rs1{1}, rs1{0}, frm{2}, frm{1}, frm{0}, rd{4}, rd{3}, rd{2}, rd{1}, rd{0}, 1, 0, 1, 0, 0, 1, 1 };

Contributor

Author

Just a ping on this.

Collaborator

Thanks for pointing this out Alex. I'm looking into a new encoding for fcvt.bf16.s.

Collaborator

fcvt.bf16.s is now encoded like FCVT.H.S, but with rs2=8
This will be updated in the spec soon.

asb reacted with thumbs up emoji

@kdockser Could I ask you to roll a new release artifact with this change and a new version number? Having a cite-able PDF with a distinct version will make it easier to do change tracking for downstream projects. In particular, this should unblock us landing experimental support in LLVM.

Collaborator

Reopened until the change is pushed (which will be soon).

Contributor

Author

I see the change is pushed now (thanks!), but a new PDF build and an incremented spec version number would be much appreciated. I could commit the experimental support for bfloat16 with reference to the current commit id, but it's not ideal that people wanting to review the implementation can't easily access a PDF.

Contributor

Author

The spec number is now incremented (thanks!) - would it be possible to get a new PDF release?

Collaborator

A new PDF has been released. This includes that changes suggested by the AR Committee.

Contributor

Author

Contributor

Author

@kdockser I'm not seeing the new PDF? The last tagged release is https://github.com/riscv/riscv-bfloat16/releases/tag/20230322 (March 22nd)

Another ping, as the PDF still doesn't seem to be posted. Many thanks.

Contributor

Author

It seems a new PDF was posted last week, so this request can be considered answered. Thank you.

Contributor

Author

With the PDF now available, I've been able to update the LLVM assembler/disassembler patches based on the v0.6 version of the spec:

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