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Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant)

Cross-posted to 2 groups.

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John Ames <commodorejohn@gmail.com> writes:
> On Wed, 02 Sep 2026 02:04:51 GMT
> MitchAlsup <user5857@newsgrouper.org.invalid> wrote:
>> For ADD, carry Rd contains only 1 bit, but for others, it contains
>> “all the bits that did not make it into the standard result”. This
>> makes CARRY suitable for all integer multiprecision arithmetic and
>> for significant quantities of FP arithmetic.
>> [...]
>> Multi-precisions shifts use carry
>
> Yes, exactly - add/subtract set the designated overflow/carry register
> to 1 on a carry/borrow (which can then be added/subtracted from the next
> more significant word) but e.g. multiply can set it to the whole most-
> significant half of the product, multi-place shifts can catch all the
> bits that fall off one end in the other so that rotate becomes shift-
> and-OR, etc. Very flexible, especially if you have the capability to
> branch on the value of a GPR.

A bit-shift operation with two source registers is sometimes called a
“funnel shift”, which is sort of the inverse of this:
<https://www.eevblog.com/forum/programming/funnel-shifter-where-are-useful/>
<https://stackoverflow.com/questions/12767113/funnel-shift-what-is-it>
<https://github.com/rust-lang/rust/issues/145686>

That is, instead of one source register (for a given bit shift size) and
two destination registers, a funnel shift has two source registers (plus
the amount to shift) and possibly just one destination register.

An interesting thing about funnel shifts is that, if both source
registers are the same, it provides bit rotation, not just shift.
Conceivably you could get the same result from this inverse funnel
shift, depending on how you handle the case where the destination
register is the same as the CARRY register.

Kragen

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adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) Kragen Javier Sitaker <kragen@canonical.org> - 2026-09-01 17:15 -0300
  Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) John Ames <commodorejohn@gmail.com> - 2026-09-01 14:31 -0700
    Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) BGB <cr88192@gmail.com> - 2026-09-01 17:18 -0500
    Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) MitchAlsup <user5857@newsgrouper.org.invalid> - 2026-09-02 02:04 +0000
      Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) John Ames <commodorejohn@gmail.com> - 2026-09-02 08:23 -0700
        Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) Kragen Javier Sitaker <kragen@canonical.org> - 2026-09-11 13:48 -0300
          Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) MitchAlsup <user5857@newsgrouper.org.invalid> - 2026-09-11 18:03 +0000
      Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) Kragen Javier Sitaker <kragen@canonical.org> - 2026-09-02 13:52 -0300
        Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) MitchAlsup <user5857@newsgrouper.org.invalid> - 2026-09-02 17:41 +0000
    Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2026-09-09 11:16 +0000
  Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) MitchAlsup <user5857@newsgrouper.org.invalid> - 2026-09-02 01:53 +0000
  Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) EricP <ThatWouldBeTelling@thevillage.com> - 2026-09-05 12:08 -0400
    Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2026-09-09 13:36 +0000
  Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2026-09-09 10:50 +0000
    Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) Paul Rubin <no.email@nospam.invalid> - 2026-09-09 22:10 -0700
    Re: adding overflow and carry to every GPR (was Re: OT: Epic RISC-V rant) Kragen Javier Sitaker <kragen@canonical.org> - 2026-09-11 14:28 -0300

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