[Libre-soc-isa] [Bug 1183] add /mrr mode (reverse mode) to Data-Dependent Fail-First CR_ops

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1183

Jacob Lifshay <programmerjake at gmail.com> changed:

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

--- Comment #1 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #0)
> it was discovered when implementing pow(x,y,mod) in the cryptoprimitives
> grant that "reverse data-dependent fail-first" is needed. this bugreport
> needs to implement that, but there is only room to do so for the 5-bit
> mode of CR_ops. fortunately this covers sv.cmpi which is a critically
> important use-case.

are you sure it covers sv.cmpi? cmpi has BF as the destination field which is 3
bits, not 5.

(In reply to Luke Kenneth Casson Leighton from bug 1044 comment #56)
> |6 | 7 |19:20|21 | 22:23   |  description     |
> |--|---|-----|---|---------|------------------|
> |/ | / |0  0 |RG | dz  sz  | simple mode                      |
> |/ | / |1  0 |RG | dz  sz  | scalar reduce mode (mapreduce) |
> |zz|SNZ|VLI 1|inv|  CR-bit | Ffirst 3-bit mode      |
> |/ |SNZ|VLI 1|inv|  dz sz  | Ffirst 5-bit mode (implies CR-bit from result) |

actually, we *can* still fit DD-FF 3-bit, all we need is to squeeze it into the
unused space of scalar reduce mode, dropping some less-common flags (SNZ and
zz):
> |6 | 7 |19:20|21 | 22:23   |  description     |
> |--|---|-----|---|---------|------------------|
> |0 | / |1  0 |RG | dz  sz  | scalar reduce mode (mapreduce) |
> |1 |VLI|1  0 |inv|  CR-bit | reversed Ffirst 3-bit mode      |
> |RG|SNZ|VLI 1|inv|  dz sz  | Ffirst 5-bit mode (implies CR-bit from result) |

though, now that I look at it more closely, do we need SNZ that much? if we
don't, we can replace it with RG

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