[libre-riscv-dev] [Bug 336] ALU CompUnit needs to recognise that RA (src1) can be zero

[bugzilla-daemon@libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=336

--- Comment #5 from Luke Kenneth Casson Leighton <lkcl@lkcl.net> ---
Cesar, hi,

just to let you know, this should *not* interfere with adding is_zero support. 
it however will be necessary to use this:

    if hasattr(self.oper_i, "zero_a"):
        # ok now we know that this operation requires zero_a support, we can    
        # mux in zero_a.  otherwise we must put register a straight through

i am doing a minor code-morph on MultiCompUnit which makes it suitable for
general use across multiple pipelines, as specified by "regspecs".  an example
is below.

note that this example - which uses CompCROpSubset **NOT** CompALUOpSubset -
is why we will need the "detection" above.

please do not let this stop you from adding zero_a detection support, let us
keep in touch and coordinate here.

i will have to add "detect if there is an operand B immediate" in the same way,
so you can see how it is done.


class CompCROpSubset(Record):
    def __init__(self, name=None):
        layout = (('insn_type', InternalOp),
                  ('fn_unit', Function),
                  ('insn', 32),
                  ('read_cr_whole', 1),
                  ('write_cr_whole', 1),
                  )

class CRInputData(IntegerData):
    regspec = [('INT', 'a', '0:63'),      # 64 bit range
               ('INT', 'b', '0:63'),      # 6B bit range
               ('CR', 'full_cr', '0:31'), # 32 bit range
               ('CR', 'cr_a', '0:3'),     # 4 bit range
               ('CR', 'cr_b', '0:3'),     # 4 bit range
               ('CR', 'cr_c', '0:3')]     # 4 bit range
class CROutputData(IntegerData):
    regspec = [('INT', 'o', '0:63'),      # 64 bit range
               ('CR', 'full_cr', '0:31'), # 32 bit range
               ('CR', 'cr_o', '0:3')]     # 4 bit range
    ...

class CRPipeSpec(CommonPipeSpec):
    regspec = (CRInputData.regspec, CROutputData.regspec)
    opsubsetkls = CompCROpSubset

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