Re: H8 cc0 conversion

[Jeff Law <law@redhat.com>]

On 11/23/20 9:49 PM, Hans-Peter Nilsson wrote:
> On Sun, 22 Nov 2020, Jeff Law via Gcc-patches wrote:
>> This is the primary patch for cc0 removal on the H8 port.? It doesn't
>> have any of the optimization work and many patterns are simply disabled
>> at this time.? It's working well enough to not regress the testsuite.
>>
>> The H8 is similar to the m68k and other ports in that the vast majority
>> of instructions clobber the condition codes, including most of the
>> arithmetic insns that reload needs to use. While there's a special
>> adds/subs that does not modify the condition codes, they only accept
>> constant addends of 1, 2 and 4. With that in mind, this port does not
>> expose cc0 until after reload. So most patterns are defined using
>> define_insn_and_split. The splitter adds the appropriate clobbers.
> JFTR (as I'm repeating a previous note for another port): if
> you'd went for exposing cc0 *before* reload (adding a clobber to
> each pattern that clobbers, then specifying exceptions removing
> them), the amount of (required) define_insn_and_splits would
> have been zero; typically much less churn in the port.  That
> approach also has the benefit of insns not "silently" changing
> behavior at reload-time.
>
> You might think a parallel with a clobber for each insn hampers
> some rtl optimizations, but I found that to be not an issue at
> all, doing that for CRIS (at least when comparing to the cc0
> version).
I've definitely seen cases where exposing the clobber early hurts, so I
went with exposing after reload.

I also had someone else doing most of the grunt work, my son :-)

Jeff