Re: [PATCHv2, rs6000] Enable have_cbranchcc4 on rs6000

[HAO CHEN GUI <guihaoc@linux.ibm.com>]

Hi Segher,

Thanks for your comments.

在 2022/11/22 7:49, Segher Boessenkool 写道:
> *cbranch_2insn is not a machine insn.  It generates a cror and a branch
> insn.  This makes no sense to have in a cbranchcc: those do a branch
> based on an existing cr field, so based on the *output* of that cror.
> 
> If ifcvt requires differently, ifcvt needs fixing.
> 
I have a question here.
For rs6000, "*cbranch_2insn" should not be generated by cbranch_optab?
I mean it gets icode from cbranch_optab and generates insn from this
icode. If so, the predicate of cbranchcc4 should be checked every time
before insn generation other than just doing an assertion.

> We want to use the output of the cror multiple times, not generate more
> cror insns.
> 
> I don't think the behaviour of ifcvt is correct here at all, no.  It
> also does not consider the cost of the code as far as I can see?  That
> could reduce the impact of this problem at least.
ifcvt tries to generate the converted sequence. Then it compares the cost
of new sequence to the cost of orginial. If it benefits, the conversion
will be done.

Thanks
Gui Haochen