Re: [PATCH 25/31] VAX: Fix predicates for widening multiply and multiply-add insns

[Jeff Law via Gcc-patches]

On 11/30/20 9:02 AM, Maciej W. Rozycki wrote:
> On Fri, 20 Nov 2020, Jeff Law wrote:
>
>> ps.  Yes, I skipped the insv/extv changes.  They're usually a rats nest
>> of special cases.  We'll come back to them.
>  I've thought of actually reducing the number of patterns to the minimum 
> possible by folding the existing ones together, and then getting the 
> alternatives sorted by fine-grained constraints at reload.
That might work, but I've generally found insv/extv to be fairly painful
through the years, more because of limitations on the generic bits
rather than the target bits.

>
>  There is that complication caused by INSV machine instruction preserving 
> condition codes (understandably), so keeping it together with alternative 
> code sequences that do set the codes in a single RTL insn would cause 
> trouble with getting a matching insn for the comparison elimination pass.  
> Or so I think.
Yea.  I think that is (in general) not a well solved problem.  We have
similar issues on the H8 where we have multiple ways to implement
certain operations -- some of which set/clobber flags while others don't
touch them.  Depending on the context one form may be preferable to the
other.  I've punted this problem so far as I strongly suspect the gains
in handling this scenario well are marginal at best.

Jeff