Re: Init df for split pass since some target use REG_NOTE in split pattern

[Jeff Law <law at redhat dot com>]

On 07/25/2016 12:35 PM, Richard Biener wrote:
> On July 25, 2016 5:56:29 PM GMT+02:00, Jeff Law <law@redhat.com> wrote:
> > On 07/25/2016 12:36 AM, Kito Cheng wrote:
> > > Hi all:
> > >
> > > Some target(for example i386, sh and h8300) use find_regno_note in
> > > split pattern but df infrastructure seem not initialize at split
> > pass,
> > > so it may got wrong note since it's out-of-date.
> > >
> > > ChangeLog
> > > 2016-07-25  Kito Cheng <kito.cheng@gmail.com>
> > >
> > >         * gcc/recog.c (split_all_insns): Initialize df for split
> > pass.
> > >         (split_all_insns_noflow): Likewise.
> > Patch was not included/attached to your message.  Please resend with
> > the
> > patch and hopefully a testcase which exhibits the problem.
>
> And maybe back ends shouldn't look at notes in their splitters?
It's certainly not ideal.    But I'm not sure what alternatives we have 
and it's bee supported for a very long time.

For example on the H8, there is no shift-by-a-variable-amount.  Instead 
you have to emit it as a loop.  However, we don't expose the loop until 
late in the RTL pipeline (via a splitter).  When we split a variable 
shift into a loop, one of the things we want to know is whether or not 
the register holding the shift count dies or not.  If it does not die, 
we'll just use it rather than copying it elsewhere.

Similarly we can generate a more efficient split for something like 
(ior (ashift (X) (const_int)) for certain integers when we know the 
input dies.   Or for some (add (mult)) expressions.

Jeff