Re: [PATCH, ping 1] Move insns without introducing new temporaries in loop2_invariant

[Jeff Law <law at redhat dot com>]

On 05/06/2015 03:47 AM, Thomas Preud'homme wrote:
> Ping?
Something to consider as future work -- I'm pretty sure PRE sets up the 
same kind of problematical pattern with a new pseudo (reaching reg) 
holding the result of the redundant expression and the original 
evaluations turned into copies from the reaching reg to the final 
destination.

That style is easy to prove correct.  There was an issue with the copies 
not propagating away that was pretty inherent in the partial redundancy 
cases that I could probably dig out of my archives if you're interested.

> > So here are the new ChangeLog entries:
> >
> > *** gcc/ChangeLog ***
> >
> > 2015-03-11  Thomas Preud'homme  <thomas.preudhomme@arm.com>
> >
> >          * loop-invariant.c (can_move_invariant_reg): New.
> >          (move_invariant_reg): Call above new function to decide whether
> >          instruction can just be moved, skipping creation of temporary
> >          register.
> >
> > *** gcc/testsuite/ChangeLog ***
> >
> > 2015-03-12  Thomas Preud'homme  <thomas.preudhomme@arm.com>
> >
> >          * gcc.dg/loop-8.c: New test.
> >          * gcc.dg/loop-9.c: New test.
It looks like there's a variety of line wrapping issues.  Please 
double-check line wrapping using an 80 column window.  Minor I know, but 
the consistency with the rest of the code is good.

> > +
> > +  /* Check that all uses reached by the def in insn would still be reached
> > +     it.  */
> > +  dest_regno = REGNO (reg);
> > +  for (use = DF_REG_USE_CHAIN (dest_regno); use; use =
> > DF_REF_NEXT_REG (use))
[ ... ]
So isn't this overly conservative if DEST_REGNO is set multiple times 
since it's going to look at all the uses, even those not necessarily 
reached by the original SET of DEST_REGNO?

Or is that not an issue for some reason?  And I'm not requiring you to 
make this optimal, but if I'm right, a comment here seems wise.


I think with the wrapping nits fixed and closure on the multi-set issue 
noted immediately above and this will be good for the trunk.

Jeff