Re: Hints for backporting gcc 4.5 powerpc fix to gcc 4.4.3?

[Simon Baldwin <simonb@google.com>]

On 22 March 2011 14:56, David Edelsohn <dje.gcc@gmail.com> wrote:
>
> On Tue, Mar 22, 2011 at 9:25 AM, Simon Baldwin <simonb@google.com> wrote:
> > I'm currently trying to backport a small part of gcc 4.5 r151729 to
> > gcc 4.4.3.  This revision fixes a problem in powerpc code generation
> > that leads to gcc not using lmw/stmw instructions in function prologue
> > and epilogues, where it could otherwise validly use them.
> >
> > On the face of things, the central piece of r151729 I seem to want is just this:
> >
> > Index: gcc/config/rs6000/rs6000.c
> > ===================================================================
> > --- gcc/config/rs6000/rs6000.c  (revision 151728)
> > +++ gcc/config/rs6000/rs6000.c  (revision 151729)
> > @@ -18033,7 +18033,8 @@ static bool
> >  no_global_regs_above (int first, bool gpr)
> >  {
> >   int i;
> > -  for (i = first; i < gpr ? 32 : 64 ; i++)
> > +  int last = gpr ? 32 : 64;
> > +  for (i = first; i < last; i++)
> >     if (global_regs[i])
> >       return false;
> >   return true;
> >
> > Taking only that and leaving out all of the rest of r151729 lets me
> > build a powerpc gcc that does use lmw/stmw instructions in function
> > prologue and epilogues as hoped.  Unfortunately it also has bad
> > codegen elsewhere.  So it seems I need more than just this little
> > piece of r151729.  Unfortunately, r151729 is a fairly large patch that
> > seems to do a number of jobs and which does not apply readily to gcc
> > 4.4.  At the moment it's not clear to me what other parts of it I
> > might need.
> >
> > Can anyone here offer any hints or pointers on how to extract from the
> > r151729 diff just the few pieces needed to fix this single powerpc
> > codegen bug in gcc 4.4.3?  Anyone recognize this issue and already
> > dealt with it in isolation?
>
> The change to no_global_regs_above() is one of the key pieces, but
> that change exposed other latent bugs, as you have encountered.  One
> needs the additional patches to the save/restore strategy routines and
> prologue/epilogue.  This is why the entire patch was committed in one
> piece.

Thanks for the reply, David.  I'll take another look and see if I can
abstract out just the required pieces.  In practice, though, it looks
like it may be easier for me to just upgrade to gcc 4.5 or 4.6.
Certainly safer.

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