Hints for backporting gcc 4.5 powerpc fix to gcc 4.4.3?

Hints for backporting gcc 4.5 powerpc fix to gcc 4.4.3?

[Simon Baldwin]

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?

Thanks.

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

[David Edelsohn]

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.

- David

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

[Simon Baldwin]

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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