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

[David Edelsohn <dje.gcc@gmail.com>]

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