On Tue, Mar 22, 2011 at 9:25 AM, Simon Baldwin <sim...@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
