Re: Shrink wrapping issues

[Jakub Jelinek <jakub@redhat.com>]

On Sat, Nov 05, 2011 at 08:58:18PM +1030, Alan Modra wrote:
> On Sat, Nov 05, 2011 at 10:50:44AM +0100, Jakub Jelinek wrote:
> > >From quick look, f1 isn't shrink-wrapped probably because of the set
> > of bb's that need prologue/epilogue around it doesn't end in a return,
> > but in a tail call.  Can't we just add a prologue before the bar call
> > and throw the epilogue away (normally the epilogue in a function that
> > ends only in a tail call is just emitted after the barrier and
> > optimized away I think, we could do the same?).
> 
> http://gcc.gnu.org/ml/gcc-patches/2011-11/msg00046.html ought to cure
> this particular problem.  With that patch, similar code on
> powerpc-linux does result in shrink wrapping.

I'll try that.

> > And f2 is something that IMHO with especially AVX/AVX2 code happens very
> > often, the prologue is expensive as it realigns the stack.  The reason
> > for that is that until reload we don't know whether something won't be
> > spilled on the stack and we need/want 32-byte aligned stack slots
> > for that spilling.
> 
> Huh?  thread_prologue_and_epilogue is after reload.  So your backend
> ought to be able to figure out whether an aligned stack is needed.

Sure, the backend could figure out.  But it would need to duplicate
parts of what function.c does.  In particular, roughly equivalent of
computing prologue_used and set_up_by_prologue hard regsets (sure, it
doesn't have to necessarily generate the prologue for that), then
the
  FOR_EACH_BB (bb)
    FOR_BB_INSNS (bb, insn)
      if (requires_stack_frame_p (insn, prologue_used,
				  set_up_by_prologue))
	goto needs_prologue;
(and perhaps only do this if -fno-omit-frame-pointer wasn't used, i.e. if
the prologue frame pointer setup was not requested by the user explicitly).
If it is just i?86/x86_64, then perhaps we just should export
requires_stack_frame_p and do it in the backend.  But if it is for more
targets, can't function.c just handle it in the generic code when it
already computes everything needed?

	Jakub