------- Additional Comments From roger at eyesopen dot com 2005-04-12 14:38
-------
Subject: Re: [PR target/20126, RFC] loop DEST_ADDR biv replacement may fail
Hi Alexandre,
On 12 Apr 2005, Alexandre Oliva wrote:
> Does any expert in rtl loop care to chime in?
I'm not sure I qualify for the title "rtl loop" expert, but setting
bl->all_reduced to zero after we fail to validate a change to the
RTL looks to be a reasonable failure mode.
I still like your fallbacks, that by trying harder we perform better
optimization, but as shown by the ARM's "stmia" instruction I suspect
there will always be cases were we can't reduce IV expressions in
some backend instructions.
Previously, we didn't even detect these cases and potentially generated
bad code. I think the ICE was an improvement over the "potentially" bad
code, but what we really need is a more graceful failure/degradation.
As you propose, I'd recommend something like (for your final clause):
/* If it wasn't a reg, create a pseudo and use that. */
rtx reg, seq;
start_sequence ();
reg = force_reg (v->mode, *v->location);
! if (validate_change_maybe_volatile (v->insn, v->location, reg))
! {
! seq = get_insns ();
! end_sequence ();
! loop_insn_emit_before (loop, 0, v->insn, seq);
! }
! else
! {
! end_sequence ();
! if (loop_dump_stream)
! fprintf (loop_dump_stream,
! "unable to reduce iv to register in insn %d\n",
! INSN_UID (v->insn));
! bl->all_reduced = 0;
! v->ignore = 1;
! continue;
! }
I think its worthwhile keeping the validate_change_maybe_volatile
calls/changes on mainline. But then for gcc 4.0.0 or 4.0.1
we can use the much simpler:
if (v->giv_type == DEST_ADDR)
/* Store reduced reg as the address in the memref where we found
this giv. */
! {
! if (!validate_change (v->insn, v->location, v->new_reg, 0))
! {
! if (loop_dump_stream)
! fprintf (loop_dump_stream,
! "unable to reduce iv to register in insn %d\n",
! INSN_UID (v->insn));
! bl->all_reduced = 0;
! v->ignore = 1;
! continue;
! }
! }
A much less intrusive regression fix than previously proposed fix for
4.0. But perhaps one of the real "rtl loop" experts would like to
comment?
Roger
--
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20126
