https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113533
Roger Sayle <roger at nextmovesoftware dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2024-01-22
Status|UNCONFIRMED |NEW
CC| |roger at nextmovesoftware dot
com
Ever confirmed|0 |1
--- Comment #6 from Roger Sayle <roger at nextmovesoftware dot com> ---
To help diagnose the problem, I came up with this simple patch:
diff --git a/gcc/fwprop.cc b/gcc/fwprop.cc
index 7872609b336..dc563ac2ca1 100644
--- a/gcc/fwprop.cc
+++ b/gcc/fwprop.cc
@@ -492,6 +492,9 @@ try_fwprop_subst_pattern (obstack_watermark &attempt,
insn_change &use_change,
" (cost %d -> cost %d)\n", old_cost, new_cost);
ok = false;
}
+ else if (dump_file)
+ fprintf (dump_file, "change is profitable"
+ " (cost %d -> cost %d)\n", old_cost, new_cost);
}
if (!ok)
which then helps reveal that on sh3-linux-gnu with -O1 we see:
propagating insn 6 into insn 12, replacing:
(set (reg:SI 174 [ _1 ])
(sign_extend:SI (reg:QI 169 [ *a_7(D) ])))
successfully matched this instruction to *extendqisi2_compact_snd:
(set (reg:SI 174 [ _1 ])
(sign_extend:SI (mem:QI (reg/v/f:SI 168 [ aD.1817 ]) [0 *a_7(D)+0 S1 A8])))
change is profitable (cost 4 -> cost 1)
which confirms Andrew's and Oleg's analyses above; the sh_rtx_costs function is
a little odd... Reading from memory is four times faster than using a pseudo!?
I'm investigating a "costs" patch for the SH backend. My apologies for the
temporary inconvenience, and thanks to Jeff for catching/spotting this.
