https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108892
Jeffrey A. Law <law at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Assignee|unassigned at gcc dot gnu.org |law at gcc dot gnu.org
--- Comment #3 from Jeffrey A. Law <law at gcc dot gnu.org> ---
This looks like a long-standing latent bug in combine.
About 20 years ago support was added to combine to allow it to utilize a
REG_EQUAL note to help simplifications. The code looks like this:
/* Temporarily replace the set's source with the
contents of the REG_EQUAL note. The insn will
be deleted or recognized by try_combine. */
rtx orig_src = SET_SRC (set);
rtx orig_dest = SET_DEST (set);
if (GET_CODE (SET_DEST (set)) == ZERO_EXTRACT)
SET_DEST (set) = XEXP (SET_DEST (set), 0);
SET_SRC (set) = note;
i2mod = temp;
i2mod_old_rhs = copy_rtx (orig_src);
i2mod_new_rhs = copy_rtx (note);
next = try_combine (insn, i2mod, NULL, NULL,
&new_direct_jump_p,
last_combined_insn);
i2mod = NULL;
if (next)
{
statistics_counter_event (cfun, "insn-with-note combine",
1);
goto retry;
}
SET_SRC (set) = orig_src;
SET_DEST (set) = orig_dest;
So assume that temp (from which SET in the above code was extracted) looks like
this:
(insn 122 117 127 2 (set (reg:DI 157 [ _46 ])
(ior:DI (reg:DI 200)
(reg:DI 251))) "j.c":14:5 -1
(expr_list:REG_EQUAL (const_int 25769803782 [0x600000006])
(nil)))
Basically the constant isn't one we can load with a single insn, so we
construct the constant using several insns and attach a REG_EQUAL note for the
final result. Totally normal.
We replace the SET_SRC with the contents of the note. This results in the
following insn that gets passed down to try_combine as I2MOD:
(insn 122 117 127 2 (set (reg:DI 157 [ _46 ])
(const_int 25769803782 [0x600000006])) "j.c":14:5 -1
(expr_list:REG_EQUAL (const_int 25769803782 [0x600000006])
(nil)))
Exactly what I would expect. try_combine will actually try to recognize that
insn and gets a match on the mvconst_internal. So when try_combine returns the
insn looks like this:
(insn 122 117 127 2 (set (reg:DI 157 [ _46 ])
(const_int 25769803782 [0x600000006])) "j.c":14:5 177
{*mvconst_internal}
(expr_list:REG_EQUAL (const_int 25769803782 [0x600000006])
(nil)))
Again, totally normal. Nothing wrong at this point. But note how we restore
the SET_SRC/SET_DEST objects after returning from try_combine. After
restoration we have:
(insn 122 117 127 2 (set (reg:DI 157 [ _46 ])
(ior:DI (reg:DI 200)
(reg:DI 251))) "j.c":14:5 177 {*mvconst_internal}
(expr_list:REG_EQUAL (const_int 25769803782 [0x600000006])
(nil)))
And that's where things have gone off the rails. When we restore the
SET_SRC/SET_DEST we need to either re-recognize or at least clear the
INSN_CODE.
This has *absolutely nothing* to do with multiple matching patterns. It's just
a latent bug in the combiner.
