From 839bc42772ba7af66af3bd16efed4a69511312ae Mon Sep 17 00:00:00 2001 From: Segher Boessenkool Date: Wed, 27 Mar 2024 14:09:52 +0000 Subject: [PATCH] combine: Don't combine if I2 does not change To: gcc-patches@gcc.gnu.org In some cases combine will "combine" an I2 and I3, but end up putting exactly the same thing back as I2 as was there before. This is never progress, so we shouldn't do it, it will lead to oscillating behaviour and the like. If we want to canonicalise things, that's fine, but this is not the way to do it. 2024-03-27 Segher Boessenkool PR rtl-optimization/101523 * combine.cc (try_combine): Don't do a 2-insn combination if it does not in fact change I2. --- gcc/combine.cc | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/gcc/combine.cc b/gcc/combine.cc index a4479f8d836..745391016d0 100644 --- a/gcc/combine.cc +++ b/gcc/combine.cc @@ -4186,6 +4186,17 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0, adjust_for_new_dest (i3); } + /* If I2 didn't change, this is not a combination (but a simplification or + canonicalisation with context), which should not be done here. Doing + it here explodes the algorithm. Don't. */ + if (rtx_equal_p (newi2pat, PATTERN (i2))) + { + if (dump_file) + fprintf (dump_file, "i2 didn't change, not doing this\n"); + undo_all (); + return 0; + } + /* We now know that we can do this combination. Merge the insns and update the status of registers and LOG_LINKS. */ -- 2.35.3