i386: Improve chaining of _{addcarry,subborrow}_u{32,64} [PR97387]

These builtins have two known issues and this patch fixes one of them.

One issue is that the builtins effectively return two results and
they make the destination addressable until expansion, which means
a stack slot is allocated for them and e.g. with -fstack-protector*
DSE isn't able to optimize that away.  I think for that we want to use
the technique of returning complex value; the patch doesn't handle that
though.  See PR93990 for that.

The other problem is optimization of successive uses of the builtin
e.g. for arbitrary precision arithmetic additions/subtractions.
As shown PR93990, combine is able to optimize the case when the first
argument to these builtins is 0 (the first instance when several are used
together), and also the last one if the last one ignores its result (i.e.
the carry/borrow is dead and thrown away in that case).
As shown in this PR, combiner refuses to optimize the rest, where it sees:
(insn 10 9 11 2 (set (reg:QI 88 [ _31 ])
        (ltu:QI (reg:CCC 17 flags)
            (const_int 0 [0]))) "include/adxintrin.h":69:10 785 {*setcc_qi}
     (expr_list:REG_DEAD (reg:CCC 17 flags)
        (nil)))
- set pseudo 88 to CF from flags, then some uninteresting insns that
don't modify flags, and finally:
(insn 17 15 18 2 (parallel [
            (set (reg:CCC 17 flags)
                (compare:CCC (plus:QI (reg:QI 88 [ _31 ])
                        (const_int -1 [0xffffffffffffffff]))
                    (reg:QI 88 [ _31 ])))
            (clobber (scratch:QI))
        ]) "include/adxintrin.h":69:10 350 {*addqi3_cconly_overflow_1}
     (expr_list:REG_DEAD (reg:QI 88 [ _31 ])
        (nil)))
to set CF in flags back to what we saved earlier.  The combiner just punts
trying to combine the 10, 17 and following addcarrydi (etc.) instruction,
because
  if (i1 && !can_combine_p (i1, i3, i0, NULL, i2, NULL, &i1dest, &i1src))
    {
      if (dump_file && (dump_flags & TDF_DETAILS))
        fprintf (dump_file, "Can't combine i1 into i3\n");
      undo_all ();
      return 0;
    }
fails - the 3 insns aren't all adjacent and
      || (! all_adjacent
          && (((!MEM_P (src)
                || ! find_reg_note (insn, REG_EQUIV, src))
               && modified_between_p (src, insn, i3))
src (flags hard register) is modified between the first and third insn - in
the second insn.

The following patch optimizes this by optimizing just the two insns,
10 and 17 above, i.e. save CF into pseudo, set CF from that pseudo, into
a nop.  The new define_insn_and_split matches how combine simplifies those
two together (except without the ix86_cc_mode change it was choosing CCmode
for the destination instead of CCCmode, so had to change that function too,
and also adjust costs so that combiner understand it is beneficial).

With this, all the testcases are optimized, so that the:
        setc    %dl
...
        addb    $-1, %dl
insns in between the ad[dc][lq] or s[ub]b[lq] instructions are all optimized
away (sure, if something would clobber flags in between they wouldn't, but
there is nothing that can be done about that).

2020-10-14  Jakub Jelinek  <jakub@redhat.com>

	PR target/97387
	* config/i386/i386.md (CC_CCC): New mode iterator.
	(*setcc_qi_addqi3_cconly_overflow_1_<mode>): New
	define_insn_and_split.
	* config/i386/i386.c (ix86_cc_mode): Return CCCmode
	for *setcc_qi_addqi3_cconly_overflow_1_<mode> pattern operands.
	(ix86_rtx_costs): Return true and *total = 0;
	for *setcc_qi_addqi3_cconly_overflow_1_<mode> pattern.  Use op0 and
	op1 temporaries to simplify COMPARE checks.

	* gcc.target/i386/pr97387-1.c: New test.
	* gcc.target/i386/pr97387-2.c: New test.

0 files changed, 0 insertions, 0 deletions