combine: Fix up -fcompare-debug issue in the combiner [PR104544]

combine: Fix up -fcompare-debug issue in the combiner [PR104544]

On the following testcase on aarch64-linux, we behave differently
with -g and -g0.

The problem is that on:
(insn 10011 10010 10012 2 (set (reg:CC 66 cc)
        (compare:CC (reg:DI 105)
            (const_int 0 [0]))) "pr104544.c":18:3 407 {cmpdi}
     (expr_list:REG_DEAD (reg:DI 105)
        (nil)))
(insn 10012 10011 10013 2 (set (reg:SI 109)
        (eq:SI (reg:CC 66 cc)
            (const_int 0 [0]))) "pr104544.c":18:3 444 {aarch64_cstoresi}
     (expr_list:REG_DEAD (reg:CC 66 cc)
        (nil)))
(insn 10013 10012 10016 2 (set (reg:DI 110)
        (zero_extend:DI (reg:SI 109))) "pr104544.c":18:3 111 {*zero_extendsidi2_aarch64}
     (expr_list:REG_DEAD (reg:SI 109)
        (nil)))
(insn 10016 10013 10017 2 (parallel [
            (set (reg:CC 66 cc)
                (compare:CC (const_int 0 [0])
                    (reg:DI 110)))
            (set (reg:DI 111)
                (neg:DI (reg:DI 110)))
        ]) "pr104544.c":18:3 281 {negdi_carryout}
     (expr_list:REG_DEAD (reg:DI 110)
        (nil)))
...
(debug_insn 6 5 7 2 (var_location:SI y (debug_expr:SI D#5)) "pr104544.c":18:3 -1
     (nil))
(debug_insn 7 6 10033 2 (debug_marker) "pr104544.c":11:3 -1
     (nil))
(insn 10033 7 10034 2 (set (reg:DI 117 [ _14 ])
        (ior:DI (reg:DI 111)
            (reg:DI 112))) "pr104544.c":11:6 496 {iordi3}
     (expr_list:REG_DEAD (reg:DI 112)
        (expr_list:REG_DEAD (reg:DI 111)
            (nil))))
we successfully split 3 insns into two:

Trying 10011, 10013 -> 10016:
 10011: cc:CC=cmp(r105:DI,0)
      REG_DEAD r105:DI
 10013: r110:DI=cc:CC==0
      REG_DEAD cc:CC
 10016: {cc:CC=cmp(0,r110:DI);r111:DI=-r110:DI;}
      REG_DEAD r110:DI
Failed to match this instruction:
(parallel [
        (set (reg:CC 66 cc)
            (compare:CC (reg:DI 105)
                (const_int 0 [0])))
        (set (reg:DI 111)
            (neg:DI (eq:DI (reg:DI 105)
                    (const_int 0 [0]))))
    ])
Failed to match this instruction:
(parallel [
        (set (reg:CC 66 cc)
            (compare:CC (reg:DI 105)
                (const_int 0 [0])))
        (set (reg:DI 111)
            (neg:DI (eq:DI (reg:DI 105)
                    (const_int 0 [0]))))
    ])
Successfully matched this instruction:
(set (reg:DI 111)
    (neg:DI (eq:DI (reg:DI 105)
            (const_int 0 [0]))))
Successfully matched this instruction:
(set (reg:CC 66 cc)
    (compare:CC (reg:DI 105)
        (const_int 0 [0])))
Successfully matched this instruction:
(set (reg:DI 112)
    (neg:DI (eq:DI (reg:CC 66 cc)
            (const_int 0 [0]))))
allowing combination of insns 10011, 10013 and 10016
original costs 4 + 4 + 4 = 16
replacement costs 4 + 4 = 12
deferring deletion of insn with uid = 10011.

but the code that searches forward for insns to update their log
links (before the change there is a link from insn 10033 to insn 10016
for pseudo 111) only finds insn 10033 and updates the log link if
-g isn't enabled, otherwise it stops earlier because there are debug insns
in between.  So, with -g LOG_LINKS of 10033 isn't updated, points eventually
to NOTE_INSN_DELETED and so we do not attempt to combine 10033 with other
insns, while with -g0 we do.

The following patch fixes that by instead ignoring debug insns during the
searching.  We can still check BLOCK_FOR_INSN (insn) on those, because
if we notice DEBUG_INSN in a following basic block, necessarily there won't
be any further normal insns in the current block after it.

2022-02-16  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/104544
	* combine.cc (try_combine): When looking for insn whose links
	should be updated from i3 to i2, don't stop on debug insns, instead
	skip over them.

	* gcc.dg/pr104544.c: New test.