[Bug optimization/12067] New: combine error

["amodra at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12067

           Summary: combine error
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: amodra at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu

A current mainline bootstrap fails with an unrecognizable insn when compiling
libjava/verify.cc if configured with:
CFLAGS="-g -O" CXXFLAGS="-g -O" /src/gcc-current/configure
This results in verify.cc being compiled with -O1 instead of the normal -O2.

What seems to be happening is that the following three insns:

(insn 138 514 139 5 /src/gcc-current/libjava/verify.cc:553 (set (reg:SI 94 [
<variable>.local_changed ])
        (mem/s/j:SI (plus:SI (reg/v/f:SI 59 [ copy ])
                (const_int 16 [0x10])) [0 <variable>.local_changed+0 S4 A32]))
-1 (nil)
    (nil))

(insn 139 138 517 5 /src/gcc-current/libjava/verify.cc:553 (set (reg:CCZ 17 flags)
        (compare:CCZ (mem:QI (plus:SI (reg:SI 94 [ <variable>.local_changed ])
                    (reg/v:SI 84 [ i ])) [0 S1 A8])
            (const_int 0 [0x0]))) -1 (insn_list 138 (nil))
    (expr_list:REG_UNUSED (reg:CCZ 17 flags)
        (nil)))

(insn 517 139 518 5 /src/gcc-current/libjava/verify.cc:553 (set (reg:CC 17 flags)
        (compare:CC (mem:QI (plus:SI (reg:SI 94 [ <variable>.local_changed ])
                    (reg/v:SI 84 [ i ])) [0 S1 A8])
            (const_int 1 [0x1]))) -1 (nil)
    (expr_list:REG_DEAD (reg:SI 94 [ <variable>.local_changed ])
        (nil)))

are combined (well, the first two), to

(gdb) p debug_rtx (newpat)
(parallel [
        (set (reg:CCZ 17 flags)
            (compare:CCZ (mem:QI (plus:SI (mem/s/j:SI (plus:SI (reg/v/f:SI 59 [
copy ])
                                (const_int 16 [0x10])) [0
<variable>.local_changed+0 S4 A32])
                        (reg/v:SI 84 [ i ])) [0 S1 A8])
                (const_int 0 [0x0])))
        (set (reg:SI 94 [ <variable>.local_changed ])
            (mem/s/j:SI (plus:SI (reg/v/f:SI 59 [ copy ])
                    (const_int 16 [0x10])) [0 <variable>.local_changed+0 S4 A32]))
    ])

which is invalid, but code at combine.c:2046 figures correctly that the
first set is setting an unused reg so can be eliminated.  The second set
is just insn 138 again and is obviously valid.  The trouble is that
reg:SI 94 is also substituted in insn 517, at the time the first two
insns are combined, giving us:

(note 138 514 139 5 NOTE_INSN_DELETED)

(insn 139 138 517 5 /src/gcc-current/libjava/verify.cc:553 (set (reg:SI 94 [
<variable>.local_changed ])
        (mem/s/j:SI (plus:SI (reg/v/f:SI 59 [ copy ])
                (const_int 16 [0x10])) [0 <variable>.local_changed+0 S4 A32]))
38 {*movsi_1} (nil)
    (nil))

(insn 517 139 518 5 /src/gcc-current/libjava/verify.cc:553 (set (reg:CC 17 flags)
        (compare:CC (mem:QI (plus:SI (mem/s/j:SI (plus:SI (reg/v/f:SI 59 [ copy ])
                            (const_int 16 [0x10])) [0 <variable>.local_changed+0
S4 A32])
                    (reg/v:SI 84 [ i ])) [0 S1 A8])
            (const_int 1 [0x1]))) -1 (nil)
    (expr_list:REG_DEAD (reg:SI 94 [ <variable>.local_changed ])
        (nil)))