[Bug target/68149] New: [ARM] ICE when splitting unaligned DImode load

[ktkachov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68149

            Bug ID: 68149
           Summary: [ARM] ICE when splitting unaligned DImode load
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: ice-on-valid-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ktkachov at gcc dot gnu.org
  Target Milestone: ---

Created attachment 36617
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36617&action=edit
Testcase

The attached testcase (reduced from a testcase in the testsuite) ICEs with
-O2 -mthumb -mcpu=cortex-a15 -mfpu=neon-vfpv4 -mfloat-abi=hard
-mfp16-format=ieee -w on arm targets.

As reported at https://gcc.gnu.org/ml/gcc-patches/2015-10/msg03074.html
this started with r229344 but it's a latend bug exposed by that commit.

internal compiler error: in change_address_1, at emit-rtl.c:2132
 }
 ^
0x7a44f4 change_address_1
        $SRC/gcc/emit-rtl.c:2132
0x7a74b3 adjust_address_1(rtx_def*, machine_mode, long, int, int, int, long)
        $SRC/gcc/emit-rtl.c:2270
0xab79f1 gen_lowpart_general(machine_mode, rtx_def*)
        $SRC/gcc/rtlhooks.c:90
0xfd7201 gen_split_47(rtx_insn*, rtx_def**)
        $SRC/gcc/config/arm/arm.md:4336
0x12473e3 split_11
        $SRC/gcc/config/arm/arm.md:4331
0x12473e3 split_insns(rtx_def*, rtx_insn*)
        $SRC/gcc/config/arm/sync.md:361
0x7af36e try_split(rtx_def*, rtx_insn*, int)
        $SRC/gcc/emit-rtl.c:3664
0xa53e89 split_insn
        $SRC/gcc/recog.c:2874
0xa5cdb9 split_all_insns()
        $SRC/gcc/recog.c:2964
0xa5cea7 rest_of_handle_split_after_reload
        $SRC/gcc/recog.c:3900
0xa5cea7 execute
        $SRC/gcc/recog.c:3929
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <http://gcc.gnu.org/bugs.html> for instructions.

This is the unaligned_loaddi splitter in arm.md
The DImode memory load it's trying to split into two is:
(insn 105 403 122 2 (set (reg:DI 0 r0 [198])
        (unspec:DI [
                (mem/u/c:DI (plus:SI (reg:SI 2 r2)
                        (const_int -256 [0xffffffffffffff00])) [0  S8 A8])
            ] UNSPEC_UNALIGNED_LOAD)) besttry.c:408 137 {unaligned_loaddi}

The splitter wants to split it into two SImode loads and -256 is not a valid
offset for such a load.

-256 was accepted as valid during reload because at that point it's seen as a
DImode load and it's accepted there because DImode is a valid
VALID_NEON_DREG_MODE, but it's not a NEON load :(