2016-10-27  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    * config/arm/arm.c (arm_split_atomic_op): Add function comment.  Add
    logic to to decide whether to copy over old value to register for new
    value.
    * config/arm/sync.md: Add comments explaning why mode and code
    attribute are not defined in iterators.md
    (thumb1_atomic_op_str): New code attribute.
    (thumb1_atomic_newop_str): Likewise.
    (thumb1_atomic_fetch_op_str): Likewise.
    (thumb1_atomic_fetch_newop_str): Likewise.
    (thumb1_atomic_fetch_oldop_str): Likewise.
    (atomic_exchange<mode>): Add new ARMv8-M Baseline only alternatives to
    mirror the more restrictive constraints of the Thumb-1 insns after
    split compared to Thumb-2 counterpart insns.
    (atomic_<sync_optab><mode>): Likewise.  Add comment to keep constraints
    in sync with non atomic version.
    (atomic_nand<mode>): Likewise.
    (atomic_fetch_<sync_optab><mode>): Likewise.
    (atomic_fetch_nand<mode>): Likewise.
    (atomic_<sync_optab>_fetch<mode>): Likewise.
    (atomic_nand_fetch<mode>): Likewise.
    * config/arm/thumb1.md (thumb1_addsi3): Add comment to keep contraint
    in sync with atomic version.
    (thumb1_subsi3_insn): Likewise.
    (thumb1_andsi3_insn): Likewise.
    (thumb1_iorsi3_insn): Likewise.
    (thumb1_xorsi3_insn): Likewise.

From-SVN: r241614