commit 19e7aca212afc7360f69f93100a157c0e659861d
Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date:   Tue May 14 11:18:34 2019 -0400

    Fix: bitfield: shift undefined/implementation defined behaviors (v3)

    bitfield.h uses the left shift operator with a left operand which
    may be negative. The C99 standard states that shifting a negative
    value is undefined.

    When building with -Wshift-negative-value, we get this gcc warning:

    In file included from /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h:44:0,
                     from /home/smarchi/src/babeltrace/ctfser/ctfser.c:42:
    /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h: In function ‘bt_ctfser_write_unsigned_int’:
    /home/smarchi/src/babeltrace/include/babeltrace/bitfield-internal.h:116:24: error: left shift of negative value [-Werror=shift-negative-value]
       mask = ~((~(type) 0) << (__start % ts));  \
                            ^
    /home/smarchi/src/babeltrace/include/babeltrace/bitfield-internal.h:222:2: note: in expansion of macro ‘_bt_bitfield_write_le’
      _bt_bitfield_write_le(ptr, type, _start, _length, _v)
      ^~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h:418:3: note: in expansion of macro ‘bt_bitfield_write_le’
       bt_bitfield_write_le(mmap_align_addr(ctfser->base_mma) +
       ^~~~~~~~~~~~~~~~~~~~

    This boils down to the fact that the expression ~((uint8_t)0) has type
    "signed int", which is used as an operand of the left shift.  This is due
    to the integer promotion rules of C99 (6.3.3.1):

        If an int can represent all values of the original type, the value is
        converted to an int; otherwise, it is converted to an unsigned int.
        These are called the integer promotions. All other types are unchanged
        by the integer promotions.

    We also need to cast the result explicitly into the left hand
    side type to deal with:

    warning: large integer implicitly truncated to unsigned type [-Woverflow]

    The C99 standard states that a right shift has implementation-defined
    behavior when shifting a signed negative value. Add a preprocessor check
    that the compiler provides the expected behavior, else provide an
    alternative implementation which guarantees the intended behavior.

    A preprocessor check is also added to ensure that the compiler
    representation for signed values is two's complement, which is expected
    by this header.

    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Change-Id: I17ba38952ddc4c27d770649f782fd249f6ea69be
    ---
    Changes since v1:
    - Fix wrong use of _$prefix$bt_make_mask() in _$prefix$bt_bitfield_write_be.
      Should be _$prefix$bt_make_mask_complement().

    Changes since v2:
    - Documentation and style updates.