Re: BYTE_BITFIELD in symtab.h

[Dan Nicolaescu <dann@godzilla.ICS.UCI.EDU>]

Andrew Cagney <ac131313@cygnus.com> writes:

  > > Hi!
  > > Is the following thing in symtab.h realy useful?
  > > /* Don't do this; it means that if some .o's are compiled with GNU C
  > >    and some are not (easy to do accidentally the way we configure
  > >    things; also it is a pain to have to "make clean" every time you
  > >    want to switch compilers), then GDB dies a horrible death.  */
  > > /* GNU C supports enums that are bitfields.  Some compilers don't. */
  > > #if 0 && defined(__GNUC__) && !defined(BYTE_BITFIELD)
  > > #define BYTE_BITFIELD   :8;
  > > #else
  > > #define BYTE_BITFIELD           /*nothing */
  > > #endif
  > > if BYTE_BITFIELD was defined to :8 the size of
  > >  "struct general_symbol_info" would decrease from 24 bytes to 20 bytes
  > > for a tipical 32 bit machine. And gdb uses quite a few of those...
  > > Isn't the price payed for being able to switch compilers too high in
  > > this case? How common are compilers that don't support enum
  > > bitfields?
  > 
  > Oh, what the heck I'll name names. ``gnu'' added this 1994-01-12 only
  > to have ``kingdon'' disable it less than a month later (1994-02-07).
  > I agree with JimK.  I think that #if is nasty asking for trouble.
  > 
  > A quick glance at my (very old) tartan labs book suggests a compiler
  > need only support unsigned bit fields.  If you think about it, that is
  > probably the only thing with vaguely well defined and fairly portable
  > semantics.
  >
  > For the above my personal preference would be to zap that macro and
  > then investigate some explicit pack/unpack or explicit (i.e. no macro)
  > unsigned bitfields.

FWIW gcc is using the following rtx_def in rtl.h


  ENUM_BITFIELD(rtx_code) code: 16;


ENUM_BITFIELD is defined in system.h:

/* Be conservative and only use enum bitfields with GCC.
   FIXME: provide a complete autoconf test for buggy enum bitfields.  */

#if (GCC_VERSION > 2000)
#define ENUM_BITFIELD(TYPE) enum TYPE
#else
#define ENUM_BITFIELD(TYPE) unsigned int
#endif

Should gdb do the same? 

  > Fortuantly, that isn't my file :-)
  > 
  > enjoy,
  > Andrew
  > 
  > PS: JimB, did you know ``struct { unsigned :1; } foo'' is valid?