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Re: BYTE_BITFIELD in symtab.h
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?