Re: [rfa] save space by using enum bitfields

[Michael Snyder <msnyder@redhat.com>]

I gotta say, this looks like uglification to me.  Is it worth it?

Michael Elizabeth Chastain wrote:

>This adds an ENUM_BITFIELD macro to defs.h and uses it in the
>symbol table to save space.
>
>Per Andrew C, this version uses the same macro that gcc 3.3.1, for the
>sake of inter-project consistency.  The macro is in defs.h so that
>any structure definition can use it, if they want.
>
>In gcc land, Zack Weinberg originally did an autoconf test, but then he
>ran into platforms where the autoconf test succeeded but the compiler
>generated buggy code.  So now gcc does an explicit test on the compiler
>used (gcc version 2 or later).  I wrote this patch the same way.
>
>If the compiler does not give us enum bitfields, then we can choose
>between integer bitfields (still save space) or full-width enum
>(chew up space, improve debugability).  The gcc code chose the
>former, so I followed it.
>
>Testing: I ran the test suite with gcc v2 and v3, dwarf-2 and stabs+,
>binutils 2.14.  I also tested the non-gcc case by putting an "#if 0"
>into the feature test in order to get the non-gcc path.
>
>Size reduction (on my vanilla native i686-pc-linux-gnu):
>
>  struct general_symbol_info  24  20
>  struct minimal_symbol       44  40
>  struct symbol               60  48
>  struct partial_symbol       32  24
>  struct symtab               68  64
>
>I also loaded a medium-sized program (a specific version of gdb) with a
>debugging version of libc and measured memory usage at the first command
>prompt with 'ps axu'.  Total memory usage dropped from 23420 kilobytes
>to 23280 kilobytes, a savings of 0.6%.  Sigh.
>
>Comments?
>
>OK to apply?
>
>Michael C
>
>2003-08-17  Michael Chastain  <mec@shout.net>
>
>	* defs.h (ENUM_BITFIELD): New macro.
>	* symtab.h (ENUM_BITFIELD): Use it.
>	(BYTE_BITFIELD): Remove old macro, which was already disabled.
>
>Index: defs.h
>===================================================================
>RCS file: /cvs/src/src/gdb/defs.h,v
>retrieving revision 1.127
>diff -c -3 -p -r1.127 defs.h
>*** defs.h	9 Aug 2003 14:57:30 -0000	1.127
>--- defs.h	17 Aug 2003 23:17:26 -0000
>*************** struct cleanup
>*** 285,290 ****
>--- 285,299 ----
>  #endif
>  #endif
>  
>+ /* Be conservative and use enum bitfields only with GCC.
>+    This is copied from gcc 3.3.1, system.h.  */
>+ 
>+ #if defined(__GNUC__) && (__GNUC__ >= 2)
>+ #define ENUM_BITFIELD(TYPE) enum TYPE
>+ #else
>+ #define ENUM_BITFIELD(TYPE) unsigned int
>+ #endif
>+ 
>  /* Needed for various prototypes */
>  
>  struct symtab;
>Index: symtab.h
>===================================================================
>RCS file: /cvs/src/src/gdb/symtab.h,v
>retrieving revision 1.77
>diff -c -3 -p -r1.77 symtab.h
>*** symtab.h	22 Jul 2003 15:41:59 -0000	1.77
>--- symtab.h	17 Aug 2003 23:17:28 -0000
>*************** struct blockvector;
>*** 35,51 ****
>  struct axs_value;
>  struct agent_expr;
>  
>- /* 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
>- 
>  /* Define a structure for the information that is common to all symbol types,
>     including minimal symbols, partial symbols, and full symbols.  In a
>     multilanguage environment, some language specific information may need to
>--- 35,40 ----
>*************** struct general_symbol_info
>*** 107,113 ****
>       This is used to select one of the fields from the language specific
>       union above. */
>  
>!   enum language language BYTE_BITFIELD;
>  
>    /* Which section is this symbol in?  This is an index into
>       section_offsets for this objfile.  Negative means that the symbol
>--- 96,102 ----
>       This is used to select one of the fields from the language specific
>       union above. */
>  
>!   ENUM_BITFIELD(language) language : 8;
>  
>    /* Which section is this symbol in?  This is an index into
>       section_offsets for this objfile.  Negative means that the symbol
>*************** struct minimal_symbol
>*** 297,304 ****
>      mst_file_text,		/* Static version of mst_text */
>      mst_file_data,		/* Static version of mst_data */
>      mst_file_bss		/* Static version of mst_bss */
>!   }
>!   type BYTE_BITFIELD;
>  
>    /* Minimal symbols with the same hash key are kept on a linked
>       list.  This is the link.  */
>--- 286,294 ----
>      mst_file_text,		/* Static version of mst_text */
>      mst_file_data,		/* Static version of mst_data */
>      mst_file_bss		/* Static version of mst_bss */
>!   };
>! 
>!   ENUM_BITFIELD(minimal_symbol_type) type : 8;
>  
>    /* Minimal symbols with the same hash key are kept on a linked
>       list.  This is the link.  */
>*************** struct minimal_symbol
>*** 321,327 ****
>  /* Different name domains for symbols.  Looking up a symbol specifies a
>     domain and ignores symbol definitions in other name domains. */
>  
>! typedef enum
>  {
>    /* UNDEF_DOMAIN is used when a domain has not been discovered or
>       none of the following apply.  This usually indicates an error either
>--- 311,317 ----
>  /* Different name domains for symbols.  Looking up a symbol specifies a
>     domain and ignores symbol definitions in other name domains. */
>  
>! typedef enum domain_enum_tag
>  {
>    /* UNDEF_DOMAIN is used when a domain has not been discovered or
>       none of the following apply.  This usually indicates an error either
>*************** struct symbol
>*** 578,588 ****
>  
>    /* Domain code.  */
>  
>!   domain_enum domain BYTE_BITFIELD;
>  
>    /* Address class */
>  
>!   enum address_class aclass BYTE_BITFIELD;
>  
>    /* Line number of definition.  FIXME:  Should we really make the assumption
>       that nobody will try to debug files longer than 64K lines?  What about
>--- 568,578 ----
>  
>    /* Domain code.  */
>  
>!   ENUM_BITFIELD(domain_enum_tag) domain : 8;
>  
>    /* Address class */
>  
>!   ENUM_BITFIELD(address_class) aclass : 8;
>  
>    /* Line number of definition.  FIXME:  Should we really make the assumption
>       that nobody will try to debug files longer than 64K lines?  What about
>*************** struct partial_symbol
>*** 655,665 ****
>  
>    /* Name space code.  */
>  
>!   domain_enum domain BYTE_BITFIELD;
>  
>    /* Address class (for info_symbols) */
>  
>!   enum address_class aclass BYTE_BITFIELD;
>  
>  };
>  
>--- 645,655 ----
>  
>    /* Name space code.  */
>  
>!   ENUM_BITFIELD(domain_enum_tag) domain : 8;
>  
>    /* Address class (for info_symbols) */
>  
>!   ENUM_BITFIELD(address_class) aclass : 8;
>  
>  };
>  
>*************** struct symtab
>*** 775,793 ****
>  
>    char *dirname;
>  
>-   /* This component says how to free the data we point to:
>-      free_contents => do a tree walk and free each object.
>-      free_nothing => do nothing; some other symtab will free
>-      the data this one uses.
>-      free_linetable => free just the linetable.  FIXME: Is this redundant
>-      with the primary field?  */
>- 
>-   enum free_code
>-   {
>-     free_nothing, free_contents, free_linetable
>-   }
>-   free_code;
>- 
>    /* A function to call to free space, if necessary.  This is IN
>       ADDITION to the action indicated by free_code.  */
>  
>--- 765,770 ----
>*************** struct symtab
>*** 803,811 ****
>  
>    int *line_charpos;
>  
>    /* Language of this source file.  */
>  
>!   enum language language;
>  
>    /* String that identifies the format of the debugging information, such
>       as "stabs", "dwarf 1", "dwarf 2", "coff", etc.  This is mostly useful
>--- 780,802 ----
>  
>    int *line_charpos;
>  
>+   /* This component says how to free the data we point to:
>+      free_contents => do a tree walk and free each object.
>+      free_nothing => do nothing; some other symtab will free
>+      the data this one uses.
>+      free_linetable => free just the linetable.  FIXME: Is this redundant
>+      with the primary field?  */
>+ 
>+   enum free_code
>+   {
>+     free_nothing, free_contents, free_linetable
>+   };
>+ 
>+   ENUM_BITFIELD(free_code) free_code : 8;
>+ 
>    /* Language of this source file.  */
>  
>!   ENUM_BITFIELD(language) language : 8;
>  
>    /* String that identifies the format of the debugging information, such
>       as "stabs", "dwarf 1", "dwarf 2", "coff", etc.  This is mostly useful
>
>  
>