Re: [Patch mach-o/gas] implement .zerofill

[Tristan Gingold <gingold at adacore dot com>]

On Jan 2, 2012, at 2:05 PM, Iain Sandoe wrote:

> we make quite heavy use of .zerofill from GCC to reduce object sizes.
> 
> most of the bulk in this patch is in splitting up the 'section' code to allow reuse - and the slightly frustrating situation that .zerofill is almost (but not quite) the same as comm/lcomm.
> 
> OK?

Ok.

Do you have tests for .zerofill ?

Tristan.

> Iain
> 
> gas:
> 
> 	* config/obj-macho.c (obj_mach_o_segT_from_bfd_name): Tidy definition.
> 	(obj_mach_o_get_section_names): New (split from obj_mach_o_section).
> 	(obj_mach_o_make_or_get_sect): Likewise.
> 	(obj_mach_o_section): Split out the functionality shared with zerofill.
> 	(obj_mach_o_zerofill): New.
> 	(obj_mach_o_common_parse): Ensure whitespace is skipped.
> 	(mach_o_pseudo_table): Add .zerofill.
> 
> gas/config/obj-macho.c |  391 +++++++++++++++++++++++++++++++++++++-----------
> 1 files changed, 306 insertions(+), 85 deletions(-)
> 
> diff --git a/gas/config/obj-macho.c b/gas/config/obj-macho.c
> index 74fb0c9..445fb2f 100644
> --- a/gas/config/obj-macho.c
> +++ b/gas/config/obj-macho.c
> @@ -45,8 +45,8 @@
> #include "mach-o/loader.h"
> #include "obj-macho.h"
> 
> -/* Forward decl.  */
> -static segT obj_mach_o_segT_from_bfd_name (const char *nam, int must_succeed);
> +/* Forward decls.  */
> +static segT obj_mach_o_segT_from_bfd_name (const char *, int);
> 
> /* TODO: Implement "-dynamic"/"-static" command line options.  */
> 
> @@ -156,6 +156,139 @@ collect_16char_name (char *dest, const char *msg, int require_comma)
>   return 0;
> }
> 
> +static int
> +obj_mach_o_get_section_names (char *seg, char *sec,
> +			      unsigned segl, unsigned secl)
> +{
> +  /* Zero-length segment and section names are allowed.  */
> +  /* Parse segment name.  */
> +  memset (seg, 0, segl);
> +  if (collect_16char_name (seg, "segment", 1))
> +    {
> +      ignore_rest_of_line ();
> +      return 0;
> +    }
> +  input_line_pointer++; /* Skip the terminating ',' */
> +
> +  /* Parse section name, which can be empty.  */
> +  memset (sec, 0, secl);
> +  collect_16char_name (sec, "section", 0);
> +  return 1;
> +}
> +
> +/* Build (or get) a section from the mach-o description - which includes
> +   optional definitions for type, attributes, alignment and stub size.
> +
> +   BFD supplies default values for sections which have a canonical name.  */
> +
> +#define SECT_TYPE_SPECIFIED 0x0001
> +#define SECT_ATTR_SPECIFIED 0x0002
> +#define SECT_ALGN_SPECIFIED 0x0004
> +
> +static segT
> +obj_mach_o_make_or_get_sect (char * segname, char * sectname,
> +			     unsigned int specified_mask,
> +			     unsigned int usectype, unsigned int usecattr,
> +			     unsigned int ualign, offsetT stub_size)
> +{
> +  unsigned int sectype, secattr, secalign;
> +  flagword oldflags, flags;
> +  const char *name;
> +  segT sec;
> +  bfd_mach_o_section *msect;
> +  const mach_o_section_name_xlat *xlat;
> +
> +  /* This provides default bfd flags and default mach-o section type and
> +     attributes along with the canonical name.  */
> +  xlat = bfd_mach_o_section_data_for_mach_sect (stdoutput, segname, sectname);
> +
> +  /* TODO: more checking of whether overides are acually allowed.  */
> +
> +  if (xlat != NULL)
> +    {
> +      name = xstrdup (xlat->bfd_name);
> +      sectype = xlat->macho_sectype;
> +      if (specified_mask & SECT_TYPE_SPECIFIED)
> +	{
> +	  if ((sectype == BFD_MACH_O_S_ZEROFILL
> +	       || sectype == BFD_MACH_O_S_GB_ZEROFILL)
> +	      && sectype != usectype)
> +	    as_bad (_("cannot overide zerofill section type for `%s,%s'"),
> +		    segname, sectname);
> +	  else
> +	    sectype = usectype;
> +	}
> +      secattr = xlat->macho_secattr;
> +      secalign = xlat->sectalign;
> +      flags = xlat->bfd_flags;
> +    }
> +  else
> +    {
> +      /* There is no normal BFD section name for this section.  Create one.
> +         The name created doesn't really matter as it will never be written
> +         on disk.  */
> +      size_t seglen = strlen (segname);
> +      size_t sectlen = strlen (sectname);
> +      char *n;
> +
> +      n = xmalloc (seglen + 1 + sectlen + 1);
> +      memcpy (n, segname, seglen);
> +      n[seglen] = '.';
> +      memcpy (n + seglen + 1, sectname, sectlen);
> +      n[seglen + 1 + sectlen] = 0;
> +      name = n;
> +      if (specified_mask & SECT_TYPE_SPECIFIED)
> +	sectype = usectype;
> +      else
> +	sectype = BFD_MACH_O_S_REGULAR;
> +      secattr = BFD_MACH_O_S_ATTR_NONE;
> +      secalign = 0;
> +      flags = SEC_NO_FLAGS;
> +    }
> +
> +  /* For now, just use what the user provided.  */
> +
> +  if (specified_mask & SECT_ATTR_SPECIFIED)
> +    secattr = usecattr;
> +
> +  if (specified_mask & SECT_ALGN_SPECIFIED)
> +    secalign = ualign;
> +
> +  /* Sub-segments don't exists as is on Mach-O.  */
> +  sec = subseg_new (name, 0);
> +
> +  oldflags = bfd_get_section_flags (stdoutput, sec);
> +  msect = bfd_mach_o_get_mach_o_section (sec);
> +
> +  if (oldflags == SEC_NO_FLAGS)
> +    {
> +      /* New, so just use the defaults or what's specified.  */
> +      if (! bfd_set_section_flags (stdoutput, sec, flags))
> +	as_warn (_("failed to set flags for \"%s\": %s"),
> +		 bfd_section_name (stdoutput, sec),
> +		 bfd_errmsg (bfd_get_error ()));
> +
> +      strncpy (msect->segname, segname, sizeof (msect->segname));
> +      strncpy (msect->sectname, sectname, sizeof (msect->sectname));
> +
> +      msect->align = secalign;
> +      msect->flags = sectype | secattr;
> +      msect->reserved2 = stub_size;
> +
> +      if (sectype == BFD_MACH_O_S_ZEROFILL
> +	  || sectype == BFD_MACH_O_S_GB_ZEROFILL)
> +        seg_info (sec)->bss = 1;
> +    }
> +  else if (flags != SEC_NO_FLAGS)
> +    {
> +      if (flags != oldflags
> +	  || msect->flags != (secattr | sectype))
> +	as_warn (_("Ignoring changed section attributes for %s"), name);
> +    }
> +
> +  return sec;
> +}
> +
> /* .section
> 
>    The '.section' specification syntax looks like:
> @@ -178,21 +311,11 @@ collect_16char_name (char *dest, const char *msg, int require_comma)
> static void
> obj_mach_o_section (int ignore ATTRIBUTE_UNUSED)
> {
> -  char *p;
> -  char c;
>   unsigned int sectype = BFD_MACH_O_S_REGULAR;
> -  unsigned int defsectype = BFD_MACH_O_S_REGULAR;
> -  unsigned int sectype_given = 0;
> +  unsigned int specified_mask = 0;
>   unsigned int secattr = 0;
> -  unsigned int defsecattr = 0;
> -  int secattr_given = 0;
> -  unsigned int secalign = 0;
>   offsetT sizeof_stub = 0;
> -  const mach_o_section_name_xlat * xlat;
> -  const char *name;
> -  flagword oldflags, flags;
> -  asection *sec;
> -  bfd_mach_o_section *msect;
> +  segT new_seg;
>   char segname[17];
>   char sectname[17];
> 
> @@ -200,23 +323,15 @@ obj_mach_o_section (int ignore ATTRIBUTE_UNUSED)
>   md_flush_pending_output ();
> #endif
> 
> -  /* Zero-length segment and section names are allowed.  */
> -  /* Parse segment name.  */
> -  memset (segname, 0, sizeof(segname));
> -  if (collect_16char_name (segname, "segment", 1))
> -    {
> -      ignore_rest_of_line ();
> -      return;
> -    }
> -  input_line_pointer++; /* Skip the terminating ',' */
> +  /* Get the User's segment annd section names.  */
> +  if (! obj_mach_o_get_section_names (segname, sectname, 17, 17))
> +    return;
> 
> -  /* Parse section name.  */
> -  memset (sectname, 0, sizeof(sectname));
> -  collect_16char_name (sectname, "section", 0);
> -
> -  /* Parse type.  */
> +  /* Parse section type, if present.  */
>   if (*input_line_pointer == ',')
>     {
> +      char *p;
> +      char c;
>       char tmpc;
>       int len;
>       input_line_pointer++;
> @@ -243,13 +358,14 @@ obj_mach_o_section (int ignore ATTRIBUTE_UNUSED)
> 	  return;
>         }
>       else
> -	sectype_given = 1;
> +	specified_mask |= SECT_TYPE_SPECIFIED;
>       /* Restore.  */
>       p[len] = tmpc;
> 
>       /* Parse attributes.
> 	 TODO: check validity of attributes for section type.  */
> -      if (sectype_given && c == ',')
> +      if ((specified_mask & SECT_TYPE_SPECIFIED)
> +	  && c == ',')
>         {
>           do
>             {
> @@ -282,7 +398,7 @@ obj_mach_o_section (int ignore ATTRIBUTE_UNUSED)
>                 }
>               else
> 		{
> -		  secattr_given = 1;
> +		  specified_mask |= SECT_ATTR_SPECIFIED;
>                   secattr |= attr;
> 		}
> 	      /* Restore.  */
> @@ -291,7 +407,8 @@ obj_mach_o_section (int ignore ATTRIBUTE_UNUSED)
>           while (*input_line_pointer == '+');
> 
>           /* Parse sizeof_stub.  */
> -          if (secattr_given && *input_line_pointer == ',')
> +          if ((specified_mask & SECT_ATTR_SPECIFIED)
> +	      && *input_line_pointer == ',')
>             {
>               if (sectype != BFD_MACH_O_S_SYMBOL_STUBS)
>                 {
> @@ -303,7 +420,8 @@ obj_mach_o_section (int ignore ATTRIBUTE_UNUSED)
> 	      input_line_pointer++;
>               sizeof_stub = get_absolute_expression ();
>             }
> -          else if (secattr_given && sectype == BFD_MACH_O_S_SYMBOL_STUBS)
> +          else if ((specified_mask & SECT_ATTR_SPECIFIED)
> +		   && sectype == BFD_MACH_O_S_SYMBOL_STUBS)
>             {
>               as_bad (_("missing sizeof_stub expression"));
> 	      ignore_rest_of_line ();
> @@ -312,70 +430,170 @@ obj_mach_o_section (int ignore ATTRIBUTE_UNUSED)
>         }
>     }
> 
> -  flags = SEC_NO_FLAGS;
> -  /* This provides default bfd flags and default mach-o section type and
> -     attributes along with the canonical name.  */
> -  xlat = bfd_mach_o_section_data_for_mach_sect (stdoutput, segname, sectname);
> -  if (xlat != NULL)
> +  new_seg = obj_mach_o_make_or_get_sect (segname, sectname, specified_mask,
> +					 sectype, secattr, 0 /*align */,
> +					 sizeof_stub);
> +  if (new_seg != NULL)
>     {
> -      name = xstrdup (xlat->bfd_name);
> -      flags = xlat->bfd_flags;
> -      defsectype = xlat->macho_sectype;
> -      defsecattr = xlat->macho_secattr;
> -      secalign = xlat->sectalign;
> +      subseg_set (new_seg, 0);
> +      demand_empty_rest_of_line ();
>     }
> -  else
> +}
> +
> +/* .zerofill segname, sectname [, symbolname, size [, align]]
> +
> +   Zerofill switches, temporarily, to a sect of type 'zerofill'.
> +
> +   If a variable name is given, it defines that in the section.
> +   Otherwise it just creates the section if it doesn't exist.  */
> +
> +static void
> +obj_mach_o_zerofill (int ignore ATTRIBUTE_UNUSED)
> +{
> +  char segname[17];
> +  char sectname[17];
> +  segT old_seg = now_seg;
> +  segT new_seg;
> +  symbolS *sym = NULL;
> +  unsigned int align = 0;
> +  unsigned int specified_mask = 0;
> +  offsetT size;
> +
> +#ifdef md_flush_pending_output
> +  md_flush_pending_output ();
> +#endif
> +
> +  /* Get the User's segment annd section names.  */
> +  if (! obj_mach_o_get_section_names (segname, sectname, 17, 17))
> +    return;
> +
> +  /* Parse variable definition, if present.  */
> +  if (*input_line_pointer == ',')
>     {
> -      /* There is no normal BFD section name for this section.  Create one.
> -         The name created doesn't really matter as it will never be written
> -         on disk.  */
> -      size_t seglen = strlen (segname);
> -      size_t sectlen = strlen (sectname);
> -      char *n;
> +      /* Parse symbol, size [.align]
> +         We follow the method of s_common_internal, with the difference
> +         that the symbol cannot be a duplicate-common.  */
> +      char *name;
> +      char c;
> +      char *p;
> +      expressionS exp;
> +
> +      input_line_pointer++; /* Skip ',' */
> +      SKIP_WHITESPACE ();
> +      name = input_line_pointer;
> +      c = get_symbol_end ();
> +      /* Just after name is now '\0'.  */
> +      p = input_line_pointer;
> +      *p = c;
> 
> -      n = xmalloc (seglen + 1 + sectlen + 1);
> -      memcpy (n, segname, seglen);
> -      n[seglen] = '.';
> -      memcpy (n + seglen + 1, sectname, sectlen);
> -      n[seglen + 1 + sectlen] = 0;
> -      name = n;
> +      if (name == p)
> +	{
> +	  as_bad (_("expected symbol name"));
> +	  ignore_rest_of_line ();
> +	  goto done;
> +	}
> +
> +      SKIP_WHITESPACE ();
> +      if (*input_line_pointer == ',')
> +	input_line_pointer++;
> +
> +      expression_and_evaluate (&exp);
> +      if (exp.X_op != O_constant
> +	  && exp.X_op != O_absent)
> +	{
> +	    as_bad (_("bad or irreducible absolute expression"));
> +	  ignore_rest_of_line ();
> +	  goto done;
> +	}
> +      else if (exp.X_op == O_absent)
> +	{
> +	  as_bad (_("missing size expression"));
> +	  ignore_rest_of_line ();
> +	  goto done;
> +	}
> +
> +      size = exp.X_add_number;
> +      size &= ((offsetT) 2 << (stdoutput->arch_info->bits_per_address - 1)) - 1;
> +      if (exp.X_add_number != size || !exp.X_unsigned)
> +	{
> +	  as_warn (_("size (%ld) out of range, ignored"),
> +		   (long) exp.X_add_number);
> +	  ignore_rest_of_line ();
> +	  goto done;
> +	}
> +
> +     *p = 0; /* Make the name into a c string for err messages.  */
> +     sym = symbol_find_or_make (name);
> +     if (S_IS_DEFINED (sym) || symbol_equated_p (sym))
> +	{
> +	  as_bad (_("symbol `%s' is already defined"), name);
> +	  *p = c;
> +	  ignore_rest_of_line ();
> +	   goto done;
> +	}
> +
> +      size = S_GET_VALUE (sym);
> +      if (size == 0)
> +	size = exp.X_add_number;
> +      else if (size != exp.X_add_number)
> +	as_warn (_("size of \"%s\" is already %ld; not changing to %ld"),
> +		   name, (long) size, (long) exp.X_add_number);
> +
> +      *p = c;  /* Restore the termination char.  */
> +
> +      SKIP_WHITESPACE ();
> +      if (*input_line_pointer == ',')
> +	{
> +	  align = (unsigned int) parse_align (0);
> +	  if (align == (unsigned int) -1)
> +	    {
> +	      as_warn (_("align value not recognized, using size"));
> +	      align = size;
> +	    }
> +	  if (align > 15)
> +	    {
> +	      as_warn (_("Alignment (%lu) too large: 15 assumed."),
> +			(unsigned long)align);
> +	      align = 15;
> +	    }
> +	  specified_mask |= SECT_ALGN_SPECIFIED;
> +	}
>     }
> + /* else just a section definition.  */
> 
> -  /* Sub-segments don't exists as is on Mach-O.  */
> -  sec = subseg_new (name, 0);
> +  specified_mask |= SECT_TYPE_SPECIFIED;
> +  new_seg = obj_mach_o_make_or_get_sect (segname, sectname, specified_mask,
> +					 BFD_MACH_O_S_ZEROFILL,
> +					 BFD_MACH_O_S_ATTR_NONE,
> +					 align, (offsetT) 0 /*stub size*/);
> +  if (new_seg == NULL)
> +    return;
> 
> -  oldflags = bfd_get_section_flags (stdoutput, sec);
> -  msect = bfd_mach_o_get_mach_o_section (sec);
> -   if (oldflags == SEC_NO_FLAGS)
> +  /* In case the user specifies the bss section by mach-o name.
> +     Create it on demand */
> +  if (strcmp (new_seg->name, BSS_SECTION_NAME) == 0
> +      && bss_section == NULL)
> +    bss_section = new_seg;
> +
> +  subseg_set (new_seg, 0);
> +
> +  if (sym != NULL)
>     {
> -      if (! bfd_set_section_flags (stdoutput, sec, flags))
> -	as_warn (_("error setting flags for \"%s\": %s"),
> -		 bfd_section_name (stdoutput, sec),
> -		 bfd_errmsg (bfd_get_error ()));
> -      strncpy (msect->segname, segname, sizeof (msect->segname));
> -      msect->segname[16] = 0;
> -      strncpy (msect->sectname, sectname, sizeof (msect->sectname));
> -      msect->sectname[16] = 0;
> -      msect->align = secalign;
> -      if (sectype_given)
> +      if (new_seg == bss_section)
> 	{
> -	  msect->flags = sectype;
> -	  if (secattr_given)
> -	    msect->flags |= secattr;
> -	  else
> -	    msect->flags |= defsecattr;
> +	  bss_alloc (sym, size, align);
> +	  S_CLEAR_EXTERNAL (sym);
> 	}
>       else
> -        msect->flags = defsectype | defsecattr;
> -      msect->reserved2 = sizeof_stub;
> -    }
> -  else if (flags != SEC_NO_FLAGS)
> -    {
> -      if (flags != oldflags
> -	  || msect->flags != (secattr | sectype))
> -	as_warn (_("Ignoring changed section attributes for %s"), name);
> +	{
> +	  S_SET_VALUE (sym, (valueT) size);
> +	  S_SET_SEGMENT (sym, new_seg);	
> +	}
>     }
> -  demand_empty_rest_of_line ();
> +
> +done:
> +  /* switch back to the section that was current before the .zerofill.  */
> +  subseg_set (old_seg, 0);
> }
> 
> static segT
> @@ -675,6 +893,8 @@ obj_mach_o_common_parse (int is_local, symbolS *symbolP,
> {
>   addressT align = 0;
> 
> +  SKIP_WHITESPACE ();
> +
>   /* Both comm and lcomm take an optional alignment, as a power
>      of two between 1 and 15.  */
>   if (*input_line_pointer == ',')
> @@ -837,6 +1057,7 @@ const pseudo_typeS mach_o_pseudo_table[] =
>   { "picsymbol_stub3", obj_mach_o_opt_tgt_section, 4}, /* extension.  */
> 
>   { "section", obj_mach_o_section, 0},
> +  { "zerofill", obj_mach_o_zerofill, 0},
> 
>   /* Symbol-related.  */
>   { "indirect_symbol", obj_mach_o_placeholder, 0},
>