Implementing -fdata-sections for uninitialised data (resubmission)

[Nick Clifton <nickc at cygnus dot com>]

Hi Guys,

  This is a resubmission of a patch which I submitted last year, but
  which appears to have fallen through the cracks and not been
  reviewed.

  This patch adds the ability to generate unique sections for data
  that is going to go in the .bss section.  Together with the
  --gc-sections command line switch this will allow the gnu linker to
  discard any uninitialised variables that are not referenced by any
  code, saving space in the resulting executable. 

  Note that this feature is only enabled if the -fdata-sections
  command line switch is given to GCC, and then only if the new target
  macros ASM_OUTPUT_SHARED_BSS or ASM_OUTPUT_SHARED_LOCAL are
  defined.  Since no port currently defines these macros, this patch
  cannot affect current code generation.  (I have an implementation of
  these macros for the arm-elf toolchain, which works for the eCos
  sources).

  This patch also needs a small addition to the linker script of
  whichever toolchain actually uses it.  I will be submitting a simple
  patch to binutils to do this for the default elf linker script.
  (And yes, Geoff, I will fix up .sbss too...) 

  Note - this patch makes no attempt to generate unique section names
  for common variables.  There are two reasons for this.  Firstly
  common variables are typically generated by issuing the .comm
  assembler pseudo op, and it is the assembler which is responsible
  for switching to the .comm section, generating the symbol and
  assigning a size to it.  Hence it would be necessary to either
  extend the assembler so that the .comm directive could take a
  section name, or else bypass the assembler completely and generate
  the .comm entry manually.

  The second reason is that the linker will treat entries in the .comm
  section specially, (ie it will treat them as common) and I was not
  sure if this behaviour would be extended to sections named, for
  example .comm.foo.

  Of course these problems could be fixed, but there is currently no
  demand for them, so why complicate this patch ?

  Is it OK to apply this patch ?

Cheers
	Nick

2000-01-03  Nick Clifton  <nickc@cygnus.com>

	* varasm.c (asm_emit_uninitialised): If flag_data_sections is
        true, then attempt to use ASM_OUTPUT_UNIQUE_BSS or
        ASM_OUTPUT_UNIQUE_LOCAL to emit the variable.

	* tm.texi (ASM_OUTPUT_UNIQUE_BSS): Document new target macro.
	(ASM_OUTPUT_UNIQUE_LOCAL): Document new target macro.

Index: gcc/varasm.c
===================================================================
RCS file: /cvs/gcc/egcs/gcc/varasm.c,v
retrieving revision 1.91
diff -p -r1.91 varasm.c
*** varasm.c	1999/12/20 13:18:16	1.91
--- varasm.c	2000/01/03 20:27:29
*************** asm_emit_uninitialised (decl, name, size
*** 1274,1279 ****
--- 1274,1299 ----
  	}
      }
  
+   if (flag_data_sections)
+     {
+       switch (destination)
+ 	{
+ #ifdef ASM_OUTPUT_UNIQUE_BSS
+ 	case asm_dest_bss:
+ 	  ASM_OUTPUT_UNIQUE_BSS (asm_out_file, decl, name, size);
+ 	  return;
+ #endif
+ #ifdef ASM_OUTPUT_UNIQUE_LOCAL
+ 	case asm_dest_local:
+ 	  ASM_OUTPUT_UNIQUE_LOCAL (asm_out_file, decl, name, size);
+ 	  return;
+ #endif
+ 	case asm_dest_common:
+ 	default:
+ 	  break;
+ 	}
+     }
+   
    switch (destination)
      {
  #ifdef ASM_EMIT_BSS

Index: gcc/tm.texi
===================================================================
RCS file: /cvs/gcc/egcs/gcc/tm.texi,v
retrieving revision 1.99
diff -p -r1.99 tm.texi
*** tm.texi	1999/11/30 12:16:55	1.99
--- tm.texi	2000/01/03 20:27:30
*************** If defined, it is similar to @code{ASM_O
*** 5479,5484 ****
--- 5479,5491 ----
  is used when @var{name} is shared.  If not defined, @code{ASM_OUTPUT_BSS}
  will be used.
  
+ @findex ASM_OUTPUT_UNIQUE_BSS
+ @item ASM_OUTPUT_UNIQUE_BSS (@var{stream}, @var{decl}, @var{name}, @var{size})
+ If defined, it is similar to @code{ASM_OUTPUT_BSS}, except that it
+ is used when @var{name} should be placed in its own uniquely named
+ section so that it can be subject to linker garbage collection.  If not
+ defined, @code{ASM_OUTPUT_BSS} will be used.
+ 
  @findex ASM_OUTPUT_LOCAL
  @item ASM_OUTPUT_LOCAL (@var{stream}, @var{name}, @var{size}, @var{rounded})
  A C statement (sans semicolon) to output to the stdio stream
*************** in place of both @code{ASM_OUTPUT_DECL} 
*** 5510,5522 ****
  @code{ASM_OUTPUT_ALIGNED_DECL}.  Define this macro when you need to see
  the variable's decl in order to chose what to output.
  
- 
  @findex ASM_OUTPUT_SHARED_LOCAL
  @item ASM_OUTPUT_SHARED_LOCAL (@var{stream}, @var{name}, @var{size}, @var{rounded})
  If defined, it is similar to @code{ASM_OUTPUT_LOCAL}, except that it
  is used when @var{name} is shared.  If not defined, @code{ASM_OUTPUT_LOCAL}
  will be used.
  @end table
  
  @node Label Output
  @subsection Output and Generation of Labels
--- 5517,5535 ----
  @code{ASM_OUTPUT_ALIGNED_DECL}.  Define this macro when you need to see
  the variable's decl in order to chose what to output.
  
  @findex ASM_OUTPUT_SHARED_LOCAL
  @item ASM_OUTPUT_SHARED_LOCAL (@var{stream}, @var{name}, @var{size}, @var{rounded})
  If defined, it is similar to @code{ASM_OUTPUT_LOCAL}, except that it
  is used when @var{name} is shared.  If not defined, @code{ASM_OUTPUT_LOCAL}
  will be used.
  @end table
+ 
+ @findex ASM_OUTPUT_UNIQUE_LOCAL
+ @item ASM_OUTPUT_UNIQUE_LOCAL (@var{stream}, @var{decl}, @var{name}, @var{size})
+ If defined, it is similar to @code{ASM_OUTPUT_LOCAL}, except that it
+ is used when @var{name} should be placed in its own uniquely named
+ section so that it can be subject to linker garbage collection.  If not
+ defined, @code{ASM_OUTPUT_LOCAL} will be used.
  
  @node Label Output
  @subsection Output and Generation of Labels