Re: [RFA] patch to skip bigtoc fixup code

[Elena Zannoni <ezannoni at cygnus dot com>]

Nicholas Duffek writes:
 > This patch is an improvement over the one I committed a couple of weeks
 > ago.  With the new patch, GDB works similarly to dbx:
 > 
 >   1. "step" silently steps through bigtoc fixup code.
 > 
 >   2. Numeric fixup code addresses resolve to symbols @FIX*.  For example,
 >      when debugging a cc1plus binary containing bigtoc fixup section @FIX1
 >      in address range 0x1001b614 through 0x1001bfc8, "x/i 0x1001b6fc"
 >      displays "<@FIX1+232>: addis r3,r2,1".
 > 
 > Without the patch, "step" stops at bigtoc fixup code because GDB detects
 > that it has stepped outside the current function into the middle of
 > another function.  A subsequent "step" would return to the original
 > function.
 > 
 > The patch uses some of the trampoline hooks in handle_inferior_event() to
 > tell GDB to skip bigtoc fixup code.  The hooks detect bigtoc fixup code by
 > looking for "@FIX" in the first 4 bytes of the code's function name.  "@"
 > symbols are no longer ignored by xcoffread.c; instead, they introduduce
 > new symbol tables and generate function minimal_symbols via the
 > misc_func_recorded mechanism.
 > 

Ah, I see. AIX has separate symbol tables for each section. If you
record the '@' symbols in their own symtab you will avoid also the
problem that prompted your previous patch, correct?

Anyway, the xcoffread.c part looks good to me.

Elena


 > ChangeLog:
 > 
 > 	* config/rs6000/tm-rs6000.h (IN_SOLIB_RETURN_TRAMPOLINE): Define.
 > 	(rs6000_in_solib_return_trampoline): Declare.
 > 	* rs6000-tdep.c (rs6000_in_solib_return_trampoline): New
 > 	function.
 > 	(rs6000_skip_trampoline_code): Skip bigtoc fixup code.
 > 	* xcoffread.c (read_xcoff_symtab): Perform the ISFCN function
 > 	check after the CSECT check rather than before it.  Allocate
 > 	separate symtabs for CSECTs whose names begin with '@'.
 > 	(scan_xcoff_symtab): Don't ignore symbols beginning with '@'.
 > 	Activate the misc_func_recorded mechanism for whose names begin
 > 	with '@'.
 > 
 > Tested on powerpc-ibm-aix4.3.3.0.  Okay to apply?
 > 
 > Nicholas Duffek
 > <nsd@redhat.com>
 > 
 > [patch follows]
 > 
 > Index: gdb/config/rs6000/tm-rs6000.h
 > ===================================================================
 > diff -up gdb/config/rs6000/tm-rs6000.h gdb/config/rs6000/tm-rs6000.h
 > --- gdb/config/rs6000/tm-rs6000.h	Fri Apr 27 13:41:03 2001
 > +++ gdb/config/rs6000/tm-rs6000.h	Fri Apr 27 12:09:00 2001
 > @@ -36,6 +36,13 @@ extern char *pc_load_segment_name (CORE_
 >  #undef CPLUS_MARKER
 >  #define CPLUS_MARKER '.'
 >  
 > +/* Return whether PC in function NAME is in code that should be skipped when
 > +   single-stepping.  */
 > +
 > +#define IN_SOLIB_RETURN_TRAMPOLINE(pc, name) \
 > +  rs6000_in_solib_return_trampoline (pc, name)
 > +extern int rs6000_in_solib_return_trampoline (CORE_ADDR, char *);
 > +
 >  /* If PC is in some function-call trampoline code, return the PC
 >     where the function itself actually starts.  If not, return NULL.  */
 >  
 > Index: gdb/rs6000-tdep.c
 > ===================================================================
 > diff -up gdb/rs6000-tdep.c gdb/rs6000-tdep.c
 > --- gdb/rs6000-tdep.c	Fri Apr 27 13:41:08 2001
 > +++ gdb/rs6000-tdep.c	Fri Apr 27 13:40:52 2001
 > @@ -1045,19 +1045,55 @@ rs6000_extract_return_value (struct type
 >  
 >  static CORE_ADDR rs6000_struct_return_address;
 >  
 > -/* Indirect function calls use a piece of trampoline code to do context
 > -   switching, i.e. to set the new TOC table. Skip such code if we are on
 > -   its first instruction (as when we have single-stepped to here). 
 > -   Also skip shared library trampoline code (which is different from
 > +/* Return whether handle_inferior_event() should proceed through code
 > +   starting at PC in function NAME when stepping.
 > +
 > +   The AIX -bbigtoc linker option generates functions @FIX0, @FIX1, etc. to
 > +   handle memory references that are too distant to fit in instructions
 > +   generated by the compiler.  For example, if 'foo' in the following
 > +   instruction:
 > +
 > +     lwz r9,foo(r2)
 > +
 > +   is greater than 32767, the linker might replace the lwz with a branch to
 > +   somewhere in @FIX1 that does the load in 2 instructions and then branches
 > +   back to where execution should continue.
 > +
 > +   GDB should silently step over @FIX code, just like AIX dbx does.
 > +   Unfortunately, the linker uses the "b" instruction for the branches,
 > +   meaning that the link register doesn't get set.  Therefore, GDB's usual
 > +   step_over_function() mechanism won't work.
 > +
 > +   Instead, use the IN_SOLIB_RETURN_TRAMPOLINE and SKIP_TRAMPOLINE_CODE hooks
 > +   in handle_inferior_event() to skip past @FIX code.  */
 > +
 > +int
 > +rs6000_in_solib_return_trampoline (CORE_ADDR pc, char *name)
 > +{
 > +  return name && !strncmp (name, "@FIX", 4);
 > +}
 > +
 > +/* Skip code that the user doesn't want to see when stepping:
 > +
 > +   1. Indirect function calls use a piece of trampoline code to do context
 > +   switching, i.e. to set the new TOC table.  Skip such code if we are on
 > +   its first instruction (as when we have single-stepped to here).
 > +
 > +   2. Skip shared library trampoline code (which is different from
 >     indirect function call trampolines).
 > +
 > +   3. Skip bigtoc fixup code.
 > +
 >     Result is desired PC to step until, or NULL if we are not in
 > -   trampoline code.  */
 > +   code that should be skipped.  */
 >  
 >  CORE_ADDR
 >  rs6000_skip_trampoline_code (CORE_ADDR pc)
 >  {
 >    register unsigned int ii, op;
 > +  int rel;
 >    CORE_ADDR solib_target_pc;
 > +  struct minimal_symbol *msymbol;
 >  
 >    static unsigned trampoline_code[] =
 >    {
 > @@ -1070,6 +1106,21 @@ rs6000_skip_trampoline_code (CORE_ADDR p
 >      0x4e800020,			/*    br                */
 >      0
 >    };
 > +
 > +  /* Check for bigtoc fixup code.  */
 > +  msymbol = lookup_minimal_symbol_by_pc (pc);
 > +  if (msymbol && rs6000_in_solib_return_trampoline (pc, SYMBOL_NAME (msymbol)))
 > +    {
 > +      /* Double-check that the third instruction from PC is relative "b".  */
 > +      op = read_memory_integer (pc + 8, 4);
 > +      if ((op & 0xfc000003) == 0x48000000)
 > +	{
 > +	  /* Extract bits 6-29 as a signed 24-bit relative word address and
 > +	     add it to the containing PC.  */
 > +	  rel = ((int)(op << 6) >> 6);
 > +	  return pc + 8 + rel;
 > +	}
 > +    }
 >  
 >    /* If pc is in a shared library trampoline, return its target.  */
 >    solib_target_pc = find_solib_trampoline_target (pc);
 > Index: gdb/xcoffread.c
 > ===================================================================
 > diff -up gdb/xcoffread.c gdb/xcoffread.c
 > --- gdb/xcoffread.c	Fri Apr 27 13:41:13 2001
 > +++ gdb/xcoffread.c	Fri Apr 27 12:52:10 2001
 > @@ -1100,14 +1100,6 @@ read_xcoff_symtab (pst)
 >  	  /* done with all files, everything from here on is globals */
 >  	}
 >  
 > -      /* if explicitly specified as a function, treat is as one. */
 > -      if (ISFCN (cs->c_type) && cs->c_sclass != C_TPDEF)
 > -	{
 > -	  bfd_coff_swap_aux_in (abfd, raw_auxptr, cs->c_type, cs->c_sclass,
 > -				0, cs->c_naux, &main_aux);
 > -	  goto function_entry_point;
 > -	}
 > -
 >        if ((cs->c_sclass == C_EXT || cs->c_sclass == C_HIDEXT)
 >  	  && cs->c_naux == 1)
 >  	{
 > @@ -1177,7 +1169,8 @@ read_xcoff_symtab (pst)
 >  						SECT_OFF_TEXT (objfile));
 >  		      file_end_addr = file_start_addr + CSECT_LEN (&main_aux);
 >  
 > -		      if (cs->c_name && cs->c_name[0] == '.')
 > +		      if (cs->c_name && (cs->c_name[0] == '.'
 > +					 || cs->c_name[0] == '@'))
 >  			{
 >  			  last_csect_name = cs->c_name;
 >  			  last_csect_val = cs->c_value;
 > @@ -1251,6 +1244,16 @@ read_xcoff_symtab (pst)
 >  	    }
 >  	}
 >  
 > +      /* If explicitly specified as a function, treat is as one.  This check
 > +	 evaluates to true for @FIX* bigtoc CSECT symbols, so it must occur
 > +	 after the above CSECT check.  */
 > +      if (ISFCN (cs->c_type) && cs->c_sclass != C_TPDEF)
 > +	{
 > +	  bfd_coff_swap_aux_in (abfd, raw_auxptr, cs->c_type, cs->c_sclass,
 > +				0, cs->c_naux, &main_aux);
 > +	  goto function_entry_point;
 > +	}
 > +
 >        switch (cs->c_sclass)
 >  	{
 >  
 > @@ -2295,14 +2298,8 @@ scan_xcoff_symtab (objfile)
 >  	    else
 >  	      csect_aux = main_aux[0];
 >  
 > -	    /* If symbol name starts with ".$" or "$", ignore it. 
 > -
 > -	       A symbol like "@FIX1" introduces a section for -bbigtoc jump
 > -	       tables, which contain anonymous linker-generated code. 
 > -	       Ignore those sections to avoid "pc 0x... in read in psymtab,
 > -	       but not in symtab" warnings from find_pc_sect_symtab.  */
 > -
 > -	    if (namestring[0] == '$' || namestring[0] == '@'
 > +	    /* If symbol name starts with ".$" or "$", ignore it.  */
 > +	    if (namestring[0] == '$'
 >  		|| (namestring[0] == '.' && namestring[1] == '$'))
 >  	      break;
 >  
 > @@ -2348,7 +2345,11 @@ scan_xcoff_symtab (objfile)
 >  			       objfile->static_psymbols.next);
 >  			  }
 >  		      }
 > -		    if (namestring && namestring[0] == '.')
 > +		    /* Activate the misc_func_recorded mechanism for
 > +		       compiler- and linker-generated CSECTs like ".strcmp"
 > +		       and "@FIX1".  */ 
 > +		    if (namestring && (namestring[0] == '.'
 > +				       || namestring[0] == '@'))
 >  		      {
 >  			last_csect_name = namestring;
 >  			last_csect_val = symbol.n_value;
 >