Re: [PATCH] i386_stab_reg_to_regnum (4 <-> 5, ebp <-> esp)

[Jim Blandy <jimb@redhat.com>]

I'm getting a bit lost, so let me try to sum up the discussion, for my
own sake.  There are two distinct questions at hand:

- Should GDB's i386_stab_reg_to_regnum be changed?
- What register numbering should Cygwin Dwarf 2 use?


For the first question: I think your original patch is correct.  The
big question is, "Why wasn't it noticed before?" but there's an answer
to that which seems pretty solid to me:

- GCC's dbx_register_map and GDB's i386_stab_reg_to_regnum simply
  aren't used on many modern systems.  Every ELF target that I see in
  GCC (except for i[34567]86-*-nto-qnx*) uses gcc/config/i386/i386.c's
  svr4_dbx_register_map.  That agrees with i386-tdep.c's
  i386_dwarf_reg_to_regnum.  In GDB, we have:

    void
    i386_elf_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
    {
      /* We typically use stabs-in-ELF with the DWARF register numbering.  */
      set_gdbarch_stab_reg_to_regnum (gdbarch, i386_dwarf_reg_to_regnum);
    }

  and i386_elf_init_abi is called by:

    - i386fbsd-tdep.c (i386fbsd_init_abi)
    - i386gnu-tdep.c (i386gnu_init_abi)
    - i386-linux-tdep.c (i386_linux_init_abi)
    - i386nbsd-tdep.c (i386nbsdelf_init_abi)
    - i386-nto-tdep.c (i386nto_init_abi)
    - i386obsd-tdep.c (i386obsd_elf_init_abi)
    - i386-tdep.c (i386_svr4_init_abi)

  So just about every ELF target uses gdb/i386.c's
  i386_dwarf_reg_to_regnum for both STABS and Dwarf 2.  So they never
  see the broken numbering.
  
- The remain non-ELF tagrets are mostly non-Dwarf 2, and thus mostly
  STABS.  And my message to Eli Zaretskii explains why %ebp / %esp
  discrepancies won't often show up in STABS.

The bit-twiddling is correct, but I'd rather see something more
direct, like:

Index: gdb/i386-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/i386-tdep.c,v
retrieving revision 1.182
diff -c -c -F'^(' -r1.182 i386-tdep.c
*** gdb/i386-tdep.c	1 Apr 2004 18:14:03 -0000	1.182
--- gdb/i386-tdep.c	5 Apr 2004 22:15:01 -0000
***************
*** 211,218 ****
    /* This implements what GCC calls the "default" register map.  */
    if (reg >= 0 && reg <= 7)
      {
!       /* General-purpose registers.  */
!       return reg;
      }
    else if (reg >= 12 && reg <= 19)
      {
--- 211,223 ----
    /* This implements what GCC calls the "default" register map.  */
    if (reg >= 0 && reg <= 7)
      {
!       /* General-purpose registers.  The debug info calls %ebp
!          register 4, and %esp register 5.  */
!       if (reg == 4)
!         return 5;
!       else if (reg == 5)
!         return 4;
!       else return reg;
      }
    else if (reg >= 12 && reg <= 19)
      {


For the second question, about what register numbering to use in
Cygwin Dwarf 2:

We agree that there are no toolchains, other than the one we're
putting together right now, that uses Dwarf 2 in PE, right?  So we
could choose any numbering we please without introducing
incompatibilities with any existing toolchain.  I'm not talking about
what would be most consistent yet; I'm just observing that we wouldn't
misread any prior existing compiler's output, or misdirect any prior
existing debugger.

So what would b the most consistent numbering to use?  It's been said
that "Dwarf 2 uses svr4_dbx_register_map."  This is true, but it's
incomplete.  The big picture, I think, is this:

- GCC doesn't switch register numberings depending on the debug format
  in use (except on rs6000).  For a given GCC, -gstabs+ and -gdwarf-2
  use the same numberings.

- Dwarf 2 is mostly widely used on ELF systems, which almost all use
  svr4_dbx_register_map --- for both STABS and Dwarf 2.

The statement "Dwarf 2 uses svr4_dbx_register_map" suggests that there
would be targets that use svr4_dbx_register_map with Dwarf 2, but a
different map for other debug formats.  But that's the exception (the
rs6000), not the rule.  In fact, it looks to me as if DJGPP uses
dbx_register_map for both STABS and Dwarf 2.  (Eli, is this right?)


It's true that the comments for svr4_dbx_register_map in
gcc/config/i386/i386.c say:

  /* Define the register numbers to be used in Dwarf debugging information.

but this comment doesn't match the code it accompanies: every i386 GCC
configuration uses either dbx_register_map or svr4_dbx_register_map
for both debug formats.