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DWARF register numbering discrepancy on SPARC between GCC and GDB


Eric and I discovered a discrepancy in the DWARF register numbering
on SPARC for floating point registers.  The problem is more visible
on SPARC 64-bit because they are used for parameter passing, whether
i0 is used on 32-bit SPARC.  Consider for instance the following code:

    volatile register float r asm("f0");
    int foo(float f)
      r = f;
    At -O0 -g:
            st      %i0, [%fp+68]
            ld      [%fp+68], %f0
            .byte   0x5     ! uleb128 0x5; (DIE (0xd2) DW_TAG_variable)
            .ascii "r\0"    ! DW_AT_name
            .byte   0x1     ! DW_AT_decl_file (t.c)
            .byte   0x1     ! DW_AT_decl_line
            .uaword 0xdf    ! DW_AT_type
            .byte   0x1     ! DW_AT_external
            .byte   0x2     ! DW_AT_location
            .byte   0x90    ! DW_OP_regx
     !!->   .byte   0x28    ! uleb128 0x28
            .byte   0x6     ! uleb128 0x6; (DIE (0xdf) DW_TAG_volatile_type)
            .uaword 0xc9    ! DW_AT_type

As you can see, GCC tells us that variable "r" is in register 0x28=40.
The problem is that GCC thinks that register 40 is f0, whereas GDB
thinks that register 32 is f0.

More generally, GCC thinks that registers f0-f31 should be numbered 40-71:

/* Define how the SPARC registers should be numbered for Dwarf output.
   The numbering provided here should be compatible with the native
   svr4 SDB debugger in the SPARC/svr4 reference port.  The numbering
   is as follows:

   Assembly name        gcc internal regno      Dwarf regno
   g0-g7                0-7                     0-7
   o0-o7                8-15                    8-15
   l0-l7                16-23                   16-23
   i0-i7                24-31                   24-31
   f0-f31               32-63                   40-71

According to Eric, this has been like that for the past since 1992.

However, when I tried to find some kind of official document
to confirm this numbering, I only found:

This is a wiki page, so I'm not sure how much we can trust the contents.
However, it does contradict the numbers above: Apparently DBX expects
f0-f31 to be numbered 32-63, not 40-71. If that information is correct,
perhaps Sun changed it since the first implementation in SDB? Does
anyone have maybe a more affirmative document?

The decision we need to make is to decide whether to change GDB
to match GCC or to change GCC. Changing GDB shouldn't be very hard,
but I think we should choose the same numbering scheme as DBX.


Thank you!

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