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Re: DWARF register numbering discrepancy on SPARC between GCC and GDB
- From: Mark Kettenis <mark dot kettenis at xs4all dot nl>
- To: brobecker at adacore dot com, ebotcazou at adacore dot com
- Cc: gcc at gcc dot gnu dot org, gdb at sourceware dot org
- Date: Wed, 21 Jan 2009 13:37:02 +0100 (CET)
- Subject: Re: DWARF register numbering discrepancy on SPARC between GCC and GDB
- References: <20090121110847.GU5709@adacore.com>
> Date: Wed, 21 Jan 2009 15:08:47 +0400
>
> Hello,
>
> 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.
Strange, since the GCC 3.2 installed on one of the Solaris boxes at
work defenitely starts numbering from 32.
> 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.
Ah, but he is overlooking that sol2.h used to have its own defenition
of DBX_REGISTER_NUMBER:
/* However it appears that Solaris 2.0 uses the same reg numbering as
the old BSD-style system did. */
/* Same as sparc.h */
#undef DBX_REGISTER_NUMBER
#define DBX_REGISTER_NUMBER(REGNO) \
(TARGET_FLAT && (REGNO) == HARD_FRAME_POINTER_REGNUM ? 31 : REGNO)
> However, when I tried to find some kind of official document
> to confirm this numbering, I only found:
>
> http://wikis.sun.com/display/SunStudio/Dwarf+Register+Numbering
>
> 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.
>
> Opinions?
Obviously the GCC folks broke backwards compatibility with themselves.
So unless we find evidence that contradicts the wiki page you cite, I
think GCC needs to be fixed.
OpenBSD and Linux are fine; they use 32-63 to number f0-f31.