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.
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:
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?
Thank you!
--
Joel
