What's the proper way to debug D programs with GDB?
robert at octarineparrot.com
Wed Mar 17 17:04:44 PDT 2010
On 17/03/10 22:50, Walter Bright wrote:
> grauzone wrote:
>> dmd producing buggy debugging information seems to be an old problem:
> The problem is that dmd produces dwarf debug info according to the dwarf
> spec. When gdb fails, I have no clue why, and nobody has ever been able
> to figure out just *what* in the dwarf info is causing gdb to fail.
> Some problems were identified a few months back, and those were fixed
> (evidently gdb cannot handle module records, and gdb also relied on some
> undocumented stuff).
> If anyone wants to peruse the gdb source and figure out exactly *what*
> it is choking on, that would be most appreciated.
I've spent about half an hour trying to track this down, and I can't
seem to reproduce the problem. The only time I can get the errors
mentioned here and in the listed bug is when using -g. When switching to
-gc the problems disappear. Does anyone have a test case that fails with
-gc too that I can play with?
The reason this happens when using -g is the D extensions to dwarf which
GDB doesn't support (even with the gdb-patches applied) are used here,
which causes the error, as gdb is unable to parse the debug info. When
using -gc, dmd doesn't use the D extensions, so the error doesn't occur.
To fix this, gdb would need patching to add support for the D
extensions, or alternatively we'll have to keep using -gc.
foo ~= 'a';
When compiled using -gc, gdb works as expected. When compiled with -g,
gdb gives the following error:
(gdb) b test.d:3
Die: DW_TAG_<unknown> (abbrev = 4, offset = 77)
has children: FALSE
DW_AT_byte_size (DW_FORM_data1) constant: 8
DW_AT_type (DW_FORM_ref4) constant ref: 69 (adjusted)
Dwarf Error: Cannot find type of die [in module /tmp/test]
Taking a look at the dwarf info for test:
% objdump --dwarf=abbrev test
-- SNIP --
4 Unknown TAG value: 41 [no children]
-- SNIP --
Unknown TAG value at 41... This isn't part of the dwarf spec, looking at
// D programming language extensions
DW_TAG_darray_type = 0x41,
DW_TAG_aarray_type = 0x42,
DW_TAG_delegate_type = 0x43,
DW_TAG_lo_user = 0x4080,
DW_TAG_hi_user = 0xFFFF,
Oh look, 0x41 is a D extension, no wonder GDB chokes :)
I'll look more into this if someone can provide me a test case which
doesn't require the D extensions to cause the error, currently this
looks like a non-bug to me as gdb doesn't support D extensions yet (yes,
this is even with a patched gdb).
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