digitalmars.D.bugs - [Issue 3987] New: [gdb] Invalid DWARF output for function pointers
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http://d.puremagic.com/issues/show_bug.cgi?id=3987
Summary: [gdb] Invalid DWARF output for function pointers
Product: D
Version: 1.055
Platform: Other
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P2
Component: DMD
AssignedTo: nobody puremagic.com
ReportedBy: robert octarineparrot.com
16:18:50 PDT ---
The following program causes dmd to produce invalid DWARF output, rendering the
program impossible to debug with gdb:
----
void function() myfunc;
void main(){}
----
When compiled with either -g or -gc, causes this error in gdb when setting a
breakpoint (eg b _Dmain):
----
Die: DW_TAG_padding (abbrev = 0, offset = 0)
has children: FALSE
attributes:
Dwarf Error: Cannot find type of die [in module /tmp/test]
----
From what I can gather, the DWARF output is correct (objdump --dwarf gives no
errors), but the padding before .debug_info is incorrect, causing gdb to be
unable to read it properly.
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Mar 19 2010
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12:01:39 PDT ---
Compile with:
$ dmd -c -gc test.d
$ gcc test.o -o test -m32
(Note: I had a dmd.conf in the same directory to make sure druntime/phobos
weren't included, I don't know if this is needed)
----
void function() foobar;
extern(C)void _Dmodule_ref(){}
extern(C)int main(int argc, char** argv)
{
return 0;
}
//void foo(){}
//void main(){}
----
Uncommenting either of the 2 functions at the bottom will trigger the bug, but
without them it does not happen. I have attached 3 files, which are the outputs
of the following 3 commands:
$ objdump --dwarf test > dwarf2
$ diff -u dwarf2 dwarf
Hopefully this should help figure out what's happening. Without reading into
the DWARF spec I cannot figure out what's wrong, but there's not a huge amount
of difference between them, so it shouldn't take too much effort for someone
who knows the spec to figure out what's wrong. The above was tested with dmd
2.042.
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Mar 27 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3987 12:12:48 PDT --- For some unknown reason (probably PEBKAC), the attachments were added to the wrong ticket... See: http://d.puremagic.com/issues/attachment.cgi?id=590 http://d.puremagic.com/issues/attachment.cgi?id=591 http://d.puremagic.com/issues/attachment.cgi?id=592 -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Mar 27 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3987 07:37:29 PDT --- I've picked up on this again today, and it seems the debug info produced is wrong either way... -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Mar 28 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3987
14:28:51 PDT ---
I've found the issue. In backend/dwarf.c:dward_typidx(), there is a switch
statement which switches on the basic type for the current type. For a function
pointer, this is a pointer, so the debug info for a pointer is generated. dmd
tries to select the type the pointer is pointing to, (for int* it would be int
for example) and seems to select the wrong value for function pointers.
Comparing the debug output with that of clang, it seems that the DW_AT_type is
not needed for function pointers. Commenting out lines 1210, 1328, 1329 and
1386 (in dmd 2.042, may be different for other versions) fixes the debug info
and all is well. Of course this breaks debug info for normal pointer types, so
a special case needs to be made.
----
void function() foobar;
extern(C)void _Dmodule_ref(){}
extern(C)int main(int argc, char** argv)
{
return 0;
}
----
When compiled with dmd -gc test.d, the debug info is as follows (irrelevant
parts removed):
.debug_info:
<1><4b>: Abbrev Number: 4 (DW_TAG_pointer_type)
<4c> DW_AT_byte_size : 4
<4d> DW_AT_type : <0x45>
.debug_abbrev:
4 DW_TAG_pointer_type [no children]
DW_AT_byte_size DW_FORM_data1
DW_AT_type DW_FORM_ref4
When removing the above lines:
.debug_info:
<1><4b>: Abbrev Number: 4 (DW_TAG_pointer_type)
<4c> DW_AT_byte_size : 4
.debug_abbrev:
4 DW_TAG_pointer_type [no children]
DW_AT_byte_size DW_FORM_data1
Removing the DW_AT_type fixes the issue. I don't know enough about the dmd
backend to create a patch to check if the type is a function pointer or not, so
I can't create a patch, but the patch should be simple if you're familiar with
the DM backend.
Looking at it again, the above isn't the right way to fix it (although it
does)... The DW_TAG_pointer_type should have a DW_AT_type (void for the above
code sample), and the DW_tag_subroutine_type shouldn't have a DW_AT_type (it is
currently set to 0x0). Sorry if this comment doesn't make complete sense, I
kept coming back to it and amending it :)
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Mar 29 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3987 15:29:47 PDT --- Created an attachment (id=593) Patch for bug 3987 -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Mar 29 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3987
Robert Clipsham <robert octarineparrot.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |patch
15:30:18 PDT ---
I've attached a patch that seems to fix this issue.
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Mar 29 2010
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Robert Clipsham <robert octarineparrot.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords|patch |
02:03:55 BST ---
The patch I created works for a selection of cases, and fixes the bug mentioned
in the bug report, however there are a lot of extremely related issues which it
doesn't fix. I've written a preliminary patch which does fix the other issues,
unfortunately it causes the debug info output to be incorrect, so it needs more
work. The current patch removes the DW_AT_type from the pointer, but to
function properly it should remove the DW_AT_type from the function. This is
rather difficult to do, as the function does not know its parent type, so
cannot know if it is a function pointer and therefore needs to leave out the
DW_AT_type. The current patch I'm working on adds another argument to
dwarf_typidx() to specify it, it seems a bit hacky though. If anyone has a
better idea of how to check if the function's parent type is a pointer, please
let me know :)
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Mar 29 2010
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Robert Clipsham <robert octarineparrot.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |patch
22:31:36 BST ---
Ignoring the above patch, and removing lines 1447 and 1469 (both DW_AT_type) of
backend/dwarf.c, fixes this and most of the function pointer related debugging
issues, and unlike the attached patch creates the correct debug info (the patch
creates debug info that works but isn't the correct way to do it). The only
situations it doesn't not fix involve:
----
// T and U can be any type
T function(U function(/*Any args here*/)) funcptr;
----
Where dmd created invalid information for .debug_range, and when attempting to
read the debug info both readelf and objdump degfault, gdb continues to DIE. If
I cannot fix this issue then this fix should be applied for now, as the
situation is still far better than it is currently (debug info for phobos and
druntime works, and most apps I've encountered don't use the above).
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Mar 30 2010
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Robert Clipsham <robert octarineparrot.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords|patch |
02:44:44 BST ---
Again, the last patch I mentioned is wrong! The correct solution is to only
output the DW_AT_type if the type isn't void. I've managed to fix the other
issue too, it needs more time to make a proper patch though. The issue is that
the DW_TAG_subroutine_type info for the .debug_info section isn't cached,
unlike all the other basic types, so when a function pointer parameter is a
function pointer, it generates the debug info again before the end of the
output for the current DW_TAG_subroutine_type (I probably didn't explain that
too well). The solution is to cache the function types, as well as the basic
types. This needs to be a bit more advanced though, as the types are combined,
so more than a basic array is needed. I haven't tested if structs/classes have
the same issue when they are parameters to function pointers, I'll check.
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Mar 31 2010
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Robert Clipsham <robert octarineparrot.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
obsolete| |
13:18:16 BST ---
Created an attachment (id=594)
This patch fixes the debug info. Tested with several test programs, including
QtD, a couple of cases from Bernard Helyer, and my own test cases (which
include the use of Phobos and DRuntime). The patch is against dmd 2.042, but it
shouldn't be too hard to adapt it to the latest svn and dmd 1.x.
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Apr 01 2010
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nfxjfg gmail.com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |nfxjfg gmail.com
Hey Walter, could you have a look at bug 3214 too? It's trivial to fix.
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Apr 01 2010
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Eldar Insafutdinov <e.insafutdinov gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |e.insafutdinov gmail.com
14:13:48 PDT ---
I confirm that the patch fixes problems with QtD.
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Apr 01 2010
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Robert Clipsham <robert octarineparrot.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
obsolete| |
21:38:13 BST ---
Created an attachment (id=604)
Updated patch to the latest dmd 2 beta, including whitespace fixes. Also
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Apr 07 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3987
Walter Bright <bugzilla digitalmars.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |bugzilla digitalmars.com
07:01:51 PDT ---
I think I figured out what was going wrong with the function type caching, and
came up with a corrected fix. Changeset 451. Please give it a try.
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Apr 27 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3987 19:43:59 BST --- Using r452 all my mini tests are working, I haven't tried QtD or other real apps, the test cases I got from them seem to work though :) -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Apr 27 2010
http://d.puremagic.com/issues/show_bug.cgi?id=3987 12:37:33 PDT --- Robert, I should add that I'm very indebted to you for figuring out what was going wrong with the dwarf output. Thank you! -- Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email ------- You are receiving this mail because: -------
Apr 27 2010
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Don <clugdbug yahoo.com.au> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
CC| |clugdbug yahoo.com.au
Resolution| |FIXED
Fixed DMD2.044 and DMD1.059.
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May 10 2010