Bug 17993 - Error in dwarf2 debug output of bitfield members
Summary: Error in dwarf2 debug output of bitfield members
Alias: None
Product: gcc
Classification: Unclassified
Component: target (show other bugs)
Version: 3.4.1
: P2 normal
Target Milestone: 4.3.0
Assignee: Not yet assigned to anyone
Depends on: 19885
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Reported: 2004-10-14 08:11 UTC by Torleif Sandnes
Modified: 2008-03-14 16:37 UTC (History)
4 users (show)

See Also:
Host: 386
Target: avr-*-*
Known to work:
Known to fail:
Last reconfirmed: 2007-04-28 17:26:32


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Description Torleif Sandnes 2004-10-14 08:11:49 UTC
Reading specs from c:/programs/WinAVR/bin/../lib/gcc/avr/3.4.1/specs
Configured with: ../gcc-3.4.1/configure --prefix=e:/avrdev/install
--build=mingw32 --host=mingw32 --target=avr --enable-languages=c,c++
Thread model: single
gcc version 3.4.1

command line:
avr-gcc -gdwarf-2 bitfieldbug.c  -o bf.elf

The debug output as shown by avr-readelf -w bf.elf shows that the first bitfield
member has a DW_AT_data_member_location of ff ff ff ff ff, which is obviously

This behaviour seems to persist no matter how I define the bitfield.

typedef struct
  int   j:5;
  int   k:6;
  int   m:5;
  int   n:8;
} faultybitfield_t; 

int main(int arcg, char **argv)
  faultybitfield_t bf;
  bf.j = 3;
  return 0;
---end source---
Comment 1 Torleif Sandnes 2004-10-14 08:24:33 UTC
This has been discussed on the gcc mailinglist:
Comment 2 Eric Weddington 2007-03-10 19:40:08 UTC
From the link in comment #1, on the gcc list, from Jim Wilson:

">    DW_AT_member_location seems to consequently equal -1 (ff ff ff ff) for the > first member of a bitfield. 

FYI You can get descriptive assembly by using the -dA option. This adds comments that make the dwarf2 debug info readable. Gcc calls this DW_AT_data_member_location.

The DWARF2 info comes from dwarf2out.c. Grepping for DW_AT_data_member_location shows that it comes from the function add_data_member_location_attribute. The offset comes from the function field_byte_offset. Looking at this code, it seems to have a general problem on targets where TYPE_ALIGN of int is smaller than TYPE_SIZE of int. This is true on avr where the size is 2 but the alignment is 1. The code computes the end of the bitfield, subtracts the type size, and then rounds up to the alignment, which gives -1 for the offset, which is not a useful number. I would think the code would work better if we started with the beginning of the bitfield, and then rounded down to the alignment. I'd suggest checking the history of the code to see if there is a reason why it was written this way. There may be an obscure reason why the code is like it is."

The issue is in field_byte_offset() in dwarf2out.c.

Can someone with Bugzilla permissions please mark this bug as NEW (confirmed)?
Comment 3 Eric Weddington 2007-08-22 17:48:54 UTC
Bug fixed in 4.3.0 20070817 snapshot.
Comment 4 Richard Biener 2008-03-14 16:37:29 UTC
Fixed then.