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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - DWARF incorrect for natively-sized bitfields"
href="https://bugs.llvm.org/show_bug.cgi?id=44601">44601</a>
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<th>Summary</th>
<td>DWARF incorrect for natively-sized bitfields
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>8.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>other
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>DebugInfo
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
<tr>
<th>Reporter</th>
<td>levon@movementarian.org
</td>
</tr>
<tr>
<th>CC</th>
<td>jdevlieghere@apple.com, keith.walker@arm.com, llvm-bugs@lists.llvm.org, paul_robinson@playstation.sony.com
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<p>
<div>
<pre>This test case comes via Robert Mustacchi.
$ llvm-config --version
8.0.1
$ clang --version
clang version 8.0.1 (tags/RELEASE_801/final)
Target: i386-pc-solaris2.11
$ cat a.c
struct bitfields {
unsigned int first:7;
unsigned int second:32;
} __attribute__((packed));
struct bitfields mb;
$ clang -gdwarf-4 -m32 -c -o a.o a.c
$ gobjdump --dwarf a.o >dwarf.out
The DWARF produced has:
74 <1><1e>: Abbrev Number: 2 (DW_TAG_variable)
75 <1f> DW_AT_name : (indirect string, offset: 0x41): mb
76 <23> DW_AT_type : <0x2f>
77 <27> DW_AT_external : 1
78 <27> DW_AT_decl_file : 1
79 <28> DW_AT_decl_line : 7
80 <29> DW_AT_location : 5 byte block: 3 0 0 0 0 (DW_OP_addr: 0)
81 <1><2f>: Abbrev Number: 3 (DW_TAG_structure_type)
82 <30> DW_AT_name : (indirect string, offset: 0x5e): bitfields
83 <34> DW_AT_byte_size : 5
84 <35> DW_AT_decl_file : 1
85 <36> DW_AT_decl_line : 2
86 <2><37>: Abbrev Number: 4 (DW_TAG_member)
87 <38> DW_AT_name : (indirect string, offset: 0x44): first
88 <3c> DW_AT_type : <0x53>
89 <40> DW_AT_decl_file : 1
90 <41> DW_AT_decl_line : 3
91 <42> DW_AT_byte_size : 4
92 <43> DW_AT_bit_size : 7
93 <44> DW_AT_bit_offset : 25
94 <45> DW_AT_data_member_location: 0
95 <2><46>: Abbrev Number: 5 (DW_TAG_member)
96 <47> DW_AT_name : (indirect string, offset: 0x57): second
97 <4b> DW_AT_type : <0x53>
98 <4f> DW_AT_decl_file : 1
99 <50> DW_AT_decl_line : 4
100 <51> DW_AT_data_member_location: 0
Note the lack of bit_offset and such for the 'second' member. GCC for the same
structure member produces (correctly I believe):
32 <2><38>: Abbrev Number: 4 (DW_TAG_member)
33 <39> DW_AT_name : (indirect string, offset: 0x23): second
34 <3d> DW_AT_decl_file : 1
35 <3e> DW_AT_decl_line : 4
36 <3f> DW_AT_type : <0x48>
37 <43> DW_AT_byte_size : 4
38 <44> DW_AT_bit_size : 32
39 <45> DW_AT_bit_offset : -7
40 <46> DW_AT_data_member_location: 0
I haven't proven it, but this line seems suspiciously pertinent:
<a href="https://github.com/llvm-mirror/llvm/blob/master/lib/CodeGen/AsmPrinter/DwarfUnit.cpp#L1481">https://github.com/llvm-mirror/llvm/blob/master/lib/CodeGen/AsmPrinter/DwarfUnit.cpp#L1481</a>
1481 bool IsBitfield = FieldSize && Size != FieldSize;
Comparing the bitfield size against the bitfield's type's size is not
sufficient in this case.</pre>
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