[LLVMbugs] [Bug 7595] New: DWARF debug info does not handle forward declarations properly

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=7595

           Summary: DWARF debug info does not handle forward declarations
                    properly
           Product: clang
           Version: trunk
          Platform: Other
        OS/Version: FreeBSD
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: kan at freebsd.org
                CC: llvmbugs at cs.uiuc.edu


Created an attachment (id=5190)
 --> (http://llvm.org/bugs/attachment.cgi?id=5190)
Create DICompositeType node with FlagFwdDecl for forward declatations of
scructs and unions

Given the following trivial example:

struct a;
union  b;

void foo(struct a *ptr_a, union b *ptr_b)
{
}

clang 2.7 and trunk both end up generating the following DWARF info fragment do
describe pointers to struct a and union b:

<1><  229>      DW_TAG_structure_type
                DW_AT_name                  fwd.type.0
                DW_AT_byte_size             0
                DW_AT_decl_file             2 /home/kan/test/ctf/test.c
                DW_AT_decl_line             1
<1><  244>      DW_TAG_pointer_type
                DW_AT_type                  <229>
                DW_AT_byte_size             8
                DW_AT_decl_file             2 /home/kan/test/ctf/test.c
                DW_AT_decl_line             0
<1><  252>      DW_TAG_union_type
                DW_AT_name                  fwd.type.1
                DW_AT_byte_size             0
                DW_AT_decl_file             2 /home/kan/test/ctf/test.c
                DW_AT_decl_line             2
<1><  267>      DW_TAG_pointer_type
                DW_AT_type                  <252>
                DW_AT_byte_size             8
                DW_AT_decl_file             2 /home/kan/test/ctf/test.c
                DW_AT_decl_line             0

Note the use of fictitious zero-sized structures fwd.type.[12].

Attached patch attempts to fix this, by emitting proper struct and enum
definitions with DW_AT_declaration=1 for forward declarations. This results in
the following (correct) DWARF fragment:

<1><  229>      DW_TAG_structure_type
                DW_AT_name                  a
                DW_AT_declaration           yes(1)
<1><  233>      DW_TAG_pointer_type
                DW_AT_type                  <229>
                DW_AT_byte_size             8
                DW_AT_decl_file             2 /home/kan/test/ctf/test.c
                DW_AT_decl_line             0
<1><  241>      DW_TAG_union_type
                DW_AT_name                  b
                DW_AT_declaration           yes(1)
<1><  245>      DW_TAG_pointer_type
                DW_AT_type                  <241>
                DW_AT_byte_size             8
                DW_AT_decl_file             2 /home/kan/test/ctf/test.c
                DW_AT_decl_line             0

Note proper names and DW_TAG_declaration use and notice that respective
pointers now actually make sense.

As a side effect, this makes OpenSolaris' ctftools work properly on FreeBSD
kernel compiled with clang, instead of taking ages to compile, mostly sifting
though tons of fwd.type.<num> fictitious types.

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