llvm dev - Jul 2008 - [LLVMdev] RFC: debug_line Emission

In CodeGen/DwarfWriter.cpp's EmitDebugLine file, these lines are
causing havoc on Mac OS X systems:

    // If there are no lines to emit (such as when we're using .loc
directives
    // to emit .debug_line information) don't emit a .debug_line
header.
    if (SectionSourceLines.empty())
      return;

Basically, if there's a file with only data in it, we still need the
debug_line prologue generated.

$ cat a.c
const char ver[] __attribute__((used)) = "Hello world\n";
$ gcc -g -O0 -c a.c -o a.gcc.o
$ dwarfdump --debug-line=0 a.gcc.o
----------------------------------------------------------------------
 File: a.gcc.o (architecture i386)
----------------------------------------------------------------------

.debug_line contents:
----------------------------------------------------------------------
debug_line[0x00000000]
----------------------------------------------------------------------
Line table prologue:
   total_length: 0x00000027
        version: 0x0002
prologue_length: 0x00000017
min_inst_length: 0x01
default_is_stmt: 0x01
      line_base: -10
     line_range: 245
    opcode_base: 0x0a
standard_opcode_lengths[ DW_LNS_copy             ] = 0
standard_opcode_lengths[ DW_LNS_advance_pc       ] = 1
standard_opcode_lengths[ DW_LNS_advance_line     ] = 1
standard_opcode_lengths[ DW_LNS_set_file         ] = 1
standard_opcode_lengths[ DW_LNS_set_column       ] = 1
standard_opcode_lengths[ DW_LNS_negate_stmt      ] = 0
standard_opcode_lengths[ DW_LNS_set_basic_block  ] = 0
standard_opcode_lengths[ DW_LNS_const_add_pc     ] = 0
standard_opcode_lengths[ DW_LNS_fixed_advance_pc ] = 1
                Dir  Mod Time   File Len   File Name
                ---- ---------- ---------- ---------------------------
file_names[  1]    0 0x00000000 0x00000000 a.c
0x00000021: DW_LNE_set_address( 0x00000000 )
0x00000028: DW_LNE_end_sequence
            0x0000000000000000      1      1      0

By removing the "early exit" from EmitDebugLine, I got LLVM to
generate this data. However, Dan pointed out that assemblers that use
the ".loc" directive can't have their debug_line generated by the
compiler. My suggestion was to have a flag that indicated that ".loc"
was used. If it was, then we would exit out of the EmitDebugLine
method early. Something like so:

    // If there are no lines to emit (such as when we're using .loc
directives
    // to emit .debug_line information) don't emit a .debug_line
header.
    if (LocDirectiveUsed())
      return;

Dan wasn't sure if this would work in all cases. We don't have a Linux
box to test this on. Does this sound like a good idea? Those who work
with Linux or other OSes, would this idea break things for you?

-bw

On Jul 17, 2008, at 3:33 PM, Bill Wendling wrote:
> In CodeGen/DwarfWriter.cpp's EmitDebugLine file, these lines are
> causing havoc on Mac OS X systems:
>
>    // If there are no lines to emit (such as when we're using .loc
> directives
>    // to emit .debug_line information) don't emit a .debug_line
> header.
>    if (SectionSourceLines.empty())
>      return;
The fix is to move the early exist to just below this line:  
EmitLabel("line_prolog_end", 0), right?

>
>
> Basically, if there's a file with only data in it, we still need the
> debug_line prologue generated.
Right.
>
> By removing the "early exit" from EmitDebugLine, I got LLVM to
> generate this data. However, Dan pointed out that assemblers that use
> the ".loc" directive can't have their debug_line generated by
the
> compiler. My suggestion was to have a flag that indicated that
".loc"
I am not sure I understand this statement?

Evan
>
> was used. If it was, then we would exit out of the EmitDebugLine
> method early. Something like so:
>
>
>    // If there are no lines to emit (such as when we're using .loc
> directives
>    // to emit .debug_line information) don't emit a .debug_line
> header.
>    if (LocDirectiveUsed())
>      return;
>
> Dan wasn't sure if this would work in all cases. We don't have a
Linux
> box to test this on. Does this sound like a good idea? Those who work
> with Linux or other OSes, would this idea break things for you?
>
> -bw
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

On Jul 17, 2008, at 3:33 PM, Bill Wendling wrote:
> In CodeGen/DwarfWriter.cpp's EmitDebugLine file, these lines are
> causing havoc on Mac OS X systems:
>
>    // If there are no lines to emit (such as when we're using .loc
> directives
>    // to emit .debug_line information) don't emit a .debug_line
> header.
>    if (SectionSourceLines.empty())
>      return;
>
> Basically, if there's a file with only data in it, we still need the
> debug_line prologue generated.
>
> $ cat a.c
> const char ver[] __attribute__((used)) = "Hello world\n";
> $ gcc -g -O0 -c a.c -o a.gcc.o
> $ dwarfdump --debug-line=0 a.gcc.o
> ----------------------------------------------------------------------
> File: a.gcc.o (architecture i386)
> ----------------------------------------------------------------------
>
> .debug_line contents:
> ----------------------------------------------------------------------
> debug_line[0x00000000]
> ----------------------------------------------------------------------
> Line table prologue:
>   total_length: 0x00000027
>        version: 0x0002
> prologue_length: 0x00000017
> min_inst_length: 0x01
> default_is_stmt: 0x01
>      line_base: -10
>     line_range: 245
>    opcode_base: 0x0a
> standard_opcode_lengths[ DW_LNS_copy             ] = 0
> standard_opcode_lengths[ DW_LNS_advance_pc       ] = 1
> standard_opcode_lengths[ DW_LNS_advance_line     ] = 1
> standard_opcode_lengths[ DW_LNS_set_file         ] = 1
> standard_opcode_lengths[ DW_LNS_set_column       ] = 1
> standard_opcode_lengths[ DW_LNS_negate_stmt      ] = 0
> standard_opcode_lengths[ DW_LNS_set_basic_block  ] = 0
> standard_opcode_lengths[ DW_LNS_const_add_pc     ] = 0
> standard_opcode_lengths[ DW_LNS_fixed_advance_pc ] = 1
>                Dir  Mod Time   File Len   File Name
>                ---- ---------- ---------- ---------------------------
> file_names[  1]    0 0x00000000 0x00000000 a.c
> 0x00000021: DW_LNE_set_address( 0x00000000 )
> 0x00000028: DW_LNE_end_sequence
>            0x0000000000000000      1      1      0
>
> By removing the "early exit" from EmitDebugLine, I got LLVM to
> generate this data. However, Dan pointed out that assemblers that use
> the ".loc" directive can't have their debug_line generated by
the
> compiler. My suggestion was to have a flag that indicated that
".loc"
> was used. If it was, then we would exit out of the EmitDebugLine
> method early. Something like so:
>
>    // If there are no lines to emit (such as when we're using .loc
> directives
>    // to emit .debug_line information) don't emit a .debug_line
> header.
>    if (LocDirectiveUsed())
>      return;
>
> Dan wasn't sure if this would work in all cases. We don't have a
Linux
> box to test this on. Does this sound like a good idea? Those who work
> with Linux or other OSes, would this idea break things for you?
I've now tested how GCC does this on Ubuntu, at least. It uses .loc and
does not emit a separate .debug_line prologue. It apparently assumes
that the .file numbers will directly correspond with file indicies in
the debug line section, suitable for use with DW_AT_decl_file.

So I think the way to fix this is to make it dependent on whether
the target uses .loc or not. Something like this:

   // If the target is using .loc/.file, the assembler will be emitting
   // the .debug_line table automatically.
   if (TAI->hasDotLocAndDotFile())
     return;

Dan

On Jul 18, 2008, at 10:45 AM, Evan Cheng wrote:
> On Jul 17, 2008, at 3:33 PM, Bill Wendling wrote:
>
>> In CodeGen/DwarfWriter.cpp's EmitDebugLine file, these lines are
>> causing havoc on Mac OS X systems:
>>
>>   // If there are no lines to emit (such as when we're using .loc
>> directives
>>   // to emit .debug_line information) don't emit a .debug_line
>> header.
>>   if (SectionSourceLines.empty())
>>     return;
>
> The fix is to move the early exist to just below this line:
> EmitLabel("line_prolog_end", 0), right?
>
No. We still need the emission of some of the labels after this. So it  
would get rid of the if-statement altogether. However, Dan came up  
with a better solution. :-)
>> By removing the "early exit" from EmitDebugLine, I got LLVM
to
>> generate this data. However, Dan pointed out that assemblers that use
>> the ".loc" directive can't have their debug_line
generated by the
>> compiler. My suggestion was to have a flag that indicated that
".loc"
>
> I am not sure I understand this statement?
>
Now that I read it, neither can I. :-) I was trying to say that  
the .loc and .file directives are there to indicate that the assembler  
should generate the debug_line section -- with line tables, etc. So  
the compiler shouldn't be outputting those sections.

-bw