llvm dev - Mar 2012 - [LLVMdev] Debug info compileunit metadata strangeness..

Hi,

I have a question regarding the metadata for compileunit debug info. I
find a few things in it a bit strange, but maybe there it is a reason
for it to be that way that I just don't understand (but if that is the
case I guess the documentation needs to be clearer).

Consider this C program: "int X;"

Compiled with "clang -g" it debug metadata along these lines:

 !llvm.dbg.cu = !{!0}
 !0 = metadata !{i32 786449, .....
                 metadata !1, ;; List of enums types
                 metadata !1, ;; List of retained types
                 metadata !1, ;; List of subprograms
                 metadata !3  ;; List of global variables
                } ; [ DW_TAG_compile_unit ]
 !1 = metadata !{metadata !2}
 !2 = metadata !{i32 0}
 !3 = metadata !{metadata !4}
 !4 = metadata !{metadata !5}
 !5 = metadata !{i32 786484, ...} ; [ DW_TAG_variable ]


Documentation says "List of global variables", but it is built as a
"One element list containing the list of global variables". Is there a
reason it is built that way?

Looking at DIBuilder::createCompileUnit it does indeed build it with
an extra indirection.

Another thing I find strange is the empty lists, such as "List of
enums types" in my example above. They apparently end up being
one-element lists containing "i32 0". I would expect that the
reference to the empty lists would be "null".

In other words, I'd expect it to look like this:

 !llvm.dbg.cu = !{!0}

 !0 = metadata !{i32 786449, ...
                 null,        ;; List of enums types
                 null,        ;; List of retained types
                 null,        ;; List of subprograms
                 metadata !1  ;; List of global variables
                } ; [ DW_TAG_compile_unit ]
 !1 = metadata !{metadata !2}
 !2 = metadata !{i32 786484, ...} ; [ DW_TAG_variable ]


I attached two patches that changes compileunit metadata to have the
layout I think makes sense. I do not expect these patches to be
applied (especially the second one is just a gross hack), I'm
attaching them because maybe they may help show what I'm trying to say
(a patch says more than 1000 words?).

 anders

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On Mar 4, 2012, at 12:58 PM, Anders Waldenborg wrote:
> Documentation says "List of global variables", but it is built as
a
> "One element list containing the list of global variables". Is
there a
> reason it is built that way?
> 
> Looking at DIBuilder::createCompileUnit it does indeed build it with
> an extra indirection.
You're right. This is weird. I haven't seen a reason why it's that
way.

-eric
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