[LLVMdev] need pointers navigating through metadata in llvm :-)

[Devang Patel]

Jason,

This looks like PR 7689.
-
Devang
On Sep 8, 2010, at 6:16 PM, Jason Kim wrote:

> Hi everyone, apologies for the mass spam, but I ran into an issue
> involving debug metadata in the bitcode linking stage (using gold)
> 
> I am interested in helping out, so would like some pointers if possible.
> 
> The proximate cause of the issue in our local version of llvm is that
> a Value is being written out, and the dump() of that value is actually
> not a value at all, but what looks like a reference to a function. So
> what happens is that  BitcodeWriter.cpp:WriteModuleConstants() is
> barfing, because this particular Value is not any of the variants that
> its made to handle.
> 
> It looks like the specific value in question is part of the debug
> metadata generated by our local version (which is somewhere between
> v2.7 and the llvm-tip, plus our local changes)
> 
> Interestingly enough, the .bc file (with debug info) generated by
> vanilla llvm-gcc 2.7 passes through the gold bitcode linking stage
> without issues - and after some futzing about, I narrowed the change
> down to a single entry diff
> 
> The first one passes through link while the latter does not.
> 
> !0 = metadata !{i32 524334, i32 0, metadata !1, metadata !"main",
> metadata !"main", metadata !"main", metadata !1, i32 5, metadata !3,
> i1 false, i1 true, i32 0, i32 0, null, i1 false, i1 false, i32 0} ; [
> DW_TAG_subprogram ]
> 
> !0 = metadata !{i32 524334, i32 0, metadata !1, metadata !"main",
> metadata !"main", metadata !"main", metadata !1, i32 5, metadata !3,
> i1 false, i1 true, i32 0, i32 0, null, i1 false, i1 false, i32 (i32,
> i8**)* @main} ; [ DW_TAG_subprogram ]
> 
> The difference is the LAST entry (0 versus (i32, i8**)* @main), which
> looks like the reference to the function itself, (in this case, it
> happens to be "main" :-)
> 
> So the question is, what is the intent of the metadata type tag 46?
> was it intended to have a reference to the actual function?  It looks
> like the corresponding node as generated by llvm-2.7 is missing this
> item. (I guess I should look up what the dwarf tag  subprogram is
> supposed to have - that's on my todo queue :-)
> 
> So is this a metadata generation bug? or is this a metadata
> consumption bug? or both?  I can maybe field a wild guess that this
> particular Value should not be written out as a Constant at all, but
> rather as a linkable reference?
> 
> 
> Thank you.
> -Jason Kim
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