[PATCH] Prevent MDNode's RAUW from introducing a reference to a void Value.

[Duncan P. N. Exon Smith]

> On Oct 16, 2014, at 6:08 PM, Eric Christopher <echristo at gmail.com> wrote:
> 
> On Thu, Oct 16, 2014 at 6:06 PM, Duncan P. N. Exon Smith
> <dexonsmith at apple.com> wrote:
>> 
>>> On Oct 16, 2014, at 3:54 PM, Frederic Riss <friss at apple.com> wrote:
>>> 
>>> If you look at the added testcase, it cast a void (*func)() to a int (*func)(). The int typed result is referenced in a debug info metadata node (an argument to the dbg.value intrinsic). Then instcombine would remove the bitcast and turn the int returning function into a void returning function that gets RAUWd over the value in the MDNode. And at this point in the current code you have invalid IR.
>>> 
>>> The code in the testcase is reduced from a huge LTO link that encountered this situation in a much more complicated setup.
>>> 
>>> http://reviews.llvm.org/D5828
>> 
>> Weird.  This LGTM (after fixing a few super minor whitespace changes);
>> not sure if Eric is still looking.
> 
> Enh, I was trying to figure out if there was some way to avoid the
> void metadata node being create rather than avoiding RAUW'ing it. It's
> probably not a huge distinction at the moment.

I don't think it is a void metadata node; it's a void `CallInst`.

We have:

    call void @llvm.dbg.declare(metadata !{i32* %deadvar}, ...)
    %call = ...
    store i32 %call, i32* %deadvar, align 4, !dbg !21

Which converts to:

    call void @llvm.dbg.value(metadata !{i32* %call}, ...)
    %call = ...

Then, when %call gets RAUW'ed, `metadata !{i32* %call}` gets updated
to `void`.