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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Aha, okay. I had noticed that the column-info hack went away. So the distinct-ness implies the scope implicit in the inlined call, which later on will be
turned into the explicit inlined_subroutine entry. That seems… indirect.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I have to say, the LangRef page's words about "merge based on content" is not really to the point. It's like saying the purpose of a street-corner STOP sign
is to make you stop. That's the mechanism it uses, but it's not why the sign is there. It would be great if somebody would clarify what distinct-ness is actually good for.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> David Blaikie [mailto:dblaikie@gmail.com]
<br>
<b>Sent:</b> Monday, April 06, 2015 10:38 AM<br>
<b>To:</b> Robinson, Paul<br>
<b>Cc:</b> LLVM Developers Mailing List (llvmdev@cs.uiuc.edu)<br>
<b>Subject:</b> Re: [LLVMdev] "distinct" metadata nodes are ...?<o:p></o:p></span></p>
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<p class="MsoNormal">On Mon, Apr 6, 2015 at 10:26 AM, Robinson, Paul <<a href="mailto:Paul_Robinson@playstation.sony.com" target="_blank">Paul_Robinson@playstation.sony.com</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">I'm encountering a merge issue whose root cause has to do with "distinct"<br>
metadata nodes. I see that distinct-ness is an intentional concept, but<br>
the explanation in the LLVM Language Reference is not very enlightening.<br>
<br>
distinct nodes are useful when nodes shouldn't be merged based on<br>
their content.<br>
<br>
The notion of "merged" metadata is not discussed elsewhere on the page,<br>
except for Objective-C garbage collection; I'm looking at debug location<br>
metadata, so that's not relevant.<br>
<br>
I understand that distinct-ness was invented as a replacement for a<br>
self-reference hack, but that just begs the question. Why is this a<br>
useful concept? What is it used for? Why shouldn't certain nodes be<br>
merged based on their content?<br>
<br>
My specific issue has to do with inlined-at chains. If I have<br>
return inlined_func() + 1;<br>
the inlined-at chain for inlined_func() [and whatever else is inlined<br>
into inlined_func()] terminates in a node that is 'distinct' from the<br>
node for the calling statement, even though they describe the same<br>
source location. This didn't used to be a problem, chasing the chain<br>
ended up with something that compared equal to the calling statement's<br>
source location.<o:p></o:p></p>
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<p class="MsoNormal"><br>
<a href="http://llvm.org/viewvc/llvm-project?rev=226736&view=rev">http://llvm.org/viewvc/llvm-project?rev=226736&view=rev</a> is the change that caused this & has some context on why it's necessary.<br>
<br>
The issue is that the scope change of debuglocs is how we build scopes, including inline scopes (DW_TAG_inlined_subroutine). If the call site locations aren't uniqued, then two calls from the same line to the same function would have the same location and thus
be the same scope - so we'd only have one DW_TAG_inlined_subroutine, instead of two.<br>
<br>
Clang worked around this for a while by putting column info on call sites to help give them unique call sites, but this was insufficient (the two calls could've come from within a macro, in which case they'd be attributed to the same line/column again).<br>
<br>
- David<br>
<o:p></o:p></p>
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Thanks,<br>
--paulr<br>
<br>
<br>
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