llvm dev - Apr 2015 - [LLVMdev] "distinct" metadata nodes are ...?

I'm encountering a merge issue whose root cause has to do with
"distinct"
metadata nodes.  I see that distinct-ness is an intentional concept, but
the explanation in the LLVM Language Reference is not very enlightening.

    distinct nodes are useful when nodes shouldn't be merged based on
    their content.

The notion of "merged" metadata is not discussed elsewhere on the
page,
except for Objective-C garbage collection; I'm looking at debug location
metadata, so that's not relevant.

I understand that distinct-ness was invented as a replacement for a
self-reference hack, but that just begs the question. Why is this a
useful concept?  What is it used for?  Why shouldn't certain nodes be
merged based on their content?

My specific issue has to do with inlined-at chains.  If I have
    return inlined_func() + 1;
the inlined-at chain for inlined_func() [and whatever else is inlined
into inlined_func()] terminates in a node that is 'distinct' from the 
node for the calling statement, even though they describe the same 
source location.  This didn't used to be a problem, chasing the chain
ended up with something that compared equal to the calling statement's
source location.

Thanks,
--paulr

On Mon, Apr 6, 2015 at 10:26 AM, Robinson, Paul <
Paul_Robinson at playstation.sony.com> wrote:
> I'm encountering a merge issue whose root cause has to do with
"distinct"
> metadata nodes.  I see that distinct-ness is an intentional concept, but
> the explanation in the LLVM Language Reference is not very enlightening.
>
>     distinct nodes are useful when nodes shouldn't be merged based on
>     their content.
>
> The notion of "merged" metadata is not discussed elsewhere on the
page,
> except for Objective-C garbage collection; I'm looking at debug
location
> metadata, so that's not relevant.
>
> I understand that distinct-ness was invented as a replacement for a
> self-reference hack, but that just begs the question. Why is this a
> useful concept?  What is it used for?  Why shouldn't certain nodes be
> merged based on their content?
>
> My specific issue has to do with inlined-at chains.  If I have
>     return inlined_func() + 1;
> the inlined-at chain for inlined_func() [and whatever else is inlined
> into inlined_func()] terminates in a node that is 'distinct' from
the
> node for the calling statement, even though they describe the same
> source location.  This didn't used to be a problem, chasing the chain
> ended up with something that compared equal to the calling statement's
> source location.
>
http://llvm.org/viewvc/llvm-project?rev=226736&view=rev is the change that
caused this & has some context on why it's necessary.

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.

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).

- David

>
> Thanks,
> --paulr
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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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.

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.
Thanks,
--paulr

From: David Blaikie [mailto:dblaikie at gmail.com]
Sent: Monday, April 06, 2015 10:38 AM
To: Robinson, Paul
Cc: LLVM Developers Mailing List (llvmdev at cs.uiuc.edu)
Subject: Re: [LLVMdev] "distinct" metadata nodes are ...?



On Mon, Apr 6, 2015 at 10:26 AM, Robinson, Paul <Paul_Robinson at
playstation.sony.com<mailto:Paul_Robinson at playstation.sony.com>>
wrote:
I'm encountering a merge issue whose root cause has to do with
"distinct"
metadata nodes.  I see that distinct-ness is an intentional concept, but
the explanation in the LLVM Language Reference is not very enlightening.

    distinct nodes are useful when nodes shouldn't be merged based on
    their content.

The notion of "merged" metadata is not discussed elsewhere on the
page,
except for Objective-C garbage collection; I'm looking at debug location
metadata, so that's not relevant.

I understand that distinct-ness was invented as a replacement for a
self-reference hack, but that just begs the question. Why is this a
useful concept?  What is it used for?  Why shouldn't certain nodes be
merged based on their content?

My specific issue has to do with inlined-at chains.  If I have
    return inlined_func() + 1;
the inlined-at chain for inlined_func() [and whatever else is inlined
into inlined_func()] terminates in a node that is 'distinct' from the
node for the calling statement, even though they describe the same
source location.  This didn't used to be a problem, chasing the chain
ended up with something that compared equal to the calling statement's
source location.

http://llvm.org/viewvc/llvm-project?rev=226736&view=rev is the change that
caused this & has some context on why it's necessary.

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.

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).

- David


Thanks,
--paulr


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