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<div class="moz-cite-prefix">On 11/05/2014 03:08 PM, Arnaud A. de
Grandmaison wrote:<br>
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<p class="MsoNormal"><span
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<p class="MsoNormal"><span
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<p class="MsoNormal" style="margin-left:36.0pt"><b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif""
lang="EN-US">From:</span></b><span
style="font-size:10.0pt;font-family:"Tahoma","sans-serif""
lang="EN-US"> Reid Kleckner [<a class="moz-txt-link-freetext" href="mailto:rnk@google.com">mailto:rnk@google.com</a>] <br>
<b>Sent:</b> 05 November 2014 22:49<br>
<b>To:</b> Philip Reames<br>
<b>Cc:</b> Arnaud De Grandmaison; LLVM Developers Mailing
List<br>
<b>Subject:</b> Re: [LLVMdev] lifetime.start/end
clarification<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt">On Wed,
Nov 5, 2014 at 11:17 AM, Philip Reames <<a
moz-do-not-send="true"
href="mailto:listmail@philipreames.com"
target="_blank">listmail@philipreames.com</a>>
wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt">On
11/05/2014 10:54 AM, Reid Kleckner wrote:<o:p></o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt">This seems fine
to me. The optimizer can (soundly)
conclude that %p is dead after the
"lifetime.end" (for the two instructions),
and dead before the "lifetime.start" (for
the *single* instruction in that basic
block, *not* for the previous BB). This
seems like the proper result for this
example, am I missing something?<o:p></o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt"><o:p> </o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt">What if I put
that in a loop, unroll it once, and prove
that the lifetime.start is unreachable? We
would end up with IR like:<o:p></o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt"><o:p> </o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt">loop:<o:p></o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt"> ... use %p<o:p></o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt"> call void
@lifetime.end( %p )<o:p></o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt"> ... use %p<o:p></o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt"> call void
@lifetime.end( %p )<o:p></o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt"> br i1 %c, label
%loop, label %exit<o:p></o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt"><o:p> </o:p></p>
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<p class="MsoNormal"
style="margin-left:36.0pt">Are the second
uses of %p uses of dead memory?<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt">It's
hard to discuss this without being specific about the
starting IR and transforms. My general response is
that either a) such a transform wouldn't be valid or
b) the behaviour of the original program was
undefined. <o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt">The
starting IR would be something that jumps into the
middle of a lifetime region, like the example that
Arnaud gave. This was assuming the current state of
the world where we haven't added a second lifetime
start call prior to the goto branch.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt">Start
with something like:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt">void
f(int x) {<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"> while
(x) {<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"> goto
skip_start;<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"> {<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt">
int y; // lifetime.start<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt">skip_start:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"> y
= g();<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"> x
-= y;<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"> //
lifetime.end<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"> }<br>
}<br>
}<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-left:36.0pt">The
block containing the lifetime start of y is
unreachable and can be deleted.<o:p></o:p></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">On
this example, I think the critical edge to the
skip_start label need to get a lifetime.start
inserted (btw, we already insert there a gep to the
alloca to solve a dominator issue), and this makes
sense because such an edge also contains (in
essence) the trivial constructors of the scope where
the label is.</span></p>
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Well said. This is exactly the point I've been trying to make. <br>
<blockquote cite="mid:000f01cff94d$59df5400$0d9dfc00$@arm.com"
type="cite">
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">This
looks to me as a hint that when they are used with
an alloca, lifetime.start/end really have to be
paired, and that all possible uses of the alloca
should be covered by a [start,end] :<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">
- In the above testcase, when the path from
lifetime.end is walked-up the cfg, it reaches the
alloca, and this is weird.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">
- In my testcase, when the paths from lifetime.start
and lifetime.end are walked-up the cfg, it becomes
inconsistent in the entry block: one path will tell
it’s dead, the other it’s alive.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">This
puts the burden onto clang to generate consistent
marker info, which sounds reasonable after all.</span></p>
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</blockquote>
My analysis is slightly different, but the result is the same. I
don't believe that a dynamic trace of {end, use, end} should be well
defined. I'm suggesting we extend the current semantics to make the
trace {end, start, use, end} meaningful. It's the frontends job to
ensure that no dynamic traces of the first type are possible if the
second is meant. <br>
<blockquote cite="mid:000f01cff94d$59df5400$0d9dfc00$@arm.com"
type="cite">
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I
can probably add some checks to enforce this in
llvm, or at least detect such cases: this will
definitely help me to spot candidates for the
miscompilation I observe, which by the way is not
caused by unnamed temporaries, but by standard
VarDecl + lifetime markers. I do not know yet where
though. It means that we have a lurking bug today
in-tree, and changing the size threshold for marker
insertion is enough to trigger it.</span></p>
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I agree with this analysis. <br>
<blockquote cite="mid:000f01cff94d$59df5400$0d9dfc00$@arm.com"
type="cite">
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<p class="MsoNormal"><span
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Cheers,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Arnaud<o:p></o:p></span></p>
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