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<div class="moz-cite-prefix">On 11/05/2014 10:10 AM, Reid Kleckner
wrote:<br>
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cite="mid:CACs=tyKpi53=7J-khTE+4w+tbyDcr_mfREVM-8kZmTyo5FW3Vg@mail.gmail.com"
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<div class="gmail_quote">On Wed, Nov 5, 2014 at 10:01 AM,
Philip Reames <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:listmail@philipreames.com" target="_blank">listmail@philipreames.com</a>></span>
wrote:<br>
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</span> Would one of you mind taking a step back and
explaining what you believe the "stack colouring
problem" to be? I'm familiar with the general meaning
of the term and even some of LLVM's implementation; I'm
just not sure what specific issue you're referring to.
Having the context would make it much easier to assess
your proposals. </div>
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<div>The goal of stack coloring is to reduce stack usage by
storing user data with non-overlapping lifetimes in the
same stack memory.</div>
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<div>C/C++ source programs usually have a naturally scoped
structure, where the lifetime of data is bounded by curly
braces. This information reduces the lifetime that stack
coloring sees, so it saves stack memory.</div>
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<div>When we go to LLVM IR, we lose all that information. We
currently try to recapture it with calls to
@lifetime.start / end intrinsic calls at the point of
declaration and when the variable falls out of scope.
Basically we're trying to figure out how to put that
scoping information back into the IR without turning it
back into a tree.</div>
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<div>Furthermore, if we had better information about this in
the IR, we could augment ASan to detect use-after-scope.</div>
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Everything you say here is general goodness. What part of this is
problematic today? My belief is that the lifetime markers give you
exactly the support you need. Where does this break down? Is the
analysis too hard? Is Clang getting the semantics wrong? What's
the actually blocking issue?<br>
<br>
Philip<br>
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