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<div class="moz-cite-prefix">On 08/17/2017 04:49 PM, Daniel Berlin
via llvm-dev wrote:<br>
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For two given access sequences we can determine if the
accessed<br>
objects are allowed to overlap by the rules of the input<br>
language.</blockquote>
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<div>Sadly, this is where this becomes "unlikely to want to
use to replace TBAA", at least for me. It may still be a
thing we want anyway.</div>
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The rules proposed by Ivan for handling C/C++ seem pretty generic.
We generally explain our current TBAA rules by saying that they're
generic but motivated by C/C++ rules. I could say the same thing
about this proposed system with its proposed rules.<br>
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<div>This scheme is really an encoding of C/C++ TBAA info so
it can be read by LLVM and requires that *LLVM* have some
set of rules that it enforces about that scheme.</div>
<div>But that scheme is still very language specific in how
it is used.</div>
<div>GCC still has something in between this and LLVM, where
the language rules are a bit encoded (but not as much as
you have).<br>
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<div>We (and gcc) have deliberately avoided such schemes, in
favor of transforming the info into abstract set trees
that then tag loads and stores.</div>
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<div>The encoding of "struct path" tbaa, is just a way of
trading space vs time in that encoding. We trade walking
time for the space of transitive closure, etc.</div>
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If the provided statistic of 15% holds up, maybe which way we go in
this trade-off space doesn't matter much?<br>
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<div>None of the TBAA in LLVM really has any *real* relation
to the original type system rules, and that is
deliberate. I've argued for years that calling it "TBAA"
just badly confuses people, and i believe here is a good
example :)</div>
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<div>So i don't think what you've written can be used to
replace TBAA.</div>
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So *if* we just take the proposed rules for C/C++ as the rules for
the scheme in general, I'm not sure this is true. There is an
isomorphism it seems (we could auto-upgrade even, if we'd like, by
mapping the current offset value onto the field id of this scheme
(we'd need to use a different root name, however, so everything
would remain consistent under LTO)).<br>
<br>
-Hal<br>
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<div>However, i *do* believe it would be useful to further
optimize C and C++ programs in LLVM by using access paths.</div>
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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