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<div class="moz-cite-prefix">On 03/01/2017 05:30 PM, Daniel Berlin
via llvm-dev wrote:<br>
</div>
<blockquote
cite="mid:CAF4BwTWnC8Ds25ABEi_+uLkpTnaRzkgMdg+WmM5i0yqeB09XkA@mail.gmail.com"
type="cite">
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<div dir="ltr">So, <a moz-do-not-send="true"
href="https://bugs.llvm.org/show_bug.cgi?id=32056">https://bugs.llvm.org/show_bug.cgi?id=32056</a>
is an example showing our current TBAA tree for union generation
is definitely irretrievably broken.
<div>I'll be honest here. I'm pretty sure your proposal doesn't
go far enough.</div>
<div>But truthfully, I would rather see us come closer to a
representation we know works, which is GCC's.</div>
<div>Let me try to simplify what you are suggesting, and what we
have.</div>
<div>Our current representation is basically inverted from GCC,
but we don't allow things that would enable it to work.<br>
</div>
<div><br>
</div>
<div>Given</div>
<div>union {int a, short b};</div>
<div><br>
</div>
<div>GCC's will be:</div>
<div><br>
</div>
<div> union</div>
<div> / \</div>
<div>short int</div>
<div><br>
</div>
<div><br>
</div>
<div>Everything is implicitly a subset of alias set 0 (which for
C/C++ is char) just to avoid representing it.</div>
<div><br>
</div>
<div>Our metadata has no child links, and only a single parent
link.</div>
<div><br>
</div>
<div>You can't represent the above form because you can't make a
single short node a child of every union/struct it needs to
be (lack of multiple parents means you can't just frob them
all to offset zero).</div>
<div><br>
</div>
<div>Your suggestion is to invert this as a struct</div>
<div><br>
</div>
<div>short int</div>
<div> | / </div>
<div>union</div>
<div><br>
</div>
<div>We don't allow multiple parents, however.</div>
<div>Because of that, you suggest we follow all nodes for
unions, special casing union-type nodes somehow</div>
</div>
</blockquote>
<br>
Now that I've spent a bunch of time looking at this, I'd like to
voice support for Steven's original proposal. In the context of what
we have, it makes sense, seems sound, and should fix the
representational mapping problem we currently have. Something
completely different (e.g. closer to what GCC uses) can work too,
but this seems unnecessary (the proposed extension to the current
semantics seem equivalently expressive).<br>
<br>
What you call "special casing of union-type nodes" does not actually
seem all that special. The rule seems quite simple: if you some
across duplicate offsets, then search all of them. This should not
be difficult to implement or use, and seems no less efficient than
any other way of encoding the concept of disjunction in the
hierarchy.<br>
<br>
-Hal<br>
<br>
<blockquote
cite="mid:CAF4BwTWnC8Ds25ABEi_+uLkpTnaRzkgMdg+WmM5i0yqeB09XkA@mail.gmail.com"
type="cite">
<div dir="ltr">
<div><br>
</div>
<div>Let me suggest something different:</div>
<div><br>
</div>
<div>We know the current structure fails us in a number of ways.</div>
<div>Instead of trying to shoehorn this into our current
structure, I suggest: we stop messing around and just have a
GCC style tree, and represent the children instead of the
parents.</div>
<div>We make contained types descendants instead of ancestors.</div>
<div>We allow multiple children at each offset and for scalar
type nodes.x`<br>
</div>
<div><br>
</div>
<div>We could do that without changing to children, but
honestly, i strongly believe nobody who ever looks at this
really understands it right now.</div>
<div>(If someone really does like the ancestor form, i'd love to
understand why :P)</div>
<div><br>
So if we are going to change it, we should change it to
something that is easier to understand.</div>
<div><br>
</div>
<div>something like:<br>
<br>
</div>
<div>scalar type node -> {name, children nodes} </div>
<div>struct node -> {name, array of {offset, child node} }</div>
<div><br>
</div>
<div>Paths are separate from the tbaa tree itself, and are just:</div>
<div>path node -> {struct node, offset} or whatever.</div>
<div><br>
</div>
<div>unions are just scalar type nodes with multiple children,
not struct nodes with special-cased offset zero.</div>
<div><br>
</div>
<div>This also has a well-defined upgrade path.</div>
<div><br>
</div>
<div>For "old-style" DAGs, like exist now, we know we have to
regen the metadata, and that it is wrong (So we could, for
example, simply disable it for correctness reasons)</div>
<div>.</div>
<div>Anything with a "new-style" DAG, we know is usable.</div>
<div><br>
</div>
<div>In this representation, the most generic tbaa node is just
the one that contains the other.</div>
<div>If neither contains the other, you can just make one that
has both as children and use that.</div>
<div>(instead of now, where you'd have to have multiple parents
again).</div>
<div><br>
</div>
<div>(The above also may be better for other languages)</div>
<div><br>
</div>
<div>--Dan</div>
<div><br>
</div>
<div><br>
</div>
<div><br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Tue, Feb 28, 2017 at 4:44 PM, Steven
Perron <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:perrons@ca.ibm.com" target="_blank">perrons@ca.ibm.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex"><font
size="2" face="sans-serif">Seems like the comments have
stopped. I'll
try to get a patch together. Then we can continue the
discussion
from there. </font><br>
<br>
<font size="2" face="sans-serif">Hubert, as for the issue
with the llvm
optimizations losing the TBAA information, it should be
the responsibility
to make sure the aliasing is changed in the correct way.
One function
related to this has already been mentioned:
getMostGenericTBAA.</font><br>
<br>
<font size="2" face="sans-serif">Later,</font><br>
<font size="2" face="sans-serif">Steven Perron</font><br>
<br>
<br>
<br>
<font color="#5f5f5f" size="1" face="sans-serif">From:
</font><font size="1" face="sans-serif">Hubert Tong <<a
moz-do-not-send="true"
href="mailto:hubert.reinterpretcast@gmail.com"
target="_blank">hubert.reinterpretcast@gmail.<wbr>com</a>></font><br>
<font color="#5f5f5f" size="1" face="sans-serif">To:
</font><font size="1" face="sans-serif">Steven
Perron/Toronto/IBM@IBMCA</font><br>
<font color="#5f5f5f" size="1" face="sans-serif">Cc:
</font><font size="1" face="sans-serif">Daniel Berlin
<<a moz-do-not-send="true"
href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</a>>,
llvm-dev <<a moz-do-not-send="true"
href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>>,
Sanjoy Das <<a moz-do-not-send="true"
href="mailto:sanjoy@playingwithpointers.com"
target="_blank">sanjoy@playingwithpointers.<wbr>com</a>></font><br>
<font color="#5f5f5f" size="1" face="sans-serif">Date:
</font><font size="1" face="sans-serif">2017/02/15 07:44
AM</font><span class=""><br>
<font color="#5f5f5f" size="1" face="sans-serif">Subject:
</font><font size="1" face="sans-serif">Re:
[llvm-dev]
RFC: Representing unions in TBAA</font><br>
</span>
<hr noshade="noshade">
<div class="HOEnZb">
<div class="h5"><br>
<br>
<br>
<font size="3">On Tue, Feb 14, 2017 at 11:22 PM, Steven
Perron <</font><a moz-do-not-send="true"
href="mailto:perrons@ca.ibm.com" target="_blank"><font
color="blue" size="3"><u>perrons@ca.ibm.com</u></font></a><font
size="3">>
wrote:</font><br>
<font size="2" face="sans-serif">3) How should we handle
a reference
directly through a union, and a reference that is not
through the union?
</font><font size="3"><br>
</font><font size="2" face="sans-serif"><br>
My solution was to look for each member of the union
overlaps the given
offset, and see if any of those members aliased the
other reference.
If no member aliases the other reference, then the
answer is no alias.
Otherwise the answer is may alias. The run time for
this would be
proportional to "distance to the root" * "number of
overlapping members". This could be slow if there are
unions
with many members or many unions of unions.</font><font
size="3"><br>
</font><font size="2" face="sans-serif"><br>
Another option is to say that they do not alias. This
would mean
that all references to unions must be explicitly
through the union.</font><br>
<font size="3">From what I gather from the thread so
far, the access
through the union may be lost because of LLVM
transformations. I am not
sure how, in the face of that, TBAA could indicate
NoAlias safely (without
the risk of functional-correctness issues in correct
programs) between
types which overlap within any union (within some
portion of the program).<br>
</font><br>
<font size="3">As for the standards, it is definitely
not true that all
references to unions must be explicitly through the
union. However, if
you are trying to perform union-based type punning
(under C11), then it
appears that it is intended that the read must be
through the union.</font><br>
<font size="3"> </font><br>
<font size="2" face="sans-serif">This would be the least
restrictive
aliasing allowing the most optimization. The
implementation would
be simple. I believe we make the parent of the TBAA
node for the
union to be "omnipotent char". This might be similar
to
treating the union TBAA node more like a scalar node
instead of a struct-path.
Then the traversal of the TBAA nodes will be quick.
I'll have
to work this out a bit more, but, if this is good
enough to meet the requirements
of the standard, I can try to think this through a
little more. I'll
need Hubert and Daniel to comment on that since I am
no expert on the C
and C++ standards.</font><font size="3"><br>
</font><font size="2" face="sans-serif"><br>
The third option is to be pessimistic and say "may
alias" all
of the time (conservatively correct), and rely on
other alias analysis
to improve it. This will have good compile time, but
could hinder
optimization. Personally I do not like this option.
Most of
the time it will not have a negative effect, but there
will be a reasonable
number of programs where this will hurt optimization
more that it needs
to.</font><br>
<br>
<br>
<br>
</div>
</div>
</blockquote>
</div>
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</div>
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</blockquote>
<br>
<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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