<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><br></div><div><just want to focus on these parts for a second. *All* of these representations are really access path representations, just encoded slightly different ways, and, as a result, with various parts of the rules in slightly different places> </div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><span style="font-size:12.8px">Imagine that we took the enhancement we previously discussed, but instead of implementing it directly, we just directly encoded for every access the path from the access type to the root. I think it would look very much like this proposal.</span></blockquote><div><br>Something like it, yes.</div><div>Note that this representation also has special vtable and union groups, for example, and assumes very c-like rules in several places around the sequencing of access types.</div><div>Also note that in the language of access paths, C allows overlapping that does not exist in, for example, Java</div><div>(This is true in both the points-to and type-based domains).</div><div>Ada has discriminated unions (and you could not use the "union" in this proposal to represent them)</div><div>etc.</div><div><br></div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div bgcolor="#FFFFFF"><span class="gmail-"><blockquote type="cite"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div> <br>
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<div bgcolor="#FFFFFF"> We generally explain our current
TBAA rules by saying that they're generic but motivated
by C/C++ rules.</div>
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<div>We do say that but that's not really what our
implementation does in any way. <br>
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Really? I thought it was motivated by C/C++ rules. When you say that
it's "not really what our implementation does", is this because it
drops a lot of potential information in the translation to our
generic form?</div></blockquote><div><br></div><div>What we do is completely and totally unrelated to types as they exist in the original language. It is a completely and totally generic implementation with no rules related to the original language. The original language, for example, has dynamic typing rules, object lifetime rules, etc. We have none of these.</div><div><br></div><div>The types do not, in fact,even always have the same relationship we give them.</div><div> <br></div><div>We have a tree and some nodes, and we happen to name them after types. Like this proposal, they also represent computed access paths, in a much simpler language.</div><div><br></div><div>The nodes represent alias set members (either one or many)</div><div>The edges represent either "access at offset" (where the default is 0 if unweighted).</div><div>We have a simple rule that that processes the access paths related to ancestry.</div><div><br></div><div>You can view it as either a grammar parsing or a graph reachability problem depending on what works for you (in fact, it's really a Dyck-CFL reachability problem on bidirected trees)</div><div><br></div><div>The reachability rule is usually given as: If node Target is reachable from node Source, offset another through either it's children or the upwards edges (or was it downwards, i always screw up which direction we go in), they may-alias<br></div><div><br></div><div>You could also implement it as a real grammar parsing problem (IE you quite literally could generate strings from tag + offset, and parse them against the grammar, and see if the terminal nodes contain your target). They are equivalent.</div><div><br></div><div>This representation would be the same in that regard.<br></div><div><br></div><div>Our current lowering for clang happens to kind of look like c/c++ structures converted to a tree. </div><div><br></div><div>However, this is actually just inefficient, space wise, and done because it's simple to lower it this way. </div><div>Because the accesses are completely unrelated to the original types, and require *no* language rules to interpret, you could also just partition the things that alias by the language rules in the frontend, and then output a tree that represents the possible unique paths.</div><div><br></div><div>IE figure out all the answers, then re-encode it as precisely as possible.</div><div><br></div><div>This trades time for space.</div><div><br></div><div>This representation is *super* generic. That is, the language being used here is super simple, and the reachability rule is super simple.</div><div><br></div><div>I could have the frontend generate these trees based on anything i like. I could, in fact, encode steensgaard points-to results as this tree without any trouble.</div><div><br></div><div>The access path language described in this proposal is more complex and complete, and directly closer to access paths you find in C/C++. It has bitfields, vtables, and unions.</div><div>The reachability rules are more complex.</div><div>Is it possible to express other languages in that set of access path rules?</div><div>Sure. For example, you can, as above, generate the set of answers, and then re-encode it into access paths.</div><div><br></div><div>Right now, the work it takes *in the front end* is minimal, and has a fairly efficient space encoding.</div><div>if i want to say two things alias, i just gotta be able to reach one from the other.</div><div>If i want to say two things do not alias, i just gotta be able to not reach one from the other only using certain types of edges</div><div>In our current language, all that takes in a frontend is "find longest no-aliasing part of tree. Go to parent, add new child".</div><div><br></div><div>In the proposed language, the lowering is more complex. Is it doable?</div><div>Sure, of course, not gonna claim otherwise. But the more "features" of the access path you add and expect the middle end to handle, instead of the front-end expanding them, and the more those feature's reachability rules are related to a specific language the more language-specific it gets.</div><div><br></div><div>That's the *only* tradeoff we are making here, representationally. How much does the frontend have to understand about how the middle end walks these things, in order to generate whatever it wants as precisely as possible</div><div><br></div><div>We can make whichever we want express everything we want in N^3 time :)</div><div>The real question is "do we try to add on to what we have in ways that work for multiple languages, and are expressed neutrally in a simple reachability language"</div><div>or "do we add language-specific kinds of nodes to the grammar, and have reachability rules that are fairly language specific".</div><div><br></div><div>IE do you add, say, discriminated_union nodes to our current representation for ada, or "vtable_access" nodes to our current representation for C++ vtable accesses</div><div>Or do you instead generate a metadata that has a unidirectional edge reachability (IE up only), or whatever it takes to do vtables generically.</div><div><br></div><div>Both are completely and totally viable paths, and it's all about which way you want to go.</div><div>But they *definitely* have a difference in terms of language-specificness.</div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div></div></div></div>