[llvm-dev] RFC: Representing unions in TBAA (original) (raw)

Krzysztof Parzyszek via llvm-dev llvm-dev at lists.llvm.org
Fri Apr 7 13:28:16 PDT 2017

I'm asking if people are ok with it. :) We've had customer reports that can be tracked down to this issue, so this is something we'd really like to working (at least in terms of correctness).

I can come up with a patch.

-Krzysztof

On 4/7/2017 3:25 PM, Daniel Berlin wrote: > Not familiar with clang enough to know. > Staring at it, it looks a bit annoying to do. > You'd basically just want to stop decorating loads/stores with tbaa info > if it's a union. >>> On Fri, Apr 7, 2017 at 12:09 PM, Krzysztof Parzyszek via llvm-dev > <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote: >> Can we turn off TBAA info for union member accesses in clang before > this gets fixed? >> -Krzysztof >>> On 3/1/2017 5:30 PM, Daniel Berlin via llvm-dev wrote: >> So, https://bugs.llvm.org/showbug.cgi?id=32056 > <https://bugs.llvm.org/showbug.cgi?id=32056> is an example showing > our current TBAA tree for union generation is definitely > irretrievably > broken. > I'll be honest here. I'm pretty sure your proposal doesn't go > far enough. > But truthfully, I would rather see us come closer to a > representation > we know works, which is GCC's. > Let me try to simplify what you are suggesting, and what we have. > Our current representation is basically inverted from GCC, but > we don't > allow things that would enable it to work. >> Given > union {int a, short b}; >> GCC's will be: >> union > _/ _ > short int >>> Everything is implicitly a subset of alias set 0 (which for C/C++ is > char) just to avoid representing it. >> Our metadata has no child links, and only a single parent link. >> 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). >> Your suggestion is to invert this as a struct >> short int > | / > union >> We don't allow multiple parents, however. > Because of that, you suggest we follow all nodes for unions, special > casing union-type nodes somehow >> Let me suggest something different: >> We know the current structure fails us in a number of ways. > 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. > We make contained types descendants instead of ancestors. > We allow multiple children at each offset and for scalar type > nodes.x` >> We could do that without changing to children, but honestly, i > strongly > believe nobody who ever looks at this really understands it > right now. > (If someone really does like the ancestor form, i'd love to > understand > why :P) >> So if we are going to change it, we should change it to > something that > is easier to understand. >> something like: >> scalar type node -> {name, children nodes} > struct node -> {name, array of {offset, child node} } >> Paths are separate from the tbaa tree itself, and are just: > path node -> {struct node, offset} or whatever. >> unions are just scalar type nodes with multiple children, not struct > nodes with special-cased offset zero. >> This also has a well-defined upgrade path. >> 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) > . > Anything with a "new-style" DAG, we know is usable. >> In this representation, the most generic tbaa node is just the > one that > contains the other. > If neither contains the other, you can just make one that has > both as > children and use that. > (instead of now, where you'd have to have multiple parents again). >> (The above also may be better for other languages) >> --Dan >>>>> On Tue, Feb 28, 2017 at 4:44 PM, Steven Perron > <perrons at ca.ibm.com <mailto:perrons at ca.ibm.com> > <mailto:perrons at ca.ibm.com <mailto:perrons at ca.ibm.com>>> wrote: >> Seems like the comments have stopped. I'll try to get a patch > together. Then we can continue the discussion from there. >> 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. >> Later, > Steven Perron >>>> _From: Hubert Tong <hubert.reinterpretcast at gmail.com_ > <mailto:hubert.reinterpretcast at gmail.com> > _<mailto:hubert.reinterpretcast at gmail.com_ > <mailto:hubert.reinterpretcast at gmail.com>>> > To: Steven Perron/Toronto/IBM at IBMCA > _Cc: Daniel Berlin <dberlin at dberlin.org_ > <mailto:dberlin at dberlin.org> > <mailto:dberlin at dberlin.org <mailto:dberlin at dberlin.org>>>, > llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > _<mailto:llvm-dev at lists.llvm.org_ > <mailto:llvm-dev at lists.llvm.org>>>, Sanjoy Das > _<sanjoy at playingwithpointers.com_ > <mailto:sanjoy at playingwithpointers.com> > _<mailto:sanjoy at playingwithpointers.com_ > <mailto:sanjoy at playingwithpointers.com>>> > Date: 2017/02/15 07:44 AM > Subject: Re: [llvm-dev] RFC: Representing unions in TBAA >> ------------------------------------------------------------------------ >>>> On Tue, Feb 14, 2017 at 11:22 PM, Steven Perron > _<perrons at ca.ibm.com <mailto:perrons at ca.ibm.com_ > <mailto:perrons at ca.ibm.com>>> wrote: > 3) How should we handle a reference directly through a > union, and a > reference that is not through the union? >> 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. >> Another option is to say that they do not alias. This would > mean > that all references to unions must be explicitly through the > union. > 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). >> 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. >> 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. >> 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. >>>>>>> _________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev> >>> -- > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, > hosted by The Linux Foundation >> _________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev> >>

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