[llvm-dev] [RFC] Refactor class hierarchy of VectorType in the IR

[Chris Tetreault via llvm-dev]

John,

   For the last several months, those of us working on the scalable vectors feature have been examining the codebase, identifying places where llvm::VectorType is used incorrectly, and fixing them. The fact is that there are many places where VectorType is correctly taken to be the generic “any vector” type. getNumElements may be being called, but it’s being called in accordance with the previously documented semantics. There are many places where it isn’t as well, and many people add new usages that are incorrect.

   This puts us in an unfortunate situation: if we were to take your proposal and have VectorType be the fixed width vector type, then all of this work is undone. Every place that has been fixed up to correctly have VectorType be used as a universal vector type will now incorrectly have the fixed width vector type being used as the universal vector type. Since VectorType will inherit from BaseVectorType, it will have inherited the getElementCount(), so the compiler will happily continue to compile this code. However, none of this code will even work with scalable vectors because the bool will always be false. There will be no compile time indication that this is going on, functions will just start mysteriously returning nullptr. Earlier this afternoon, I set about seeing how much work it would be to change the type names as you have suggested. I do not see any way forward other than painstakingly auditing the code.

   On the other hand, creating a new fixed width vector type has a clear upgrade path. 1) delete getNumElements() from the base class locally. 2) try to build 3) fix the failures, uploading patches for these fixes 4) once step 3 is completed throughout the codebase, merge the patch to remove getNumElements() from VectorType. Downstream and out-of-tree codebases have this upgrade path as well. There exists no easy upgrade path if we go the other way and have VectorType become a specifically fixed-width vector type.

   Basically, I believe that the best thing to do is to move forward with the type names that I have proposed:


  1.  The type names more accurately represent the usage of the types.
  2.  Changing course now would result in a tremendous amount of rework being done in the upstream codebase. This will have a significant impact on the pace of development in upstream.
  3.  The process for completing the changes would be much easier if we use the types I propose. The compiler can tell you if you’re using getNumElements() on a potentially scalable vector. The compiler cannot tell you that SomeFixedVector->getElementCount().Scalable and isa<ScalableVectorType>(SomeFixedVector) are always false.

   Additionally, for those who disagree that the LLVM developer policy is to disregard the needs of downstream codebases when making changes to upstream, I submit that not throwing away months of work by everybody working to fix the codebase to handle scalable vectors represents a real expected benefit. I personally have been spending most of my time on this since January.

Thanks,
   Christopher Tetreault

From: John McCall <rjmccall at apple.com>
Sent: Thursday, May 21, 2020 3:32 PM
To: Chris Tetreault <ctetreau at quicinc.com>
Cc: llvm-dev at lists.llvm.org
Subject: [EXT] Re: [llvm-dev] [RFC] Refactor class hierarchy of VectorType in the IR


On 21 May 2020, at 17:22, Chris Tetreault wrote:

John,

This is not categorically true, no. When we make changes that require large-scale updates for downstream codebases, we do so because there’s a real expected benefit to it. For the most part, we do make some effort to keep existing source interfaces stable.

While I’m at a loss to find a documented policy, I recall this thread (http://lists.llvm.org/pipermail/llvm-dev/2020-February/139207.html) where this claim was made and not disputed. The expected real benefits to this change are: 1) The names now match the semantics 2) It is now statically impossible to accidentally get the fixed number of elements from a scalable vector and 3) It forces everybody to fix their broken code. If we provided stability guarantees to downstream and out-of-tree codebases, then I might not agree that 3 is a benefit, but my understanding is that we do not provide this guarantee.

… Probably 99% of the code using VectorType semantically requires it to be a fixed-width vector.

This code is all broken already. I don’t think supporting common misuse of APIs in a codebase that does not provide stability guarantees is something we should be doing.

The generalization of VectorType to scalable vector types was a representational shortcut that should never have been allowed

I agree. Unfortunately it happened, and our choices are to fix it or accept the technical debt forever.

Perhaps this is part of the root of our disagreement. I consider scalable vector types to be an experimental/unstable feature, as many features are when they’re first added to the compiler. I have much lower standards for disrupting the early adopters of features like that; if scalable vectors need to be split out of VectorType, we should just do it.

Analogously, when we upstream the pointer-authentication feature, we will be upstreaming a rather flawed representation that I definitely expect us to revise after the fact. That will be problematic for early adopters of LLVM’s pointer authentication support, but that’s totally acceptable.

John.

the burden of generalization should usually fall on the people who need to use the generalization, and otherwise we should aim to keep APIs stable when there’s no harm to it.

The burden of creating the generalization should be placed on those who need it, I agree. However, once the generalization is in place, the burden is on everybody to use it correctly. The reason I’ve undertaken this refactor is because I found myself playing whack-a-mole with people adding new broken code after the fact. The previous API was very easy to misuse, so I don’t blame those people.

Are you actually auditing and testing them all to work for scalable vector types, or are you just fixing the obvious compile failures?

Everywhere that VectorType::getNumElements() is being called, we are either changing the code to cast to FixedVectorType, or we are updating the code to handle scalable vectors correctly. If the call site does not have test coverage with scalable vectors, this test coverage is being added. Even for obviously correct transformations such as `VectorType::get(SomeTy, SomeVecTy->getNumElements())` -> `VectorType::get(SomeTy, SomeVecTy->getElementCount())`, I have been required in code review to provide test coverage. We are taking this seriously.

“Vector” has a traditional and dominant meaning as a fixed-width SIMD type, and the fact that you’ve introduced a generalization doesn’t change that. Clang supports multiple kinds of pointer, but we still reserve clang::PointerType for C pointers instead of making it an abstract superclass, thus letting our sense of logic introduce a million bugs through accidental generalization throughout the compiler.

Various languages implemented on top of LLVM have various different pointer types. However, the LLVM IR language only has one pointer type. The LLVM IR language has two types of vectors, and I think it’s reasonable to model them as a class hierarchy in this manner. I’m not familiar with the clang::PointerType situation, so I cannot pass judgement on it.

Thanks,
Christopher Tetreault

From: John McCall <rjmccall at apple.com<mailto:rjmccall at apple.com>>
Sent: Thursday, May 21, 2020 1:47 PM
To: Chris Tetreault <ctetreau at quicinc.com<mailto:ctetreau at quicinc.com>>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: [EXT] Re: [llvm-dev] [RFC] Refactor class hierarchy of VectorType in the IR


On 21 May 2020, at 16:01, Chris Tetreault wrote:

Hi John,

I’d like to address some points in your message.

Practically speaking, this is going to break every out-of-tree frontend, backend, or optimization pass that supports SIMD types.

My understanding is that the policy in LLVM development is that we do not let considerations for downstream and out-of-tree codebases affect the pace of development.

This is not categorically true, no. When we make changes that require large-scale updates for downstream codebases, we do so because there’s a real expected benefit to it. For the most part, we do make some effort to keep existing source interfaces stable.

The C++ API is explicitly unstable. I maintain a downstream fork of LLVM myself, so I know the pain that this is causing because I get to fix all the issues in my internal codebase. However, the fix for downstream codebases is very simple: Just find all the places where it says VectorType, and change it to say FixedVectorType.

… by having the VectorType type semantically repurposed out from under them.

The documented semantics of VectorType prior to my RFC were that it is a generalization of all vector types. The VectorType contains an ElementCount, which is a pair of (bool, unsigned). If the bool is true, then the return value of getNumElements() is the minimum number of vector elements. If the bool is false, then it is the actual number of elements. My RFC has not changed these semantics. It will eventually delete a function that has been pervasively misused throughout the codebase, but the semantics remain the same. You are proposing a semantic change to VectorType to have it specifically be a fixed width vector.

Probably 99% of the code using VectorType semantically requires it to be a fixed-width vector. The generalization of VectorType to scalable vector types was a representational shortcut that should never have been allowed; it should always have used a different type. Honestly, I’m not convinced your abstract base type is even going to be very useful in practice vs. just having a getVectorElementType() accessor that checks for both and otherwise returns null.

… a particular largely-vendor-specific extension …

All SIMD vectors are vendor specific extensions. Just because most of the most popular architectures have them does not make this not true. AArch64 and RISC-V have scalable vectors, so it is not just one architecture. It is the responsibility of all developers to ensure that they use the types correctly. It would be nice if the obvious thing to do is the correct thing to do.

I’m not saying that we shouldn’t support scalable vector types. I’m saying that the burden of generalization should usually fall on the people who need to use the generalization, and otherwise we should aim to keep APIs stable when there’s no harm to it.

… it’s much better for code that does support both to explicitly opt in by checking for and handling the more general type …

This is how it will work. I am in the process of fixing up call sites that make fixed width assumptions so that they use FixedVectorType.

Are you actually auditing and testing them all to work for scalable vector types, or are you just fixing the obvious compile failures? Because scalable vector types impose some major restrictions that aren’t imposed on normal vectors, and the static type system isn’t going to catch most of them.

I think that it is important to ensure that things have clear sensible names, and to clean up historical baggage when the opportunity presents.

“Vector” has a traditional and dominant meaning as a fixed-width SIMD type, and the fact that you’ve introduced a generalization doesn’t change that. Clang supports multiple kinds of pointer, but we still reserve clang::PointerType for C pointers instead of making it an abstract superclass, thus letting our sense of logic introduce a million bugs through accidental generalization throughout the compiler.

You have resigned yourself to doing a lot of work in pursuit of something that I really don’t think is actually an improvement.

John.
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