[cfe-dev] Adding Support for APINotes

James Y Knight via cfe-dev cfe-dev at lists.llvm.org
Wed Oct 14 18:49:05 PDT 2020

On Wed, Oct 14, 2020 at 12:19 PM Saleem Abdulrasool <compnerd at compnerd.org>

> On Mon, Oct 12, 2020 at 8:50 AM James Y Knight <jyknight at google.com>
> wrote:
>> On Fri, Oct 9, 2020 at 10:21 AM David Rector <davrecthreads at gmail.com>
>> wrote:
>>> IIUC APINotes wants to add attributes during parsing so that the those
>>> attributes are then treated just like real parsed attributes would be by
>>> Sema.
>>> Ideally this sort of thing should be able to be contained in an
>>> ASTConsumer, but it presently cannot: the only real option ASTConsumer
>>> provides is to traverse the AST post-parsing in an override of
>>> `HandleTranslationUnit()`.  This leaves APINotes no choice but to add its
>>> details directly to Sema and Parser, with the result that Swift-specific
>>> details would necessarily show up in the guts of mainline clang were this
>>> to be upstreamed.
>>> If ASTConsumer instead provided a comprehensive virtual interface (but
>>> with negligible overhead, somehow) to perform custom consumer actions at
>>> strategic points during parsing/Sema (e.g., call consumer virtuals in each
>>> Sema::ActOn*), this conflict probably could be, or at least could have
>>> been, avoided.
>>> Note too that this issue resonates with one a user faced just the other
>>> day: he wanted to call `Sema::LookupName` within an ASTMatchers callback
>>> (during HandleTranslationUnit, as usual) to determine if a certain node
>>> could be rewritten w/o changing its semantics — but of course the necessary
>>> Sema/Parser scope info is long gone by that point.  If only he had the
>>> option to perform his logic on the node immediately after it was parsed,
>>> there would be no issue. Because he cannot, his only option was to
>>> laboriously recreate that state data in order to call LookupName.
>>> In sum, the dearth of ASTConsumer virtuals hinder encapsulation, causing
>>> 1) Sema and Parser to be polluted with what should be a consumer’s details,
>>> and 2) consumers to be polluted with what should be Sema & Parser details,
>>> and making it difficult to upstream useful tools.
>> This is an *excellent* point!
>> It looks like the primary change in Clang which is required for APINotes
>> is to place "ProcessAPINotes" calls into each of the Sema::ActOn* functions
>> which creates a (subclass of) Decl. This needs to occur after creating the
>> Decl object and processing the attributes in the source, but prior to
>> actually acting on it (e.g. see apple's SemaDeclObjC.cpp
>> <https://github.com/apple/llvm-project/blob/apple/main/clang/lib/Sema/SemaDeclObjC.cpp>).
>> This call could instead be a call to a generic hook for modifying the decl,
>> rather than specifically for processing API notes. We already even have a
>> container for such hooks in Sema, "ExternalSemaSource
>> <https://github.com/llvm/llvm-project/blob/master/clang/include/clang/Sema/ExternalSemaSource.h>",
>> which seems like a reasonable place to add a virtual method which may
>> modify newly-created Decls.
>> If such a hook mechanism is added, it then seems entirely plausible that
>> SemaAPINotes.cpp
>> <https://github.com/apple/llvm-project/blob/apple/main/clang/lib/Sema/SemaAPINotes.cpp> and
>> lib/APINotes/*
>> <https://github.com/apple/llvm-project/tree/apple/main/clang/lib/APINotes> can
>> be moved from the clang-fork into swift itself without requiring
>> significant modifications of the code, or any change in functionality of
>> the feature.
> Well, the APINotes also provide nullability information for ObjC
> interfaces, which is not part of Swift, but rather part of ObjC, so that
> still needs to be in clang.  That is, the functionality is useful for ObjC,
> and that doesn't make sense to be in Swift.

Theoretically one could imagine doing so -- but TTBOMK, it's never actually
been used like this. All nullability annotations used by objc compilations
live in the headers themselves, and the apinotes command-line options are
passed only by Swift, for creating a Swift-ObjC-bridge module.
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