[cfe-dev] Avoiding unnecessary calls to Decl::getASTContext

Bruno Ricci via cfe-dev cfe-dev at lists.llvm.org
Wed Jan 2 06:31:18 PST 2019


I don't understand the part about caching the AST context in Sema since Sema already store
a reference to the AST context.

What I would like to propose is the pass a reference to the AST context more often as
a function parameter. For example instead of having "lookup_result lookup(DeclarationName Name) const;"
we would have "lookup_result lookup(ASTContext &Context, DeclarationName Name) const;" (with ASTContext
possibly const-qualified).

This is a little bit annoying since it require updating all the callers and propagating the changes
along the call chains. I would only suggest doing this when the changes are not too invasive.
For example Decl::getAttrs is used in a lot of places and requiring the AST context would probably
not be a good idea. (although possibly getAttrs could have two versions, one with a reference to the
AST context and another one without).

The other alternatives I considered are:

1.) Store a reference to the AST context in each DeclContext. This has in my opinion
    an unacceptable memory cost. In addition when I experimented with this the speedup
    obtained from avoiding the lookup of the AST context are almost entirely negated by
    the increased memory usage (this might of course vary depending on the specific test setup).

2.) Cache the AST context in Decl::getASTContext. I did not experiment with this but it seems to
    me that this will not work when there are multiple AST contexts. Or if it can be made to work
    there is a need to store some data in Decl for the invalidation, but Decl has not a single bit
    available.

Bruno

On 02/01/2019 13:33, Jan Korous wrote:
> Hi,
> 
> Sorry for being late to the discussion.
> 
> Do I understand it right that you’d like to use a local DeclContext* variable and make sure it’s identical to S.getASTContext() result (or similar for Parent)?
> 
> What do you think about caching ASTContext and/or ParentASTContext in Sema?
> 
> Jan
> 
>> On Dec 17, 2018, at 9:51 AM, Richard Smith via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>
>> On Sun, 16 Dec 2018, 14:27 Bruno Ricci via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> wrote:
>>
>>     On 16/12/2018 18:55, Richard Smith wrote:
>>     > On Sat, 15 Dec 2018, 09:57 Bruno Ricci via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> <mailto:cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>>>     >     I have added the profile data to the first patch (https://reviews.llvm.org/D55658)
>>>>     >     The top 20 callers of getASTContext and getParentASTContext are:
>>     >     (with the number of calls to getTranslationUnitDecl, edited for readability)
>>>>     >     5399586 : DeclContext::lookup
>>     >     801684 : VarDecl::getMemberSpecializationInfo
>>     >     551928 : FunctionDecl::getMinRequiredArguments
>>     >     401464 : CXXRecordDecl::lookupInBases
>>     >     338614 : Decl::getAttrs
>>     >     311651 : DeclContext::makeDeclVisibleInContextImpl
>>     >     173684 : Decl::getASTMutationListener
>>     >     164264 : CXXConstructorDecl::isSpecializationCopyingObject
>>     >     154066 : CXXMethodDecl::size_overridden_methods
>>     >     141622 : CXXRecordDecl::getVisibleConversionFunctions
>>     >     120440 : ClassTemplateSpecializationDecl::Profile
>>     >     116303 : VarDecl::isThisDeclarationADefinition
>>     >     106208 : CXXRecordDecl::conversion_begin
>>     >     106208 : CXXRecordDecl::conversion_end
>>     >     99075 : FunctionDecl::setParams
>>     >     95910 : RedeclarableTemplateDecl::findSpecializationImpl
>>     >     57043 : CXXRecordDecl::getDestructor
>>     >     56668 : VarDecl::getDefinition
>>     >     52140 : TagDecl::startDefinition
>>     >     48266 : CXXConstructorDecl::isCopyOrMoveConstructor
>>     >     37407 : FunctionDecl::getBody
>>>>>>     > Given your data that perhaps 1.5-2% of the time building boost is finding the AST context, it definitely seems worthwhile to avoid doing so for lookup. The next three seem reasonable to address too, if you feel so inclined, but I'd not go any further than that -- the long tail seems negligible in comparison.
>>
>>     Just to clarify, the 1.5%-2% figure is an estimation for the time spent obtaining the
>>     AST context when parsing all of Boost without doing any code generation (ie: -fsyntax-only).
>>
>>     I generally agree about not bothering too much with the long tail, with perhaps two exceptions:
>>
>>     1.) Some of these are really easy to fix. For example FunctionDecl::setParams is almost only
>>         called in Sema and so the context is already easily available.
>>
>>
>> Sure, non-disruptive cleanups for this seem good where possible. (But for example, getAttrs is called from a large number of places and it'd be really awkward to require an ASTContext, so I'd leave that one.)
>>
>>     2.) The numbers shown above are the number of calls to getTranslationUnitDecl. A better metric
>>         would be the number of iterations needed to reach the translation unit declaration. With
>>         that in mind it would also make sense to pass the AST context to some of the above where
>>         the declaration is likely to occur nested deep in the declaration context chain
>>         (VarDecls for example).
>>
>>     If there are no objections I will submit some patches next week to do this change, hopefully
>>     in small increments. In any case this will require small (but straightforward) changes to at
>>     least clang-tools-extra and lldb. (Am I forgetting any project here ?)
>>
>>
>> I think it's just those two. Thank you!
>>
>>     Bruno
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