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

Bruno Ricci via cfe-dev cfe-dev at lists.llvm.org
Sat Dec 15 09:57:24 PST 2018


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

Bruno

On 15/12/2018 16:38, David Blaikie wrote:
> Seems reasonable and some profile data would help motivate/support/justify which bits are worth changing and which aren't
> 
> On Sat, Dec 15, 2018, 8:27 AM Bruno Ricci via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> wrote:
> 
>     Hi all,
> 
>     I have been looking at Decl::getASTContext and DeclContext::getParentASTContext,
>     and it turns out that they are not that cheap.
> 
>     The reason for this is that they must walk back to the translation unit declaration
>     along what is essentially a linked list. This is amplified by the fact that it does
>     look like a simple getter and therefore is called extremely often (the most egregious
>     case by far being DeclContext::lookup), even when most of the times the context is
>     already easily available.
> 
>     To put some numbers on this I experimented with removing about half of the iterations
>     in Decl::getTranslationUnitDecl. This gives a speed up of about 1% when parsing all
>     of Boost. Extrapolating from this I get that about 1.5%-2% of the time spent
>     parsing Boost is spent getting the ASTContext, which seems a bit unfortunate.
> 
>     Now to be clear 1.5%-2% is not that much, but the fix is simply to pass a ref to
>     the ASTContext more often as a function parameter. The downside is that this would
>     cause a some amount of churn in the C++ api. To be clear I am *not* suggesting
>     removing all of the calls to Decl::getASTContext, but only removing the ones causing
>     the most iterations in Decl::getTranslationUnitDecl (say the top 10 or 20).
> 
>     What do you think ?
> 
>     Bruno
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