[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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