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

[David Blaikie via cfe-dev]

Ah, I was hoping/looking for more of a timing/cpu-counter type profile, to
demonstrate the overall impact (% runtime) of these changes, if possible?

On Sat, Dec 15, 2018 at 9:57 AM Bruno Ricci <riccibrun at gmail.com> 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
>
> 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
> >     _______________________________________________
> >     cfe-dev mailing list
> >     cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
> >     http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >
>
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