[cfe-commits] r139358 - /cfe/trunk/lib/Serialization/ASTReader.cpp

[Argyrios Kyrtzidis]

On Sep 9, 2011, at 9:12 AM, Douglas Gregor wrote:

> 
> On Sep 9, 2011, at 9:05 AM, Argyrios Kyrtzidis wrote:
> 
>> On Sep 9, 2011, at 8:15 AM, Douglas Gregor wrote:
>> 
>>> 
>>> On Sep 8, 2011, at 11:44 PM, Argyrios Kyrtzidis wrote:
>>> 
>>>> Author: akirtzidis
>>>> Date: Fri Sep  9 01:44:17 2011
>>>> New Revision: 139358
>>>> 
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=139358&view=rev
>>>> Log:
>>>> [PCH] When loading the decls linked to an identifier, also make them visible
>>>> in the translation unit.
>>>> 
>>>> Modified:
>>>> cfe/trunk/lib/Serialization/ASTReader.cpp
>>>> 
>>>> Modified: cfe/trunk/lib/Serialization/ASTReader.cpp
>>>> URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Serialization/ASTReader.cpp?rev=139358&r1=139357&r2=139358&view=diff
>>>> ==============================================================================
>>>> --- cfe/trunk/lib/Serialization/ASTReader.cpp (original)
>>>> +++ cfe/trunk/lib/Serialization/ASTReader.cpp Fri Sep  9 01:44:17 2011
>>>> @@ -4841,8 +4841,16 @@
>>>>  return;
>>>> }
>>>> 
>>>> +  ASTContext &Ctx = *getContext();
>>>> for (unsigned I = 0, N = DeclIDs.size(); I != N; ++I) {
>>>>  NamedDecl *D = cast<NamedDecl>(GetDecl(DeclIDs[I]));
>>>> +
>>>> +    // In C++, translation unit's visible decls map gets emitted in the AST
>>>> +    // file, but not on C; make the decl visible so it can be looked up.
>>>> +    if (!Ctx.getLangOptions().CPlusPlus)
>>>> +      SetExternalVisibleDeclsForName(Ctx.getTranslationUnitDecl(),
>>>> +                                     DeclarationName(II), D);
>>>> +
>>>> 
>>> 
>>> This is a bit unfortunate. Creating the lookup table in the translation unit forces the AST to continually maintain that lookup table (e.g., adding entries to it every time a visible declaration is added to the translation unit), which is a cost we'd like to avoid in C. I presume that you only made this change to support the new lookup used in  r139359? I have an alternative suggestion for that...
>> 
>> But adding entries always updated the lookup table, or the lookup in r139359 would not work in non-PCH mode, right ? 
>> In general I'd find it bad for the AST usefulness if you could not do a lookup in translation unit unless it is on C++.
> 
> 
> It's more subtle than that. The lookup table for a DeclContext is constructed lazily, when one performs the first lookup, by walking the declarations lexically in that context and populating the table. Once a table is there, it is maintained as declarations are added to the DeclContext.

I see the error of my ways now. Note however that, outside of this fix, it's very unfortunate that lookup does not work on a loaded C AST.

> 
> A call to SetExternalVisibleDeclsForName creates the lookup table, so we end up paying the cost of maintaining the declarations in the DeclContext even if we never want to perform a lookup in the context.
> 
> 	- Doug