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

[Douglas Gregor]

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.

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