[PATCH] D12624: Top-level anonymous namespaces are missing import DW_TAG_imported_module and nested anonymous namespaces are not

[Adrian Prantl via cfe-commits]

> On Sep 8, 2015, at 3:08 PM, Paul Robinson <Paul_Robinson at playstation.sony.com> wrote:
> 
> probinson added inline comments.
> 
> ================
> Comment at: lib/CodeGen/CGDebugInfo.cpp:3263-3264
> @@ +3262,4 @@
> +  const NamespaceDecl *NSDecl = UD.getNominatedNamespace();
> +  if (!NSDecl->isAnonymousNamespace() || 
> +      CGM.getTarget().getTriple().isPS4CPU()) { 
> +    DBuilder.createImportedModule(
> ----------------
> rsmith wrote:
>> I think we should do this unconditionally, to better match the source language semantics, but I'm curious what David, Eric, and other folks on the DWARF side think.
> David (in previous discussions and review comments) has said he thinks it is unnecessary as the debugger already must know so much about C++ to get various things right, it might as well know that it has to implicitly import the anonymous namespace contents.  One example debugger UI allows the user to type source-like syntax, and requires the debugger to apply (for example) C++ parameter-type matching rules to distinguish between overloaded functions.  Compared to this, implicit imports are child's play.
> 
> I believe Eric agrees with David; I don't remember whether Adrian said anything in the previous iterations of this patch.

The information is redundant but otherwise harmless and there are other compilers that emit it and debuggers that came to expect it; this makes it look like a perfect candidate for a debugger tuning setting.

> 
> I believe the explicit (although artificial) import is a good thing, because it matches the source language semantics.  I find an important distinction between "which declarations are available in this scope" and "how does the user disambiguate declarations in this scope."  As a counterpart to the above debugger UI example, I postulate a GUI drop-down list of symbols available in-scope; this UI needs to know nothing about language semantics and automatic imports, if the DWARF provides the correct explicit import.  This suggests to me that the DWARF should provide it.
> 
> There's also the piddly detail that debuggers are not the only consumers of DWARF information, and presenting the DWARF in a more source-language-neutral way (i.e., with the explicit artificial import) could be beneficial for those other consumers, who might not necessarily want to learn language-specific scoping rules.
> 
> No debugger will be thrown for a loop if it sees the explicit import; however for some debuggers it would be redundant (because they implicitly import the anonymous namespace already).  There is a pretty trivial space savings if it's omitted.
> 
> Katya has mentioned the GCC and ICC precedent; in fairness I will say GCC didn't used to emit this, and GDB tolerated that.
> 
> Note that the DWARF standard does not tell us what to do; it merely tells us how to emit the import, if we want to emit one.  Whether we want to emit one is up to us.

Appendix D.3 does give an (albeit non-normative) example of an anonymous top-level namespace without an explicit import.

-- adrian

> 
> http://reviews.llvm.org/D12624
> 
> 
>