[cfe-dev] Decls are not synonyms for the symbols they represent

steve naroff snaroff at apple.com
Tue Sep 16 15:30:58 PDT 2008


I couldn't get clang to crash on the code you referenced below...

[steve-naroffs-imac-2:llvm/tools/clang] snaroff% cat xx.c
void f (void)
   extern double *p;
   extern double *p;

[steve-naroffs-imac-2:llvm/tools/clang] snaroff% ../../Debug/bin/clang  
xx.c:4:18: error: redefinition of 'p'
   extern double *p;
xx.c:3:18: error: previous definition is here
   extern double *p;
2 diagnostics generated.

Do I need to run clang with any special switches?


On Sep 16, 2008, at 5:41 PM, Ted Kremenek wrote:

> A few weeks ago I had a conversation with Daniel about the fact that  
> the ASTs (or other clang data structures) have no notion of the  
> "entity" (for lack of a better word) that a declaration represents.
> Here are a couple examples of what I mean:
> (example 1)
>   extern double x;
>   extern double x;
> Both of these are variable declarations that reference the same  
> variable.  There is no notion of the variable itself other than the  
> declarations, which is conflated, particularly since we have  
> multiple declarations in this case (i.e., there is no unique  
> "entity" for the variable).
> (incidentally, clang crashes on this input: http://llvm.org/bugs/show_bug.cgi?id=2760)
> (example 2)
>   struct s;
>   struct s { int a; };
>   struct s;
> Until a few weeks ago, these struct declarations were represented by  
> a single RecordDecl with a unique RecordType.  Now they are  
> represented by three separate RecordDecls with a shared, unique  
> RecordType.
> With structures, the unique RecordType indeed can be treated as  
> representing the "struct" itself, which seems fine since the given  
> declarations are just type declarations.  So in this case, we *do*  
> have a unique "entity" in the ASTs to represent what the  
> declarations refer to.  There are still some issues with this  
> representation, but I will delay mentioning them until after the  
> next example.
> (example 3)
> int f();
> int f();
> int f() { return 0; }
> int g();
> int g() { return 1; }
> For this example, we have separate FunctionDecls for each one of  
> these declarations.  In this example, all of the declarations both  
> 'f' and 'g' share the same type (note that this is different from  
> the case with structs).  For the case of 'f', all of its  
> FunctionDecls are chained together, and the same goes for 'g'.   
> There is, however, no notion of an entity or concept in the ASTs or  
> other clang data structures that represent 'f' itself.
> Here is an example of why not having an explicit concept for 'f',  
> 'g', or any symbol is problematic.
> Consider:
>   extern int h(int* x) __attribute__((nonnull));
>   extern int h(int *x);
>   extern int h(int* x) __attribute__((noreturn));
> This code is completely valid.  In the ASTs we create three  
> FunctionDecls, the first having the attribute "nonnull" attached to  
> it (and object of type NonNullAttr) and the third having the  
> attribute "noreturn" attached to it (an object of type NoReturnAttr).
> Suppose I had a client (e.g., code generation, static analysis) that  
> wanted to know all the attributes attached to a given function.  How  
> would I go about doing this?  Given one of these FunctionDecls, I  
> would have to iterate the chain of FunctionDecls and query each one  
> of its attributes.  This seems a little cumbersome, and causes  
> separate clients to have to implement their own logic for querying  
> information about "symbols" in a translation unit.  It also causes  
> clients to think about internal representations such as the fact  
> that FunctionDecls are chained, something we may wish to change at  
> any moment in the future.
> This email isn't really a proposal of a solution; I'm just raising  
> an issue to see if anyone has any comments.  After the last few  
> weeks I've been excited by our discussions of DeclGroups and  
> TypeSpecifiers that will solve many of the remaining issues with  
> faithfully representing syntax in the ASTs.  At the same time, I  
> think we need to pay a little more attention to the semantics, and  
> providing infrastructure that would be useful for many clients.
> Indeed, some of our changes to improve our capturing of syntax have  
> actually weakened some of our clients reasoning about semantics.   
> For example, by splitting separate struct declarations into multiple  
> RecordDecls we actually (initally) broke CodeGen because the CodeGen  
> library assumed that there was a direct 1-1 mapping between a  
> RecordDecl and the concept it represented.  That particular case was  
> easily resolved by using the RecordType instead of the RecordDecl to  
> represent the 'struct', but I'd be willing to wager that there are  
> other issues that haven't surfaced yet because RecordTypes are being  
> used in this way (by all clients).
> Thoughts?
> _______________________________________________
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> cfe-dev at cs.uiuc.edu
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