[llvm] r205137 - Don't mark the declarations of the TSan annotation functions as weak.

[Nick Lewycky]

Chandler Carruth wrote:
> Author: chandlerc
> Date: Sun Mar 30 06:20:25 2014
> New Revision: 205137
>
> URL: http://llvm.org/viewvc/llvm-project?rev=205137&view=rev
> Log:
> Don't mark the declarations of the TSan annotation functions as weak.
> That causes references to them to be weak references which can collapse
> to null if no definition is provided.

Whoops! I didn't realize that attribute weak on a declaration meant 
something different from attribute weak on a definition. Thanks for the fix!

Nick

  We call these functions
> unconditionally, so a definition *must* be provided. Make the
> definitions provided in the .cpp file weak by re-declaring them as weak
> just prior to defining them. This should keep compilers which cannot
> attach the weak attribute to the definition happy while actually
> resolving the symbols correctly during the link.
>
> You might ask yourself upon reading this commit log: how did *any* of
> this work before? Well, fun story. It turns out we have some code in
> Support (BumpPtrAllocator) which both uses virtual dispatch and has
> out-of-line vtables used by that virtual dispatch. If you move the
> virtual dispatch into its header in *just* the right way, the optimizer
> gets to devirtualize, and remove all references to the vtable. Then the
> sad part: the references to this one vtable were the only strong symbol
> uses in the support library for llvm-tblgen AFAICT. At least, after
> doing something just like this, these symbols stopped getting their weak
> definition and random calls to them would segfault instead.
>
> Yay software.
>
> Modified:
>      llvm/trunk/include/llvm/Support/Valgrind.h
>      llvm/trunk/lib/Support/Valgrind.cpp
>
> Modified: llvm/trunk/include/llvm/Support/Valgrind.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Support/Valgrind.h?rev=205137&r1=205136&r2=205137&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Support/Valgrind.h (original)
> +++ llvm/trunk/include/llvm/Support/Valgrind.h Sun Mar 30 06:20:25 2014
> @@ -24,12 +24,10 @@
>   // tsan (Thread Sanitizer) is a valgrind-based tool that detects these exact
>   // functions by name.
>   extern "C" {
> -LLVM_ATTRIBUTE_WEAK void AnnotateHappensAfter(const char *file, int line,
> -                                              const volatile void *cv);
> -LLVM_ATTRIBUTE_WEAK void AnnotateHappensBefore(const char *file, int line,
> -                                               const volatile void *cv);
> -LLVM_ATTRIBUTE_WEAK void AnnotateIgnoreWritesBegin(const char *file, int line);
> -LLVM_ATTRIBUTE_WEAK void AnnotateIgnoreWritesEnd(const char *file, int line);
> +void AnnotateHappensAfter(const char *file, int line, const volatile void *cv);
> +void AnnotateHappensBefore(const char *file, int line, const volatile void *cv);
> +void AnnotateIgnoreWritesBegin(const char *file, int line);
> +void AnnotateIgnoreWritesEnd(const char *file, int line);
>   }
>   #endif
>
>
> Modified: llvm/trunk/lib/Support/Valgrind.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Valgrind.cpp?rev=205137&r1=205136&r2=205137&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Support/Valgrind.cpp (original)
> +++ llvm/trunk/lib/Support/Valgrind.cpp Sun Mar 30 06:20:25 2014
> @@ -55,13 +55,21 @@ void llvm::sys::ValgrindDiscardTranslati
>
>   #if LLVM_ENABLE_THREADS != 0&&  !defined(NDEBUG)
>   // These functions require no implementation, tsan just looks at the arguments
> -// they're called with.
> +// they're called with. However, they are required to be weak as some other
> +// application or library may already be providing these definitions for the
> +// same reason we are.
>   extern "C" {
> +LLVM_ATTRIBUTE_WEAK void AnnotateHappensAfter(const char *file, int line,
> +                                              const volatile void *cv);
> +void AnnotateHappensAfter(const char *file, int line, const volatile void *cv) {
> +}
> +LLVM_ATTRIBUTE_WEAK void AnnotateHappensBefore(const char *file, int line,
> +                                               const volatile void *cv);
>   void AnnotateHappensBefore(const char *file, int line,
>                              const volatile void *cv) {}
> -void AnnotateHappensAfter(const char *file, int line,
> -                          const volatile void *cv) {}
> +LLVM_ATTRIBUTE_WEAK void AnnotateIgnoreWritesBegin(const char *file, int line);
>   void AnnotateIgnoreWritesBegin(const char *file, int line) {}
> +LLVM_ATTRIBUTE_WEAK void AnnotateIgnoreWritesEnd(const char *file, int line);
>   void AnnotateIgnoreWritesEnd(const char *file, int line) {}
>   }
>   #endif
>
>
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