[lld] r241420 - COFF: Make ArchiveFile::getMember thread-safe.

[David Majnemer]

On Sun, Jul 5, 2015 at 3:50 PM, Rui Ueyama <ruiu at google.com> wrote:

> Author: ruiu
> Date: Sun Jul  5 17:50:00 2015
> New Revision: 241420
>
> URL: http://llvm.org/viewvc/llvm-project?rev=241420&view=rev
> Log:
> COFF: Make ArchiveFile::getMember thread-safe.
>
> This function is called SymbolTable::readObjects, so in order to
> parallelize that function, we have to make this function thread-safe.
>
> Modified:
>     lld/trunk/COFF/InputFiles.cpp
>
> Modified: lld/trunk/COFF/InputFiles.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/InputFiles.cpp?rev=241420&r1=241419&r2=241420&view=diff
>
> ==============================================================================
> --- lld/trunk/COFF/InputFiles.cpp (original)
> +++ lld/trunk/COFF/InputFiles.cpp Sun Jul  5 17:50:00 2015
> @@ -18,6 +18,7 @@
>  #include "llvm/Support/Debug.h"
>  #include "llvm/Support/Endian.h"
>  #include "llvm/Support/raw_ostream.h"
> +#include <mutex>
>
>  using namespace llvm::object;
>  using namespace llvm::support::endian;
> @@ -76,7 +77,9 @@ std::error_code ArchiveFile::parse() {
>  }
>
>  // Returns a buffer pointing to a member file containing a given symbol.
> +// This function is thread-safe.
>  ErrorOr<MemoryBufferRef> ArchiveFile::getMember(const Archive::Symbol
> *Sym) {
> +  static std::mutex Mu;
>

I don't believe that MSVC 2013 will correctly initialize this in a
thread-safe way.  The compiler uses a bitfield-per-function to indicate
whether or not a function's static variable has been initialized,  MSVC
2013 doesn't implement this in a thread-safe way.


>    auto ItOrErr = Sym->getMember();
>    if (auto EC = ItOrErr.getError())
>      return EC;
> @@ -84,7 +87,9 @@ ErrorOr<MemoryBufferRef> ArchiveFile::ge
>
>    // Return an empty buffer if we have already returned the same buffer.
>    const char *StartAddr = It->getBuffer().data();
> +  Mu.lock();
>    auto Pair = Seen.insert(StartAddr);
> +  Mu.unlock();
>    if (!Pair.second)
>      return MemoryBufferRef();
>    return It->getMemoryBufferRef();
>
>
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