[llvm-commits] [llvm] r73636 - /llvm/trunk/lib/VMCore/Pass.cpp

[Howard Su]

I don't think this is worth for this sort of optimization. Double check lock
only optimize out a lock. However since this function is not a freqently
called function, this optimization is not that useful. I suggest you just
lock and check the flag.



On Thu, Jun 18, 2009 at 5:16 AM, Owen Anderson <resistor at mac.com> wrote:

> Author: resistor
> Date: Wed Jun 17 16:16:20 2009
> New Revision: 73636
>
> URL: http://llvm.org/viewvc/llvm-project?rev=73636&view=rev
> Log:
> We need to use double-checked locking for lazy initialization in this case
> when running multithreaded.
>
> Modified:
>    llvm/trunk/lib/VMCore/Pass.cpp
>
> Modified: llvm/trunk/lib/VMCore/Pass.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/VMCore/Pass.cpp?rev=73636&r1=73635&r2=73636&view=diff
>
>
> ==============================================================================
> --- llvm/trunk/lib/VMCore/Pass.cpp (original)
> +++ llvm/trunk/lib/VMCore/Pass.cpp Wed Jun 17 16:16:20 2009
> @@ -19,6 +19,8 @@
>  #include "llvm/ModuleProvider.h"
>  #include "llvm/ADT/STLExtras.h"
>  #include "llvm/Support/ManagedStatic.h"
> +#include "llvm/Support/Threading.h"
> +#include "llvm/System/Atomic.h"
>  #include <algorithm>
>  #include <map>
>  #include <set>
> @@ -192,8 +194,20 @@
>  // ressurection after llvm_shutdown is run.
>  static PassRegistrar *getPassRegistrar() {
>   static PassRegistrar *PassRegistrarObj = 0;
> +
> +  // Use double-checked locking to safely initialize the registrar when
> +  // we're running in multithreaded mode.
>   if (!PassRegistrarObj)
> -    PassRegistrarObj = new PassRegistrar();
> +    if (llvm_is_multithreaded()) {
> +      llvm_acquire_global_lock();
> +      if (!PassRegistrarObj) {
> +        PassRegistrar* tmp = new PassRegistrar();
> +        sys::MemoryFence();
> +        PassRegistrarObj = tmp;
> +      }
> +      llvm_release_global_lock();
> +    } else
> +      PassRegistrarObj = new PassRegistrar();
>   return PassRegistrarObj;
>  }
>
>
>
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-- 
-Howard
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