[llvm] r239566 - [Support] Fix a race initializing a static local in MSVC

[Mehdi Amini]

Hi Reid, 

Can you explain how the change makes it thread safe? It’s not obvious to me right now.

Thanks,

Mehdi

> On Jun 11, 2015, at 3:22 PM, Reid Kleckner <reid at kleckner.net> wrote:
> 
> Author: rnk
> Date: Thu Jun 11 17:22:45 2015
> New Revision: 239566
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=239566&view=rev
> Log:
> [Support] Fix a race initializing a static local in MSVC
> 
> static local initialization isn't thread safe with MSVC and a race was
> reported in PR23817. We can't use std::atomic because it's not trivially
> constructible, so instead do some lame volatile global integer
> manipulation.
> 
> Modified:
>    llvm/trunk/lib/Support/Windows/Memory.inc
> 
> Modified: llvm/trunk/lib/Support/Windows/Memory.inc
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/Windows/Memory.inc?rev=239566&r1=239565&r2=239566&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Support/Windows/Memory.inc (original)
> +++ llvm/trunk/lib/Support/Windows/Memory.inc Thu Jun 11 17:22:45 2015
> @@ -78,7 +78,15 @@ MemoryBlock Memory::allocateMappedMemory
>   // While we'd be happy to allocate single pages, the Windows allocation
>   // granularity may be larger than a single page (in practice, it is 64K)
>   // so mapping less than that will create an unreachable fragment of memory.
> -  static const size_t Granularity = getAllocationGranularity();
> +  // Avoid using one-time initialization of static locals here, since they
> +  // aren't thread safe with MSVC.
> +  static volatile size_t GranularityCached;
> +  size_t Granularity = GranularityCached;
> +  if (Granularity == 0) {
> +    Granularity = getAllocationGranularity();
> +    GranularityCached = Granularity;
> +  }
> +
>   const size_t NumBlocks = (NumBytes+Granularity-1)/Granularity;
> 
>   uintptr_t Start = NearBlock ? reinterpret_cast<uintptr_t>(NearBlock->base()) +
> 
> 
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