[PATCH, libstdc++] Improve slightly __cxa_guard_acquire

[Jakub Jelinek]

On Thu, Sep 06, 2012 at 01:33:11PM -0700, Benjamin De Kosnik wrote:
> Here's the patch as applied to trunk in rev. 191042. I'll apply it to
> 4.7 this weekend as long as nobody yelps.

> 2012-09-06  Thiago Macieira  <thiago.macieira@intel.com>
> 
> 	PR libstdc++/54172
>         * libsupc++/guard.cc (__cxa_guard_acquire): Exit the loop earlier if
>         we detect that another thread has had success. Don't compare_exchange
>         from a finished state back to a waiting state. Comment.
> 
> diff --git a/libstdc++-v3/libsupc++/guard.cc b/libstdc++-v3/libsupc++/guard.cc
> index adc9608..60165cd 100644
> --- a/libstdc++-v3/libsupc++/guard.cc
> +++ b/libstdc++-v3/libsupc++/guard.cc
> @@ -244,13 +244,13 @@ namespace __cxxabiv1
>      if (__gthread_active_p ())
>        {
>  	int *gi = (int *) (void *) g;
> -	int expected(0);
>  	const int guard_bit = _GLIBCXX_GUARD_BIT;
>  	const int pending_bit = _GLIBCXX_GUARD_PENDING_BIT;
>  	const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
>  
>  	while (1)
>  	  {
> +	    int expected(0);
>  	    if (__atomic_compare_exchange_n(gi, &expected, pending_bit, false,
>  					    __ATOMIC_ACQ_REL,
>  					    __ATOMIC_RELAXED))

Shouldn't this __ATOMIC_RELAXED be also __ATOMIC_ACQUIRE?  If expected ends
up being guard_bit, then the code will return 0; right away.

> @@ -264,13 +264,26 @@ namespace __cxxabiv1
>  		// Already initialized.
>  		return 0;	
>  	      }
> +
>  	     if (expected == pending_bit)
>  	       {
> +		 // Use acquire here.
>  		 int newv = expected | waiting_bit;
>  		 if (!__atomic_compare_exchange_n(gi, &expected, newv, false,
>  						  __ATOMIC_ACQ_REL, 
> -						  __ATOMIC_RELAXED))
> -		   continue;
> +						  __ATOMIC_ACQUIRE))
> +		   {
> +		     if (expected == guard_bit)
> +		       {
> +			 // Make a thread that failed to set the
> +			 // waiting bit exit the function earlier,
> +			 // if it detects that another thread has
> +			 // successfully finished initialising.
> +			 return 0;
> +		       }
> +		     if (expected == 0)
> +		       continue;
> +		   }
>  		 
>  		 expected = newv;
>  	       }


	Jakub