[llvm-commits] patch: CXAGuardElimination pass.

[Nick Lewycky]

This pass eliminates dead calls to __cxa_guard_acquire and 
__cxa_guard_release. Those two functions are defined by the Itanium ABI 
as part of the thread-safe one-time construction ABI for static 
variables in functions.

For example, this program:

   struct X {
     X(int) {}
   };

   void foo() {
     static X x(10);
   }

would have calls to cxa_guard_* in foo, and this pass detects that there 
is no real work being done between the guards and therefore it is safe 
to eliminate them.

However, it doesn't simplify it down all the way. See llvm.org/PR4261 
for an example of what happens after this optimization is applied on the 
above program. We may decide that PR4261 is too hard to fix in general 
and just add some extra logic to this pass, but I'd rather have this 
committed for a start.

Please review!

Nick
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