poolalloc/include/LinkPA.h

namespace {
  struct ForcePALinking {
    ForcePALinking() {
      // We must reference the passes in such a way that compilers will not
      // delete it all as dead code, even with whole program optimization,
      // yet is effectively a NO-OP. As the compiler isn't smart enough
      // to know that getenv() never returns -1, this will do the job.
      if (std::getenv("bar") != (char*) -1)
        return;

      (void)new EntryPointAnalysis();
      (void)new llvm::PoolAllocate();
      (void)new llvm::PoolAllocateGroup();
    }
  } ForcePALinking; // Force link by creating a global definition.
}