Add ROOT::GetAvailableThreads() helper.

core/base/inc/TROOT.h

@@ -73,7 +73,7 @@ namespace Internal {
       TParBranchProcessingRAII()  { EnableParBranchProcessing();  }
       ~TParBranchProcessingRAII() { DisableParBranchProcessing(); }
    };
-      
+
    // Manage parallel tree processing
    void EnableParTreeProcessing();
    void DisableParTreeProcessing();
@@ -95,6 +95,7 @@ namespace ROOT {
    void DisableImplicitMT();
    Bool_t IsImplicitMTEnabled();
    UInt_t GetImplicitMTPoolSize();
+   UInt_t GetAvailableThreads();
 }
 
 class TROOT : public TDirectory {

core/base/src/TROOT.cxx

@@ -577,6 +577,21 @@ namespace Internal {
 #endif
    }
 
+   ////////////////////////////////////////////////////////////////////////////////
+   /// Returns the number of concurrent threads supported by the implementation.
+   /// In the IMT case returns the number of logical CPUs available to the current process in accordance with its TBB affinity mask.
+   UInt_t GetNThreadsAvailable()
+   {
+#ifdef R__USE_IMT
+      static UInt_t (*sym)() = (UInt_t(*)())Internal::GetSymInLibImt("ROOT_TImplicitMT_GetNThreadsAvailable");
+      if (sym)
+         return sym();
+      else
+         return 0;
+#else
+      return std::thread::hardware_concurrency();
+#endif
+   }
 }
 
 TROOT *ROOT::Internal::gROOTLocal = ROOT::GetROOT();

core/imt/inc/ROOT/TPoolManager.hxx

@@ -72,6 +72,11 @@ namespace ROOT {
       /// The number of threads will be able to change calling the factory function again after the last
       /// remaining shared_ptr owning the object is destroyed or reasigned, which will trigger the destructor of the manager.
       std::shared_ptr<TPoolManager> GetPoolManager(UInt_t nThreads = 0);
+
+
+      /// Get the number of logical CPUs in the system.
+      /// In case of having an affinity mask (TBB), return the number of logical CPUs available to the current process in accordance with it.
+      Int_t GetNThreadsAvailable();
    }
 }
 

core/imt/src/TImplicitMT.cxx

@@ -80,6 +80,10 @@ extern "C" UInt_t ROOT_TImplicitMT_GetImplicitMTPoolSize()
    return ROOT::Internal::TPoolManager::GetPoolSize();
 };
 
+extern "C" UInt_t ROOT_TImplicitMT_GetNThreadsAvailable()
+{
+   return ROOT::Internal::GetNThreadsAvailable();
+};
 
 extern "C" void ROOT_TImplicitMT_EnableParBranchProcessing()
 {

core/imt/src/TPoolManager.cxx

@@ -55,5 +55,10 @@ namespace ROOT {
          }
          return GetWP().lock();
       }
+
+      Int_t GetNThreadsAvailable()
+      {
+         return tbb::task_scheduler_init::default_num_threads();
+      }
    }
 }