[OFFLOAD] Stricter enforcement of user offload disable (#133470)

If user specifies offload is disabled (e.g.,
OMP_TARGET_OFFLOAD=disable), disable library almost completely. This
reduces resources spent to a minimum and ensures all APIs behave as if
the only available device is the host device.

Currently some of the APIs behave as if there were devices avaible for
offload even when under OMP_TARGET_OFFLOAD=disable.

---------

Co-authored-by: Joseph Huber <[email protected]>

Author: adurang

offload/include/OffloadPolicy.h

@@ -46,10 +46,15 @@ class OffloadPolicy {
         Kind = DISABLED;
       }
       return;
-    };
+    }
   }
 
 public:
+  static bool isOffloadDisabled() {
+    return static_cast<kmp_target_offload_kind_t>(
+               __kmpc_get_target_offload()) == tgt_disabled;
+  }
+
   static const OffloadPolicy &get(PluginManager &PM) {
     static OffloadPolicy OP(PM);
     return OP;

offload/libomptarget/PluginManager.cpp

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "PluginManager.h"
+#include "OffloadPolicy.h"
 #include "Shared/Debug.h"
 #include "Shared/Profile.h"
 #include "device.h"
@@ -30,6 +31,11 @@ PluginManager *PM = nullptr;
 
 void PluginManager::init() {
   TIMESCOPE();
+  if (OffloadPolicy::isOffloadDisabled()) {
+    DP("Offload is disabled. Skipping plugin initialization\n");
+    return;
+  }
+
   DP("Loading RTLs...\n");
 
   // Attempt to create an instance of each supported plugin.