[OFFLOAD] Stricter enforcement of user offload disable

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.

Author: adurang

offload/include/OffloadPolicy.h

@@ -26,7 +26,7 @@ extern "C" int __kmpc_get_target_offload(void) __attribute__((weak));
 
 class OffloadPolicy {
 
-  OffloadPolicy(PluginManager &PM) {
+  OffloadPolicy() {
     // TODO: Check for OpenMP.
     switch ((kmp_target_offload_kind_t)__kmpc_get_target_offload()) {
     case tgt_disabled:
@@ -36,6 +36,17 @@ class OffloadPolicy {
       Kind = MANDATORY;
       return;
     default:
+      // delay DEFAULT policy until PluginManager is ready
+      UserValue = false;
+      return;
+    };
+  }
+
+  OffloadPolicy(PluginManager &PM) {
+    const OffloadPolicy &OP = get();
+    if (!OP.UserValue) {
+      // User didn't specify a policy, decide
+      // based on number of devices discovered
       if (PM.getNumDevices()) {
         DP("Default TARGET OFFLOAD policy is now mandatory "
            "(devices were found)\n");
@@ -46,10 +57,16 @@ class OffloadPolicy {
         Kind = DISABLED;
       }
       return;
-    };
+    }
+    Kind = OP.Kind;
   }
 
 public:
+  static const OffloadPolicy &get() {
+    static OffloadPolicy OP;
+    return OP;
+  }
+
   static const OffloadPolicy &get(PluginManager &PM) {
     static OffloadPolicy OP(PM);
     return OP;
@@ -58,6 +75,7 @@ class OffloadPolicy {
   enum OffloadPolicyKind { DISABLED, MANDATORY };
 
   OffloadPolicyKind Kind = MANDATORY;
+  bool UserValue = true;
 };
 
 #endif // OMPTARGET_OFFLOAD_POLICY_H

offload/libomptarget/OffloadRTL.cpp

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "OffloadPolicy.h"
 #include "OpenMP/OMPT/Callback.h"
 #include "PluginManager.h"
 
@@ -31,6 +32,12 @@ void initRuntime() {
   if (PM == nullptr)
     PM = new PluginManager();
 
+  if (OffloadPolicy::get().Kind == OffloadPolicy::DISABLED) {
+    DP("Offload is disabled. Skipping library initialization\n");
+    // Do only absolutely needed initialization
+    return;
+  }
+
   RefCount++;
   if (RefCount == 1) {
     DP("Init offload library!\n");