[clang-offload-wrapper] Lower priority of __tgt_register_lib in favor of __tgt_register_requires

Lower priority of __tgt_register_lib in order to make sure that __tgt_register_requires is called before loading a libomptarget plugin.
We want to know beforehand which requirements the user has asked for so that upon loading the plugin libomptarget can report how many devices there are that can satisfy these requirements.

Differential Revision: https://reviews.llvm.org/D75223

Author: George Rokos

clang/test/Driver/clang-offload-wrapper.c

@@ -39,8 +39,8 @@
 
 // CHECK-IR: [[DESC:@.+]] = internal constant [[DESCTY]] { i32 1, [[IMAGETY]]* getelementptr inbounds ([1 x [[IMAGETY]]], [1 x [[IMAGETY]]]* [[IMAGES]], i64 0, i64 0), [[ENTTY]]* [[ENTBEGIN]], [[ENTTY]]* [[ENTEND]] }
 
-// CHECK-IR: @llvm.global_ctors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 0, void ()* [[REGFN:@.+]], i8* null }]
-// CHECK-IR: @llvm.global_dtors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 0, void ()* [[UNREGFN:@.+]], i8* null }]
+// CHECK-IR: @llvm.global_ctors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 1, void ()* [[REGFN:@.+]], i8* null }]
+// CHECK-IR: @llvm.global_dtors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 1, void ()* [[UNREGFN:@.+]], i8* null }]
 
 // CHECK-IR: define internal void [[REGFN]]()
 // CHECK-IR:   call void @__tgt_register_lib([[DESCTY]]* [[DESC]])

clang/tools/clang-offload-wrapper/ClangOffloadWrapper.cpp

@@ -262,7 +262,12 @@ class BinaryWrapper {
     Builder.CreateRetVoid();
 
     // Add this function to constructors.
-    appendToGlobalCtors(M, Func, 0);
+    // Set priority to 1 so that __tgt_register_lib is executed AFTER
+    // __tgt_register_requires (we want to know what requirements have been
+    // asked for before we load a libomptarget plugin so that by the time the
+    // plugin is loaded it can report how many devices there are which can
+    // satisfy these requirements).
+    appendToGlobalCtors(M, Func, /*Priority*/ 1);
   }
 
   void createUnregisterFunction(GlobalVariable *BinDesc) {
@@ -283,7 +288,8 @@ class BinaryWrapper {
     Builder.CreateRetVoid();
 
     // Add this function to global destructors.
-    appendToGlobalDtors(M, Func, 0);
+    // Match priority of __tgt_register_lib
+    appendToGlobalDtors(M, Func, /*Priority*/ 1);
   }
 
 public: