Generate global openmp metadata to trigger llvm openmp-opt pass

Author: ZuseZ4

compiler/rustc_codegen_llvm/src/builder/gpu_offload.rs

@@ -55,6 +55,10 @@ impl<'ll> OffloadGlobals<'ll> {
         let init_ty = cx.type_func(&[], cx.type_void());
         let init_rtls = declare_offload_fn(cx, "__tgt_init_all_rtls", init_ty);
 
+        // We want LLVM's openmp-opt pass to pick up and optimize this module, since it covers both
+        // openmp and offload optimizations.
+        llvm::add_module_flag_u32(cx.llmod(), llvm::ModuleFlagMergeBehavior::Max, "openmp", 51);
+
         OffloadGlobals {
             launcher_fn,
             launcher_ty,

tests/codegen-llvm/gpu_offload/gpu_host.rs

@@ -104,3 +104,5 @@ pub fn _kernel_1(x: &mut [f32; 256]) {
 // CHECK-NEXT:   call void @__tgt_unregister_lib(ptr nonnull %EmptyDesc)
 // CHECK-NEXT:   ret void
 // CHECK-NEXT: }
+
+// CHECK: !{i32 7, !"openmp", i32 51}