Rollup merge of #151071 - gen-openmp-metadata, r=nnethercote

Generate openmp metadata

LLVM has an openmp-opt pass, which is part of the default O3 pipeline.
The pass bails if we don't have a global called openmp, so let's generate it if people enable our experimental offload feature. openmp is a superset of the offload feature, so they share optimizations.
In follow-up PRs I'll start verifying that LLVM optimizes Rust the way we want it.

r? compiler

compiler/rustc_codegen_llvm/src/builder/gpu_offload.rs

@@ -55,6 +55,10 @@ pub(crate) fn declare(cx: &CodegenCx<'ll, '_>) -> Self {
         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 @@ fn main() {
 // CHECK-NEXT:   call void @__tgt_unregister_lib(ptr nonnull %EmptyDesc)
 // CHECK-NEXT:   ret void
 // CHECK-NEXT: }
+
+// CHECK: !{i32 7, !"openmp", i32 51}