Manually set dso_local when its valid to do so

This should have no real effect in most cases, as e.g. `hidden`
visibility already implies `dso_local` (or at least LLVM IR does not
preserve the `dso_local` setting if the item is already `hidden`), but
it should fix `-Crelocation-model=static` and improve codegen in
executables.

Note that this PR does not exhaustively port the logic in [clang]. Only
the obviously correct portion and what is necessary to fix a regression
from LLVM 12 that relates to `-Crelocation_model=static`.

Fixes #83335

[clang]: https://github.com/llvm/llvm-project/blob/3001d080c813da20b329303bf8f45451480e5905/clang/lib/CodeGen/CodeGenModule.cpp#L945-L1039

1 files changed, 11 insertions, 8 deletions

diff --git a/compiler/rustc_codegen_llvm/src/callee.rs b/compiler/rustc_codegen_llvm/src/callee.rs
index 367c1f4811c..b26969a5012 100644
--- a/compiler/rustc_codegen_llvm/src/callee.rs
+++ b/compiler/rustc_codegen_llvm/src/callee.rs
@@ -14,6 +14,7 @@ use tracing::debug;
 
 use rustc_middle::ty::layout::{FnAbiExt, HasTyCtxt};
 use rustc_middle::ty::{self, Instance, TypeFoldable};
+use rustc_target::spec::RelocModel;
 
 /// Codegens a reference to a fn/method item, monomorphizing and
 /// inlining as it goes.
@@ -170,17 +171,19 @@ pub fn get_fn(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'tcx>) -> &'ll Value
                     }
                 }
             }
-        }
 
-        // MinGW: For backward compatibility we rely on the linker to decide whether it
-        // should use dllimport for functions.
-        if cx.use_dll_storage_attrs
-            && tcx.is_dllimport_foreign_item(instance_def_id)
-            && tcx.sess.target.env != "gnu"
-        {
-            unsafe {
+            // MinGW: For backward compatibility we rely on the linker to decide whether it
+            // should use dllimport for functions.
+            if cx.use_dll_storage_attrs
+                && tcx.is_dllimport_foreign_item(instance_def_id)
+                && tcx.sess.target.env != "gnu"
+            {
                 llvm::LLVMSetDLLStorageClass(llfn, llvm::DLLStorageClass::DllImport);
             }
+
+            if cx.tcx.sess.relocation_model() == RelocModel::Static {
+                llvm::LLVMRustSetDSOLocal(llfn, true);
+            }
         }
 
         llfn