Fix symbol resolution in get_fn to prefer dll imports

Author: RobertBColton

compiler/rustc_codegen_llvm/src/callee.rs

@@ -31,47 +31,45 @@ pub(crate) fn get_fn<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'t
 
     let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
 
-    let llfn = if let Some(llfn) = cx.get_declared_value(sym) {
+    let instance_def_id = instance.def_id();
+    let existing_decl = cx.get_declared_value(sym);
+
+    let llfn = if tcx.sess.target.arch == "x86"
+            && let Some(dllimport) = get_dllimport(tcx, instance_def_id, sym) {
+        // When calling functions in generated import libraries, MSVC needs
+        // the fully decorated name (as would have been in the declaring
+        // object file), but MinGW wants the name as exported (as would be
+        // in the def file) which may be missing decorations.
+        let mingw_gnu_toolchain = common::is_mingw_gnu_toolchain(&tcx.sess.target);
+        let llfn = cx.declare_fn(
+            &common::i686_decorated_name(
+                dllimport,
+                mingw_gnu_toolchain,
+                true,
+                !mingw_gnu_toolchain,
+            ),
+            fn_abi,
+            Some(instance),
+        );
+
+        // Fix for https://github.com/rust-lang/rust/issues/104453
+        // On x86 Windows, LLVM uses 'L' as the prefix for any private
+        // global symbols, so when we create an undecorated function symbol
+        // that begins with an 'L' LLVM misinterprets that as a private
+        // global symbol that it created and so fails the compilation at a
+        // later stage since such a symbol must have a definition.
+        //
+        // To avoid this, we set the Storage Class to "DllImport" so that
+        // LLVM will prefix the name with `__imp_`. Ideally, we'd like the
+        // existing logic below to set the Storage Class, but it has an
+        // exemption for MinGW for backwards compatibility.
+        llvm::set_dllimport_storage_class(llfn);
         llfn
     } else {
-        let instance_def_id = instance.def_id();
-        let llfn = if tcx.sess.target.arch == "x86"
-            && let Some(dllimport) = crate::common::get_dllimport(tcx, instance_def_id, sym)
-        {
-            // When calling functions in generated import libraries, MSVC needs
-            // the fully decorated name (as would have been in the declaring
-            // object file), but MinGW wants the name as exported (as would be
-            // in the def file) which may be missing decorations.
-            let mingw_gnu_toolchain = common::is_mingw_gnu_toolchain(&tcx.sess.target);
-            let llfn = cx.declare_fn(
-                &common::i686_decorated_name(
-                    dllimport,
-                    mingw_gnu_toolchain,
-                    true,
-                    !mingw_gnu_toolchain,
-                ),
-                fn_abi,
-                Some(instance),
-            );
-
-            // Fix for https://github.com/rust-lang/rust/issues/104453
-            // On x86 Windows, LLVM uses 'L' as the prefix for any private
-            // global symbols, so when we create an undecorated function symbol
-            // that begins with an 'L' LLVM misinterprets that as a private
-            // global symbol that it created and so fails the compilation at a
-            // later stage since such a symbol must have a definition.
-            //
-            // To avoid this, we set the Storage Class to "DllImport" so that
-            // LLVM will prefix the name with `__imp_`. Ideally, we'd like the
-            // existing logic below to set the Storage Class, but it has an
-            // exemption for MinGW for backwards compatibility.
-            llvm::set_dllimport_storage_class(llfn);
-            llfn
-        } else {
-            cx.declare_fn(sym, fn_abi, Some(instance))
-        };
-        debug!("get_fn: not casting pointer!");
+        existing_decl.unwrap_or_else(|| cx.declare_fn(sym, fn_abi, Some(instance)))
+    };
 
+    if existing_decl.map_or(true, |existing| existing != llfn) {
         // Apply an appropriate linkage/visibility value to our item that we
         // just declared.
         //
@@ -152,7 +150,7 @@ pub(crate) fn get_fn<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'t
         cx.assume_dso_local(llfn, true);
 
         llfn
-    };
+    }
 
     cx.instances.borrow_mut().insert(instance, llfn);