Add a "don't override" mapping for -fvisibility-from-dllstorageclass (#74629)

`-fvisibility-from-dllstorageclass` allows for overriding the visibility
of globals from their DLL storage class. The visibility to apply can be
customised for the different classes of globals via a set of dependent
options that specify the mapping values:
- `-fvisibility-dllexport=<value>`
- `-fvisibility-nodllstorageclass=<value>`
- `-fvisibility-externs-dllimport=<value>`
- `-fvisibility-externs-nodllstorageclass=<value>` 

Currently, one of the existing LLVM visibilities, `hidden`, `protected`,
`default`, can be used as a mapping value. This change adds a new
mapping value: `keep`, which specifies that the visibility should not be
overridden for that class of globals. The behaviour of
`-fvisibility-from-dllstorageclass` is otherwise unchanged and existing
uses of this set of options will be unaffected.

The background to this change is that currently the PS4 and PS5
compilers effectively ignore visibility - dllimport/export is the
supported method for export control in C/C++ source code. Now, we would
like to support visibility attributes and options in our frontend, in
addition to dllimport/export. To support this, we will override the
visibility of globals with explicit dllimport/export annotations but use
the `keep` setting for globals which do not have an explicit
dllimport/export.

There are also some minor improvements to the existing options:
- Make the `LANGOPS` `BENIGN` as they don't involve the AST.
- Correct/clarify the help text for the options.

1 files changed, 54 insertions, 27 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 482c210..77c9318 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -722,43 +722,70 @@ void InstrProfStats::reportDiagnostics(DiagnosticsEngine &Diags,
   }
 }
 
+static std::optional<llvm::GlobalValue::VisibilityTypes>
+getLLVMVisibility(clang::LangOptions::VisibilityFromDLLStorageClassKinds K) {
+  // Map to LLVM visibility.
+  switch (K) {
+  case clang::LangOptions::VisibilityFromDLLStorageClassKinds::Keep:
+    return std::nullopt;
+  case clang::LangOptions::VisibilityFromDLLStorageClassKinds::Default:
+    return llvm::GlobalValue::DefaultVisibility;
+  case clang::LangOptions::VisibilityFromDLLStorageClassKinds::Hidden:
+    return llvm::GlobalValue::HiddenVisibility;
+  case clang::LangOptions::VisibilityFromDLLStorageClassKinds::Protected:
+    return llvm::GlobalValue::ProtectedVisibility;
+  }
+  llvm_unreachable("unknown option value!");
+}
+
+void setLLVMVisibility(llvm::GlobalValue &GV,
+                       std::optional<llvm::GlobalValue::VisibilityTypes> V) {
+  if (!V)
+    return;
+
+  // Reset DSO locality before setting the visibility. This removes
+  // any effects that visibility options and annotations may have
+  // had on the DSO locality. Setting the visibility will implicitly set
+  // appropriate globals to DSO Local; however, this will be pessimistic
+  // w.r.t. to the normal compiler IRGen.
+  GV.setDSOLocal(false);
+  GV.setVisibility(*V);
+}
+
 static void setVisibilityFromDLLStorageClass(const clang::LangOptions &LO,
                                              llvm::Module &M) {
   if (!LO.VisibilityFromDLLStorageClass)
     return;
 
-  llvm::GlobalValue::VisibilityTypes DLLExportVisibility =
-      CodeGenModule::GetLLVMVisibility(LO.getDLLExportVisibility());
-  llvm::GlobalValue::VisibilityTypes NoDLLStorageClassVisibility =
-      CodeGenModule::GetLLVMVisibility(LO.getNoDLLStorageClassVisibility());
-  llvm::GlobalValue::VisibilityTypes ExternDeclDLLImportVisibility =
-      CodeGenModule::GetLLVMVisibility(LO.getExternDeclDLLImportVisibility());
-  llvm::GlobalValue::VisibilityTypes ExternDeclNoDLLStorageClassVisibility =
-      CodeGenModule::GetLLVMVisibility(
-          LO.getExternDeclNoDLLStorageClassVisibility());
+  std::optional<llvm::GlobalValue::VisibilityTypes> DLLExportVisibility =
+      getLLVMVisibility(LO.getDLLExportVisibility());
+
+  std::optional<llvm::GlobalValue::VisibilityTypes>
+      NoDLLStorageClassVisibility =
+          getLLVMVisibility(LO.getNoDLLStorageClassVisibility());
+
+  std::optional<llvm::GlobalValue::VisibilityTypes>
+      ExternDeclDLLImportVisibility =
+          getLLVMVisibility(LO.getExternDeclDLLImportVisibility());
+
+  std::optional<llvm::GlobalValue::VisibilityTypes>
+      ExternDeclNoDLLStorageClassVisibility =
+          getLLVMVisibility(LO.getExternDeclNoDLLStorageClassVisibility());
 
   for (llvm::GlobalValue &GV : M.global_values()) {
     if (GV.hasAppendingLinkage() || GV.hasLocalLinkage())
       continue;
 
-    // Reset DSO locality before setting the visibility. This removes
-    // any effects that visibility options and annotations may have
-    // had on the DSO locality. Setting the visibility will implicitly set
-    // appropriate globals to DSO Local; however, this will be pessimistic
-    // w.r.t. to the normal compiler IRGen.
-    GV.setDSOLocal(false);
-
-    if (GV.isDeclarationForLinker()) {
-      GV.setVisibility(GV.getDLLStorageClass() ==
-                               llvm::GlobalValue::DLLImportStorageClass
-                           ? ExternDeclDLLImportVisibility
-                           : ExternDeclNoDLLStorageClassVisibility);
-    } else {
-      GV.setVisibility(GV.getDLLStorageClass() ==
-                               llvm::GlobalValue::DLLExportStorageClass
-                           ? DLLExportVisibility
-                           : NoDLLStorageClassVisibility);
-    }
+    if (GV.isDeclarationForLinker())
+      setLLVMVisibility(GV, GV.getDLLStorageClass() ==
+                                    llvm::GlobalValue::DLLImportStorageClass
+                                ? ExternDeclDLLImportVisibility
+                                : ExternDeclNoDLLStorageClassVisibility);
+    else
+      setLLVMVisibility(GV, GV.getDLLStorageClass() ==
+                                    llvm::GlobalValue::DLLExportStorageClass
+                                ? DLLExportVisibility
+                                : NoDLLStorageClassVisibility);
 
     GV.setDLLStorageClass(llvm::GlobalValue::DefaultStorageClass);
   }