Add uwtable annotation to modules when required

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -165,6 +165,8 @@ pub(crate) fn uwtable_attr(llcx: &llvm::Context, use_sync_unwind: Option<bool>)
     // NOTE: We should determine if we even need async unwind tables, as they
     // take have more overhead and if we can use sync unwind tables we
     // probably should.
+    //
+    // Similar logic exists for the per-module uwtable annotation in `context.rs`.
     let async_unwind = !use_sync_unwind.unwrap_or(false);
     llvm::CreateUWTableAttr(llcx, async_unwind)
 }

compiler/rustc_codegen_llvm/src/context.rs

@@ -311,6 +311,25 @@ pub(crate) unsafe fn create_module<'ll>(
         );
     }
 
+    if sess.must_emit_unwind_tables() {
+        // This assertion checks that Max is the correct merge behavior.
+        // Async unwind tables are strictly more useful than sync uwtables.
+        const {
+            assert!((llvm::UWTableKind::None as u32) < (llvm::UWTableKind::Sync as u32));
+            assert!((llvm::UWTableKind::Sync as u32) < (llvm::UWTableKind::Async as u32));
+        }
+
+        llvm::add_module_flag_u32(
+            llmod,
+            llvm::ModuleFlagMergeBehavior::Max,
+            "uwtable",
+            match sess.opts.unstable_opts.use_sync_unwind {
+                Some(true) => llvm::UWTableKind::Sync as u32,
+                Some(false) | None => llvm::UWTableKind::Async as u32,
+            },
+        );
+    }
+
     // Add "kcfi" module flag if KCFI is enabled. (See https://reviews.llvm.org/D119296.)
     if sess.is_sanitizer_kcfi_enabled() {
         llvm::add_module_flag_u32(llmod, llvm::ModuleFlagMergeBehavior::Override, "kcfi", 1);

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -240,6 +240,18 @@ pub(crate) enum DLLStorageClass {
     DllExport = 2, // Function to be accessible from DLL.
 }
 
+/// Must match the layout of `llvm::UWTableKind`.
+#[derive(Copy, Clone)]
+#[repr(C)]
+pub(crate) enum UWTableKind {
+    /// No unwind table requested
+    None = 0,
+    /// "Synchronous" unwind tables
+    Sync = 1,
+    /// "Asynchronous" unwind tables (instr precise)
+    Async = 2,
+}
+
 /// Must match the layout of `LLVMRustAttributeKind`.
 /// Semantically a subset of the C++ enum llvm::Attribute::AttrKind,
 /// though it is not ABI compatible (since it's a C++ enum)

tests/codegen-llvm/force-no-unwind-tables.rs

@@ -9,3 +9,5 @@
 fn foo() {
     panic!();
 }
+
+// CHECK-NOT: !"uwtable"

tests/codegen-llvm/force-unwind-tables.rs

@@ -4,3 +4,5 @@
 
 // CHECK: attributes #{{.*}} uwtable
 pub fn foo() {}
+
+// CHECK: !{{[0-9]+}} = !{i32 7, !"uwtable", i32 2}