Allow not emitting `uwtable` on Android

compiler/rustc_session/src/session.rs

@@ -863,7 +863,7 @@ pub fn must_emit_unwind_tables(&self) -> bool {
         } else if self.target.requires_uwtable {
             true
         } else {
-            self.opts.cg.force_unwind_tables.unwrap_or(false)
+            self.opts.cg.force_unwind_tables.unwrap_or(self.target.default_uwtable)
         }
     }
 

compiler/rustc_target/src/spec/android_base.rs

@@ -12,7 +12,10 @@ pub fn opts() -> TargetOptions {
     base.dwarf_version = Some(2);
     base.position_independent_executables = true;
     base.has_elf_tls = false;
-    base.requires_uwtable = true;
+    // This is for backward compatibility, see https://github.com/rust-lang/rust/issues/49867
+    // for context. (At that time, there was no `-C force-unwind-tables`, so the only solution
+    // was to always emit `uwtable`).
+    base.default_uwtable = true;
     base.crt_static_respected = false;
     base
 }

compiler/rustc_target/src/spec/mod.rs

@@ -1111,6 +1111,10 @@ pub struct TargetOptions {
     /// unwinders.
     pub requires_uwtable: bool,
 
+    /// Whether or not to emit `uwtable` attributes on functions if `-C force-unwind-tables`
+    /// is not specified and `uwtable` is not required on this target.
+    pub default_uwtable: bool,
+
     /// Whether or not SIMD types are passed by reference in the Rust ABI,
     /// typically required if a target can be compiled with a mixed set of
     /// target features. This is `true` by default, and `false` for targets like
@@ -1248,6 +1252,7 @@ fn default() -> TargetOptions {
             default_hidden_visibility: false,
             emit_debug_gdb_scripts: true,
             requires_uwtable: false,
+            default_uwtable: false,
             simd_types_indirect: true,
             limit_rdylib_exports: true,
             override_export_symbols: None,
@@ -1711,6 +1716,7 @@ pub fn from_json(obj: Json) -> Result<Target, String> {
         key!(default_hidden_visibility, bool);
         key!(emit_debug_gdb_scripts, bool);
         key!(requires_uwtable, bool);
+        key!(default_uwtable, bool);
         key!(simd_types_indirect, bool);
         key!(limit_rdylib_exports, bool);
         key!(override_export_symbols, opt_list);
@@ -1947,6 +1953,7 @@ fn to_json(&self) -> Json {
         target_option_val!(default_hidden_visibility);
         target_option_val!(emit_debug_gdb_scripts);
         target_option_val!(requires_uwtable);
+        target_option_val!(default_uwtable);
         target_option_val!(simd_types_indirect);
         target_option_val!(limit_rdylib_exports);
         target_option_val!(override_export_symbols);

src/test/codegen/default-requires-uwtable.rs

+// revisions: WINDOWS ANDROID
+// needs-llvm-components: x86 arm
+// compile-flags: -C panic=abort
+// [WINDOWS] compile-flags: --target=x86_64-pc-windows-msvc
+// [ANDROID] compile-flags: --target=armv7-linux-androideabi
+
+#![feature(no_core, lang_items)]
+#![crate_type = "lib"]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+// CHECK: attributes #{{.*}} uwtable
+pub fn foo() {}

src/test/codegen/force-no-unwind-tables.rs

+// compile-flags: -C no-prepopulate-passes -C panic=abort -C force-unwind-tables=n
+// ignore-windows
+
+#![crate_type="lib"]
+
+// CHECK-NOT: attributes #{{.*}} uwtable
+pub fn foo() {}