Add -Zindirect-branch-cs-prefix (from draft PR)

Author: Darksonn

compiler/rustc_codegen_llvm/src/context.rs

@@ -463,6 +463,15 @@ pub(crate) unsafe fn create_module<'ll>(
         }
     }
 
+    if sess.opts.unstable_opts.indirect_branch_cs_prefix {
+        llvm::add_module_flag_u32(
+            llmod,
+            llvm::ModuleFlagMergeBehavior::Override,
+            "indirect_branch_cs_prefix",
+            1,
+        );
+    }
+
     match (sess.opts.unstable_opts.small_data_threshold, sess.target.small_data_threshold_support())
     {
         // Set up the small-data optimization limit for architectures that use

compiler/rustc_interface/src/tests.rs

@@ -807,6 +807,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(hint_mostly_unused, true);
     tracked!(human_readable_cgu_names, true);
     tracked!(incremental_ignore_spans, true);
+    tracked!(indirect_branch_cs_prefix, true);
     tracked!(inline_mir, Some(true));
     tracked!(inline_mir_hint_threshold, Some(123));
     tracked!(inline_mir_threshold, Some(123));

compiler/rustc_session/src/options.rs

@@ -2295,6 +2295,8 @@ options! {
         - hashes of green query instances
         - hash collisions of query keys
         - hash collisions when creating dep-nodes"),
+    indirect_branch_cs_prefix: bool = (false, parse_bool, [TRACKED TARGET_MODIFIER],
+        "add cs prefix to call and jmp to indirect thunk (default: no)"),
     inline_llvm: bool = (true, parse_bool, [TRACKED],
         "enable LLVM inlining (default: yes)"),
     inline_mir: Option<bool> = (None, parse_opt_bool, [TRACKED],

tests/codegen-llvm/indirect-branch-cs-prefix.rs

@@ -0,0 +1,18 @@
+// Test that the `indirect_branch_cs_prefix` module attribute is (not) emitted when the
+// `-Zindirect-branch-cs-prefix` flag is (not) set.
+
+//@ add-core-stubs
+//@ revisions: unset set
+//@ needs-llvm-components: x86
+//@ compile-flags: --target x86_64-unknown-linux-gnu
+//@ [set] compile-flags: -Zindirect-branch-cs-prefix
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+// unset-NOT: !{{[0-9]+}} = !{i32 4, !"indirect_branch_cs_prefix", i32 1}
+// set: !{{[0-9]+}} = !{i32 4, !"indirect_branch_cs_prefix", i32 1}