Rollup merge of #140740 - ojeda:indirect-branch-cs-prefix, r=davidtwco

Add `-Zindirect-branch-cs-prefix`

Cc: ``@azhogin`` ``@Darksonn``

This goes on top of https://github.com/rust-lang/rust/pull/135927, i.e. please skip the first commit here. Please feel free to inherit it there.

In fact, I am not sure if there is any use case for the flag without `-Zretpoline*`. GCC and Clang allow it, though.

There is a `FIXME` for two `ignore`s in the test that I took from another test I did in the past -- they may be needed or not here since I didn't run the full CI. Either way, it is not critical.

Tracking issue: https://github.com/rust-lang/rust/issues/116852.
MCP: https://github.com/rust-lang/compiler-team/issues/868.

6 files changed, 24 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index 2d91ae64e2d..4a7de7d2e69 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -471,6 +471,15 @@ 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
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index 0a764808f95..4425877308a 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/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));
diff --git a/compiler/rustc_session/messages.ftl b/compiler/rustc_session/messages.ftl
index 528c52eace7..80754964c43 100644
--- a/compiler/rustc_session/messages.ftl
+++ b/compiler/rustc_session/messages.ftl
@@ -49,6 +49,8 @@ session_hexadecimal_float_literal_not_supported = hexadecimal float literal is n
 session_incompatible_linker_flavor = linker flavor `{$flavor}` is incompatible with the current target
     .note = compatible flavors are: {$compatible_list}
 
+session_indirect_branch_cs_prefix_requires_x86_or_x86_64 = `-Zindirect-branch-cs-prefix` is only supported on x86 and x86_64
+
 session_instrumentation_not_supported = {$us} instrumentation is not supported for this target
 
 session_int_literal_too_large = integer literal is too large
diff --git a/compiler/rustc_session/src/errors.rs b/compiler/rustc_session/src/errors.rs
index bf95014843d..9471807df01 100644
--- a/compiler/rustc_session/src/errors.rs
+++ b/compiler/rustc_session/src/errors.rs
@@ -472,6 +472,10 @@ pub(crate) struct FunctionReturnRequiresX86OrX8664;
 pub(crate) struct FunctionReturnThunkExternRequiresNonLargeCodeModel;
 
 #[derive(Diagnostic)]
+#[diag(session_indirect_branch_cs_prefix_requires_x86_or_x86_64)]
+pub(crate) struct IndirectBranchCsPrefixRequiresX86OrX8664;
+
+#[derive(Diagnostic)]
 #[diag(session_unsupported_regparm)]
 pub(crate) struct UnsupportedRegparm {
     pub(crate) regparm: u32,
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 0e112edc733..6d5be2d92cd 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/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 thunks (default: no)"),
     inline_llvm: bool = (true, parse_bool, [TRACKED],
         "enable LLVM inlining (default: yes)"),
     inline_mir: Option<bool> = (None, parse_opt_bool, [TRACKED],
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index c6956cf5f23..c8f4b511a7e 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -1368,6 +1368,12 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
         }
     }
 
+    if sess.opts.unstable_opts.indirect_branch_cs_prefix {
+        if sess.target.arch != "x86" && sess.target.arch != "x86_64" {
+            sess.dcx().emit_err(errors::IndirectBranchCsPrefixRequiresX86OrX8664);
+        }
+    }
+
     if let Some(regparm) = sess.opts.unstable_opts.regparm {
         if regparm > 3 {
             sess.dcx().emit_err(errors::UnsupportedRegparm { regparm });