Rollup merge of #124655 - Darksonn:fixed-x18, r=lqd,estebank

Add `-Zfixed-x18`

This PR is a follow-up to #124323 that proposes a different implementation. Please read the description of that PR for motivation.

See the equivalent flag in [the clang docs](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-ffixed-x18).

MCP: https://github.com/rust-lang/compiler-team/issues/748
Fixes https://github.com/rust-lang/rust/issues/121970
r? rust-lang/compiler

2 files changed, 16 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_llvm/src/errors.rs b/compiler/rustc_codegen_llvm/src/errors.rs
index e15eda7c66c..9d83dc81163 100644
--- a/compiler/rustc_codegen_llvm/src/errors.rs
+++ b/compiler/rustc_codegen_llvm/src/errors.rs
@@ -254,3 +254,9 @@ pub struct MismatchedDataLayout<'a> {
 pub(crate) struct InvalidTargetFeaturePrefix<'a> {
     pub feature: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_fixed_x18_invalid_arch)]
+pub(crate) struct FixedX18InvalidArch<'a> {
+    pub arch: &'a str,
+}
diff --git a/compiler/rustc_codegen_llvm/src/llvm_util.rs b/compiler/rustc_codegen_llvm/src/llvm_util.rs
index 5552b381025..53b9b530e9b 100644
--- a/compiler/rustc_codegen_llvm/src/llvm_util.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm_util.rs
@@ -1,6 +1,6 @@
 use crate::back::write::create_informational_target_machine;
 use crate::errors::{
-    InvalidTargetFeaturePrefix, PossibleFeature, TargetFeatureDisableOrEnable,
+    FixedX18InvalidArch, InvalidTargetFeaturePrefix, PossibleFeature, TargetFeatureDisableOrEnable,
     UnknownCTargetFeature, UnknownCTargetFeaturePrefix, UnstableCTargetFeature,
 };
 use crate::llvm;
@@ -615,6 +615,15 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
         .flatten();
     features.extend(feats);
 
+    // -Zfixed-x18
+    if sess.opts.unstable_opts.fixed_x18 {
+        if sess.target.arch != "aarch64" {
+            sess.dcx().emit_fatal(FixedX18InvalidArch { arch: &sess.target.arch });
+        } else {
+            features.push("+reserve-x18".into());
+        }
+    }
+
     if diagnostics && let Some(f) = check_tied_features(sess, &featsmap) {
         sess.dcx().emit_err(TargetFeatureDisableOrEnable {
             features: f,