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

1 files changed, 22 insertions, 0 deletions

diff --git a/tests/codegen/fixed-x18.rs b/tests/codegen/fixed-x18.rs
new file mode 100644
index 00000000000..4997a39a726
--- /dev/null
+++ b/tests/codegen/fixed-x18.rs
@@ -0,0 +1,22 @@
+// Test that the `reserve-x18` target feature is (not) emitted when
+// the `-Zfixed-x18` flag is (not) set.
+
+//@ revisions: unset set
+//@ needs-llvm-components: aarch64
+//@ compile-flags: --target aarch64-unknown-none
+//@ [set] compile-flags: -Zfixed-x18
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+#[no_mangle]
+pub fn foo() {
+    // CHECK: @foo() unnamed_addr #0
+
+    // unset-NOT: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+reserve-x18{{.*}} }
+    // set: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+reserve-x18{{.*}} }
+}