add rustc option -Zpacked-stack

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -312,6 +312,18 @@ fn backchain_attr<'ll>(cx: &SimpleCx<'ll>, sess: &Session) -> Option<&'ll Attrib
     if found_positive { Some(llvm::CreateAttrString(cx.llcx, "backchain")) } else { None }
 }
 
+fn packed_stack_attr<'ll>(cx: &SimpleCx<'ll>, sess: &Session) -> Option<&'ll Attribute> {
+    if sess.target.arch != Arch::S390x {
+        return None;
+    }
+
+    if sess.opts.unstable_opts.packed_stack {
+        Some(llvm::CreateAttrString(cx.llcx, "packed-stack"))
+    } else {
+        None
+    }
+}
+
 pub(crate) fn target_cpu_attr<'ll>(cx: &SimpleCx<'ll>, sess: &Session) -> &'ll Attribute {
     let target_cpu = llvm_util::target_cpu(sess);
     llvm::CreateAttrStringValue(cx.llcx, "target-cpu", target_cpu)
@@ -533,6 +545,9 @@ pub(crate) fn llfn_attrs_from_instance<'ll, 'tcx>(
     if let Some(backchain) = backchain_attr(cx, sess) {
         to_add.push(backchain);
     }
+    if let Some(packed_stack) = packed_stack_attr(cx, sess) {
+        to_add.push(packed_stack);
+    }
     to_add.extend(patchable_function_entry_attrs(
         cx,
         sess,

compiler/rustc_session/src/errors.rs

@@ -537,3 +537,7 @@ pub(crate) struct UnexpectedBuiltinCfg {
     pub(crate) cfg_name: Symbol,
     pub(crate) controlled_by: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag("`-Zpacked-stack` is only supported on s390x")]
+pub(crate) struct UnsupportedPackedStack;

compiler/rustc_session/src/options.rs

@@ -2536,6 +2536,8 @@ options! {
         "pass `-install_name @rpath/...` to the macOS linker (default: no)"),
     packed_bundled_libs: bool = (false, parse_bool, [TRACKED],
         "change rlib format to store native libraries as archives"),
+    packed_stack: bool = (false, parse_bool, [TRACKED],
+        "use packed stack frames (s390x only) (default: no)"),
     panic_abort_tests: bool = (false, parse_bool, [TRACKED],
         "support compiling tests with panic=abort (default: no)"),
     panic_in_drop: PanicStrategy = (PanicStrategy::Unwind, parse_panic_strategy, [TRACKED],

compiler/rustc_session/src/session.rs

@@ -1348,6 +1348,12 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
             sess.dcx().emit_warn(errors::SoftFloatIgnored);
         }
     }
+
+    if sess.opts.unstable_opts.packed_stack {
+        if sess.target.arch != Arch::S390x {
+            sess.dcx().emit_err(errors::UnsupportedPackedStack);
+        }
+    }
 }
 
 /// Holds data on the current incremental compilation session, if there is one.

compiler/rustc_target/src/spec/mod.rs

@@ -3258,6 +3258,16 @@ impl Target {
                     ));
                 }
             }
+            // If both `packed-stack` and `backchain` are set we need to use soft-float ABI.
+            if self.arch == Arch::S390x
+                && features_enabled.contains("packed-stack")
+                && features_enabled.contains("backchain")
+                && !matches!(self.abi, Abi::SoftFloat)
+            {
+                return Err(format!(
+                    "Enabling both `packed-stack` and `backchain` attributes is incompatible with the default s390x target. Switch to s390x-unknown-none-softfloat if you need both attributes."
+                ));
+            }
         }
 
         Ok(())

src/doc/rustc/src/codegen-options/index.md

@@ -487,6 +487,11 @@ one of the following values:
 If not specified, overflow checks are enabled if
 [debug-assertions](#debug-assertions) are enabled, disabled otherwise.
 
+## packed-stack
+
+This flag enables packed StackFrames on s390x. Enabling both `packed-stack` and `backchain` attributes is incompatible
+with the default s390x target. Switch to s390x-unknown-none-softfloat if you need both attributes.
+
 ## panic
 
 This option lets you control what happens when the code panics.