From 45adab87a56b12b411d3cce66b8a18942f7ac179 Mon Sep 17 00:00:00 2001
From: Guy Bedford <gbedford@cloudflare.com>
Date: Wed, 13 May 2026 12:16:15 -0700
Subject: [PATCH] Pass user-provided llvm_args after target spec llvm_args.

This ensures that user arguments can override target arguments without being silently ignored.
---
 compiler/rustc_codegen_llvm/src/llvm_util.rs |  5 +-
 tests/assembly-llvm/wasm_legacy_eh.rs        | 80 ++++++++++++++++++++
 2 files changed, 84 insertions(+), 1 deletion(-)
 create mode 100644 tests/assembly-llvm/wasm_legacy_eh.rs

diff --git a/compiler/rustc_codegen_llvm/src/llvm_util.rs b/compiler/rustc_codegen_llvm/src/llvm_util.rs
index bf9db092ab7f5..eb1efa31ae4ab 100644
--- a/compiler/rustc_codegen_llvm/src/llvm_util.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm_util.rs
@@ -66,7 +66,10 @@ unsafe fn configure_llvm(sess: &Session) {
 
     let cg_opts = sess.opts.cg.llvm_args.iter().map(AsRef::as_ref);
     let tg_opts = sess.target.llvm_args.iter().map(AsRef::as_ref);
-    let sess_args = cg_opts.chain(tg_opts);
+    // Target-spec args are passed to LLVM before user `-Cllvm-args`. LLVM's
+    // `cl::opt` parser is last-wins, so this lets `-Cllvm-args=...` override
+    // a value already set in the target spec (e.g. `-wasm-use-legacy-eh`).
+    let sess_args = tg_opts.chain(cg_opts);
 
     let user_specified_args: FxHashSet<_> =
         sess_args.clone().map(|s| llvm_arg_to_arg_name(s)).filter(|s| !s.is_empty()).collect();
diff --git a/tests/assembly-llvm/wasm_legacy_eh.rs b/tests/assembly-llvm/wasm_legacy_eh.rs
new file mode 100644
index 0000000000000..fa4332904cad6
--- /dev/null
+++ b/tests/assembly-llvm/wasm_legacy_eh.rs
@@ -0,0 +1,80 @@
+//@ only-wasm32
+//@ assembly-output: emit-asm
+//@ compile-flags: -C target-feature=+exception-handling
+//@ compile-flags: -C panic=unwind
+//@ compile-flags: -C llvm-args=-wasm-use-legacy-eh=true
+
+// Verifies that user-supplied `-Cllvm-args` can override an LLVM argument that
+// was set in the target spec. The `wasm32-unknown-unknown` target spec pins
+// `-wasm-use-legacy-eh=false`; this test asserts that passing the opposite
+// value via `-Cllvm-args` switches code generation back to the legacy
+// exception-handling instructions.
+
+#![crate_type = "lib"]
+#![feature(core_intrinsics)]
+
+extern "C-unwind" {
+    fn may_panic();
+}
+
+extern "C" {
+    fn log_number(number: usize);
+}
+
+struct LogOnDrop;
+
+impl Drop for LogOnDrop {
+    fn drop(&mut self) {
+        unsafe {
+            log_number(0);
+        }
+    }
+}
+
+// CHECK-LABEL: test_cleanup:
+#[no_mangle]
+pub fn test_cleanup() {
+    let _log_on_drop = LogOnDrop;
+    unsafe {
+        may_panic();
+    }
+
+    // CHECK-NOT: try_table
+    // CHECK-NOT: catch_all_ref
+    // CHECK-NOT: throw_ref
+    // CHECK-NOT: call
+    // CHECK: try
+    // CHECK: call may_panic
+    // CHECK: catch_all
+    // CHECK: rethrow
+    // CHECK: end_try
+}
+
+// CHECK-LABEL: test_rtry:
+#[no_mangle]
+pub fn test_rtry() {
+    unsafe {
+        core::intrinsics::catch_unwind(
+            |_| {
+                may_panic();
+            },
+            core::ptr::null_mut(),
+            |data, exception| {
+                log_number(data as usize);
+                log_number(exception as usize);
+            },
+        );
+    }
+
+    // CHECK-NOT: try_table
+    // CHECK-NOT: catch_all_ref
+    // CHECK-NOT: throw_ref
+    // CHECK-NOT: call
+    // CHECK: try
+    // CHECK: call may_panic
+    // CHECK: catch
+    // CHECK: call log_number
+    // CHECK: call log_number
+    // CHECK-NOT: rethrow
+    // CHECK: end_try
+}