[SEH] Ignore async exception flag when the environment is not MSVC

[phoebewang]

Fixes #62449

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-driver

Author: Phoebe Wang (phoebewang)

Changes

Fixes #62449

---

Full diff: https://github.com/llvm/llvm-project/pull/88101.diff

2 Files Affected:

• (modified) clang/lib/Driver/ToolChains/Clang.cpp (+20-8)
• (added) clang/test/Driver/windows-seh-async-verify.cpp (+24)

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 766a9b91e3c0ad..c1ed4bd2dcda06 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -346,11 +346,14 @@ static bool addExceptionArgs(const ArgList &Args, types::ID InputType,
   bool EH = Args.hasFlag(options::OPT_fexceptions, options::OPT_fno_exceptions,
                          false);
 
-  bool EHa = Args.hasFlag(options::OPT_fasync_exceptions,
-                          options::OPT_fno_async_exceptions, false);
-  if (EHa) {
-    CmdArgs.push_back("-fasync-exceptions");
-    EH = true;
+  // Async exceptions are Windows MSVC only.
+  if (Triple.isWindowsMSVCEnvironment()) {
+    bool EHa = Args.hasFlag(options::OPT_fasync_exceptions,
+                            options::OPT_fno_async_exceptions, false);
+    if (EHa) {
+      CmdArgs.push_back("-fasync-exceptions");
+      EH = true;
+    }
   }
 
   // Obj-C exceptions are enabled by default, regardless of -fexceptions. This
@@ -8084,7 +8087,8 @@ struct EHFlags {
 ///      The 'a' modifier is unimplemented and fundamentally hard in LLVM IR.
 /// - c: Assume that extern "C" functions are implicitly nounwind.
 /// The default is /EHs-c-, meaning cleanups are disabled.
-static EHFlags parseClangCLEHFlags(const Driver &D, const ArgList &Args) {
+static EHFlags parseClangCLEHFlags(const Driver &D, const ArgList &Args,
+                                   bool isWindowsMSVC) {
   EHFlags EH;
 
   std::vector<std::string> EHArgs =
@@ -8094,8 +8098,15 @@ static EHFlags parseClangCLEHFlags(const Driver &D, const ArgList &Args) {
       switch (EHVal[I]) {
       case 'a':
         EH.Asynch = maybeConsumeDash(EHVal, I);
-        if (EH.Asynch)
+        if (EH.Asynch) {
+          // Async exceptions are Windows MSVC only.
+          if (!isWindowsMSVC) {
+            EH.Asynch = false;
+            D.Diag(clang::diag::warn_drv_unused_argument) << "/EHa" << EHVal;
+            continue;
+          }
           EH.Synch = false;
+        }
         continue;
       case 'c':
         EH.NoUnwindC = maybeConsumeDash(EHVal, I);
@@ -8159,7 +8170,8 @@ void Clang::AddClangCLArgs(const ArgList &Args, types::ID InputType,
 
   const Driver &D = getToolChain().getDriver();
 
-  EHFlags EH = parseClangCLEHFlags(D, Args);
+  bool IsWindowsMSVC = getToolChain().getTriple().isWindowsMSVCEnvironment();
+  EHFlags EH = parseClangCLEHFlags(D, Args, IsWindowsMSVC);
   if (!isNVPTX && (EH.Synch || EH.Asynch)) {
     if (types::isCXX(InputType))
       CmdArgs.push_back("-fcxx-exceptions");
diff --git a/clang/test/Driver/windows-seh-async-verify.cpp b/clang/test/Driver/windows-seh-async-verify.cpp
new file mode 100644
index 00000000000000..5fda6a77dba049
--- /dev/null
+++ b/clang/test/Driver/windows-seh-async-verify.cpp
@@ -0,0 +1,24 @@
+// RUN: %clang --target=x86_64-pc-windows -fasync-exceptions -fsyntax-only %s -### 2>&1 | FileCheck %s
+// RUN: %clang_cl --target=x86_64-pc-windows /EHa -fsyntax-only %s -### 2>&1 | FileCheck %s
+// RUN: %clang --target=x86_64-pc-windows-gnu -fasync-exceptions -fsyntax-only %s -### 2>&1 | FileCheck %s --check-prefixes=GNU-ALL,GNU
+// RUN: %clang_cl --target=x86_64-pc-windows-gnu /EHa -fsyntax-only %s -### 2>&1 | FileCheck %s --check-prefixes=GNU-ALL,CL-GNU
+
+// CHECK-NOT: warning
+// GNU: warning: argument unused during compilation: '-fasync-exceptions' [-Wunused-command-line-argument]
+// CL-GNU: warning: argument unused during compilation: '/EHa' [-Wunused-command-line-argument]
+
+// CHECK: -fasync-exceptions
+// GNU-ALL-NOT: -fasync-exceptions
+struct S {
+    union _Un {
+        ~_Un() {}
+        char _Buf[12];
+    };
+    _Un _un;
+};
+
+struct Embed {
+    S v2;
+};
+
+void PR62449() { Embed v{}; }

[MaxEW707]

LGTM!

[nico]

Looks like breaks check-clang on macOS: http://45.33.8.238/macm1/83520/step_6.txt

Please take a look and revert for now if it takes a while to fix.

[nico]

Oh, I think it's just the old clang: warning: '/Users/thakis/src/llvm-project/clang/test/Driver/windows-seh-async-verify.cpp' treated as the '/U' option [-Wslash-u-filename]. I'll try pushing a fix.

[nico]

e272c37 might have fixed this. (Didn't test locally, will check what the bot says.)

[phoebewang]

Thanks a lot @nico! I see the bot is still red, but not failed on this test any more.

[nico]

Yes, #83124 broke it in the meantime 😅