[clang] 6ea75ec - clang: enable `swiftasynccall` for Wasm (#203330)

[via cfe-commits]

Author: Max Desiatov
Date: 2026-06-18T15:22:10+01:00
New Revision: 6ea75ecbac5337e7f8f1eb4e7647482da75f0ba7

URL: https://github.com/llvm/llvm-project/commit/6ea75ecbac5337e7f8f1eb4e7647482da75f0ba7
DIFF: https://github.com/llvm/llvm-project/commit/6ea75ecbac5337e7f8f1eb4e7647482da75f0ba7.diff

LOG: clang: enable `swiftasynccall` for Wasm (#203330)

Follow-up to https://github.com/llvm/llvm-project/pull/188296, where in
LLVM `swiftasynccall` is lowered to Wasm `return_call` and
`return_call_indirect` instructions when tail calls are enabled. This
still needed to be enabled at the Clang level in
`checkCallingConvention` in `lib/Basic/Targets/WebAssembly.h`.

Added: 
    clang/test/CodeGen/WebAssembly/wasm-swiftasynccall.c
    clang/test/Sema/wasm-swiftasynccall.c

Modified: 
    clang/lib/Basic/Targets/WebAssembly.h

Removed: 
    


################################################################################
diff  --git a/clang/lib/Basic/Targets/WebAssembly.h b/clang/lib/Basic/Targets/WebAssembly.h
index 6085197498163..fa0a2b9b505e0 100644
--- a/clang/lib/Basic/Targets/WebAssembly.h
+++ b/clang/lib/Basic/Targets/WebAssembly.h
@@ -186,7 +186,7 @@ class LLVM_LIBRARY_VISIBILITY WebAssemblyTargetInfo : public TargetInfo {
     case CC_Swift:
       return CCCR_OK;
     case CC_SwiftAsync:
-      return CCCR_Error;
+      return HasTailCall ? CCCR_OK : CCCR_Error;
     default:
       return CCCR_Warning;
     }

diff  --git a/clang/test/CodeGen/WebAssembly/wasm-swiftasynccall.c b/clang/test/CodeGen/WebAssembly/wasm-swiftasynccall.c
new file mode 100644
index 0000000000000..991f187aaf5b5
--- /dev/null
+++ b/clang/test/CodeGen/WebAssembly/wasm-swiftasynccall.c
@@ -0,0 +1,42 @@
+// REQUIRES: webassembly-registered-target
+// RUN: %clang_cc1 -no-enable-noundef-analysis -triple wasm32-unknown-unknown \
+// RUN:   -target-feature +tail-call -emit-llvm -o - %s | FileCheck %s --check-prefix=IR
+// RUN: %clang_cc1 -triple wasm32-unknown-unknown -target-feature +tail-call \
+// RUN:   -S -o - %s | FileCheck %s --check-prefix=ASM
+
+// swiftasynccall uses the swifttailcc CC and musttail calls, which the
+// WebAssembly backend lowers to return_call / return_call_indirect with the
+// tail-call feature. Sema acceptance: Sema/wasm-swiftasynccall.c.
+
+#define SWIFTASYNCCALL __attribute__((swiftasynccall))
+#define ASYNC_CONTEXT  __attribute__((swift_async_context))
+
+// Definition uses swifttailcc.
+// IR-LABEL: define {{.*}}swifttailcc void @async_leaf(ptr swiftasync
+SWIFTASYNCCALL void async_leaf(char *ASYNC_CONTEXT ctx) {
+  *ctx += 1;
+}
+
+// Direct tail call lowers to return_call.
+// IR-LABEL: define {{.*}}swifttailcc void @async_direct(ptr swiftasync
+// IR: musttail call swifttailcc void @async_leaf(ptr swiftasync
+// IR-NEXT: ret void
+//
+// ASM-LABEL: async_direct:
+// ASM: return_call async_leaf
+SWIFTASYNCCALL void async_direct(char *ASYNC_CONTEXT ctx) {
+  return async_leaf(ctx);
+}
+
+typedef SWIFTASYNCCALL void (*async_fn_t)(char *ASYNC_CONTEXT);
+
+// Indirect tail call lowers to return_call_indirect.
+// IR-LABEL: define {{.*}}swifttailcc void @async_indirect(ptr
+// IR: musttail call swifttailcc void %{{.*}}(ptr swiftasync
+// IR-NEXT: ret void
+//
+// ASM-LABEL: async_indirect:
+// ASM: return_call_indirect
+SWIFTASYNCCALL void async_indirect(async_fn_t fn, char *ASYNC_CONTEXT ctx) {
+  return fn(ctx);
+}

diff  --git a/clang/test/Sema/wasm-swiftasynccall.c b/clang/test/Sema/wasm-swiftasynccall.c
new file mode 100644
index 0000000000000..838a9a2e00227
--- /dev/null
+++ b/clang/test/Sema/wasm-swiftasynccall.c
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -triple wasm32-unknown-unknown -target-feature +tail-call -fsyntax-only -verify=tail %s
+// RUN: %clang_cc1 -triple wasm32-unknown-unknown -fsyntax-only -verify=notail %s
+
+// tail-no-diagnostics
+
+// swiftasynccall is accepted only with the tail-call feature, which lets the
+// backend lower its musttail calls to return_call.
+
+// notail-error at +1 {{'swiftasynccall' calling convention is not supported for this target}}
+void __attribute__((swiftasynccall)) async_func(char *__attribute__((swift_async_context)) ctx) {}