[clang] [CIR] Add support for lowering __builtin_c23_va_start in ClangIR (PR #208741)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Akshay K (kumarak)

<details>
<summary>Changes</summary>

C23 `va_start `expands to `__builtin_c23_va_start`, which was previously routed to the unimplemented-builtin NYI path in ClangIR. As a result, variadic code failed to compile with -fclangir in -std=c23 mode.

This change lowers `__builtin_c23_va_start` the same way as `__builtin_va_start`. For both builtins, the va_list operand is always argument 0, so the existing lowering logic can be reused directly.

---
Full diff: https://github.com/llvm/llvm-project/pull/208741.diff


2 Files Affected:

- (modified) clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp (+1) 
- (added) clang/test/CIR/CodeGen/var_arg-c23.c (+45) 


``````````diff
diff --git a/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp b/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp
index 3c48b8b67d9ee..84bb33663de05 100644
--- a/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp
@@ -1059,6 +1059,7 @@ RValue CIRGenFunction::emitBuiltinExpr(const GlobalDecl &gd, unsigned builtinID,
   // C stdarg builtins.
   case Builtin::BI__builtin_stdarg_start:
   case Builtin::BI__builtin_va_start:
+  case Builtin::BI__builtin_c23_va_start:
   case Builtin::BI__va_start: {
     mlir::Value vaList = builtinID == Builtin::BI__va_start
                              ? emitScalarExpr(e->getArg(0))
diff --git a/clang/test/CIR/CodeGen/var_arg-c23.c b/clang/test/CIR/CodeGen/var_arg-c23.c
new file mode 100644
index 0000000000000..729b93bc6ceb3
--- /dev/null
+++ b/clang/test/CIR/CodeGen/var_arg-c23.c
@@ -0,0 +1,45 @@
+// RUN: %clang_cc1 -std=c23 -triple x86_64-unknown-linux-gnu -Wno-unused-value -fclangir -emit-cir %s -o %t.cir
+// RUN: FileCheck --input-file=%t.cir %s -check-prefix=CIR
+// RUN: %clang_cc1 -std=c23 -triple x86_64-unknown-linux-gnu -Wno-unused-value -fclangir -emit-llvm %s -o %t-cir.ll
+// RUN: FileCheck --input-file=%t-cir.ll %s -check-prefix=LLVM
+// RUN: %clang_cc1 -std=c23 -triple x86_64-unknown-linux-gnu -Wno-unused-value -emit-llvm %s -o %t.ll
+// RUN: FileCheck --input-file=%t.ll %s -check-prefix=LLVM
+
+// Ensure that __builtin_va_start(list, 0) and __builtin_c23_va_start(list)
+// have the same codegen, mirroring clang/test/CodeGen/varargs.c. The LLVM
+// checks are shared between the ClangIR pipeline and OG CodeGen.
+
+void noargs(...) {
+  __builtin_va_list list;
+  __builtin_va_start(list, 0);
+  __builtin_c23_va_start(list);
+  __builtin_va_end(list);
+}
+
+// CIR-LABEL: cir.func {{.*}} @noargs(
+// CIR:   %[[VAAREA:.+]] = cir.alloca "list" {{.*}} : !cir.ptr<!cir.array<!rec___va_list_tag x 1>>
+// CIR:   %[[VA_PTR0:.+]] = cir.cast array_to_ptrdecay %[[VAAREA]]
+// CIR-NEXT:   cir.va_start %[[VA_PTR0]] : !cir.ptr<!rec___va_list_tag>
+// CIR:   %[[VA_PTR1:.+]] = cir.cast array_to_ptrdecay %[[VAAREA]]
+// CIR-NEXT:   cir.va_start %[[VA_PTR1]] : !cir.ptr<!rec___va_list_tag>
+// CIR:   cir.va_end %{{.+}} : !cir.ptr<!rec___va_list_tag>
+
+// LLVM-LABEL: define {{.*}}void @noargs(...)
+// LLVM:   %[[VAAREA:.+]] = alloca [1 x %struct.__va_list_tag]
+// LLVM:   call void @llvm.va_start.p0(ptr %{{.+}})
+// LLVM:   call void @llvm.va_start.p0(ptr %{{.+}})
+// LLVM:   call void @llvm.va_end.p0(ptr %{{.+}})
+
+void with_param(int count, ...) {
+  __builtin_va_list list;
+  __builtin_c23_va_start(list, count);
+  __builtin_va_end(list);
+}
+
+// CIR-LABEL: cir.func {{.*}} @with_param(
+// CIR:   cir.va_start %{{.+}} : !cir.ptr<!rec___va_list_tag>
+// CIR:   cir.va_end %{{.+}} : !cir.ptr<!rec___va_list_tag>
+
+// LLVM-LABEL: define {{.*}}void @with_param(i32 noundef %{{.+}}, ...)
+// LLVM:   call void @llvm.va_start.p0(ptr %{{.+}})
+// LLVM:   call void @llvm.va_end.p0(ptr %{{.+}})

``````````

</details>


https://github.com/llvm/llvm-project/pull/208741