[clang] [clang][SYCL] Add sycl_external attribute and restrict emitting device code (PR #140282)

[Tom Honermann via cfe-commits]

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@@ -202,6 +202,25 @@ void SemaSYCL::handleKernelAttr(Decl *D, const ParsedAttr &AL) {
   handleSimpleAttribute<DeviceKernelAttr>(*this, D, AL);
 }
 
+void SemaSYCL::handleExternalAttr(Decl *D, const ParsedAttr &AL) {
+  auto *FD = cast<FunctionDecl>(D);
+  if (!FD->isExternallyVisible()) {
+    Diag(AL.getLoc(), diag::err_sycl_attribute_invalid_linkage);
+    return;
+  }
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tahonermann wrote:

I think the check for external linkage is better done in `Sema::CheckFunctionDeclaration()` so that other semantic checking of the function is done first. See the call to `CheckSYCLEntryPointFunctionDecl()` there; perhaps it makes sense to replicate that redirection to a `CheckSYCLExternalFunctionDecl()` function defined here.

https://github.com/llvm/llvm-project/pull/140282