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

[Erich Keane via cfe-commits]

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@@ -12251,6 +12264,9 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
   if (NewFD->hasAttr<SYCLKernelEntryPointAttr>())
     SYCL().CheckSYCLEntryPointFunctionDecl(NewFD);
 
+  if (NewFD->hasAttr<SYCLExternalAttr>())
+    SYCL().CheckSYCLExternalFunctionDecl(NewFD);
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erichkeane wrote:

Can you confirm that linkage isn't computed at that point? I would expect us to (since the entire declaration is read before we handle attributes) have it there, so it is a little surprising.  

Also, I didn't see instantiation of this attribute, do we prevent it on function templates? 

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