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

[via cfe-commits]

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@@ -0,0 +1,73 @@
+// RUN: %clang_cc1 -fsycl-is-device -triple spir64-unknown-unknown -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s
+
+// This test code generation when sycl_external attribute is used
+
+// Function defined and not used - symbols emitted
+[[clang::sycl_external]] int square(int x) { return x*x; }
+// CHECK: define dso_local spir_func noundef i32 @_Z6squarei
+
+// Function defined and used - symbols emitted
+[[clang::sycl_external]] int squareUsed(int x) { return x*x; }
+// CHECK: define dso_local spir_func noundef i32 @_Z10squareUsedi
+
+// Constexpr function defined and not used - symbols emitted
+[[clang::sycl_external]] constexpr int squareInlined(int x) { return x*x; }
+// CHECK: define linkonce_odr spir_func noundef i32 @_Z13squareInlinedi
+
+// Function declared but not defined or used - no symbols emitted
+[[clang::sycl_external]] int decl();
+
+// FIXME: Function declared and used but not defined - emit external reference
+[[clang::sycl_external]] void declused(int y);
+
+// Function overload with definition - symbols emitted
+[[clang::sycl_external]] int func1(int arg);
+int func1(int arg) { return arg; }
+// CHECK: define dso_local spir_func noundef i32 @_Z5func1i
+
+class A {
+// Defaulted special member functions - no symbols emitted
+  [[clang::sycl_external]] A& operator=(A& a) = default;
+};
+
+class B {
+  [[clang::sycl_external]] virtual void BFunc1WithAttr() { int i = 1; }
+// CHECK: define linkonce_odr spir_func void @_ZN1B14BFunc1WithAttrEv
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schittir wrote:

Thank you for this comment, Tom. 
Per our discussion, I am noting this to address in a subsequent PR since this requires addressing filtering for inline and virtual functions elsewhere, potentially in ASTContext.cpp. 

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