[SYCL] Allow non-evaluated globals to be used in a kernel. (#3323)

As requested to support SYCL-2020 specialization constants, this makes
our global variable diagnostics not happen unless it is in a runtime
evaluated context.

This is OK, because the SYCL standard prohibits ODR uses, of which
non-runtime-evaluated uses are not.

Author: Erich Keane

clang/lib/Sema/SemaExpr.cpp

@@ -215,17 +215,21 @@ bool Sema::DiagnoseUseOfDecl(NamedDecl *D, ArrayRef<SourceLocation> Locs,
   if (getLangOpts().SYCLIsDevice) {
     if (auto VD = dyn_cast<VarDecl>(D)) {
       bool IsConst = VD->getType().isConstant(Context);
-      if (!IsConst && VD->getStorageClass() == SC_Static)
+      bool IsRuntimeEvaluated =
+          ExprEvalContexts.empty() ||
+          (!isUnevaluatedContext() && !isConstantEvaluated());
+      if (IsRuntimeEvaluated && !IsConst && VD->getStorageClass() == SC_Static)
         SYCLDiagIfDeviceCode(*Locs.begin(), diag::err_sycl_restrict)
             << Sema::KernelNonConstStaticDataVariable;
       // Non-const globals are allowed for SYCL explicit SIMD.
-      else if (!isSYCLEsimdPrivateGlobal(VD) && !IsConst &&
-               VD->hasGlobalStorage() && !isa<ParmVarDecl>(VD))
+      else if (IsRuntimeEvaluated && !isSYCLEsimdPrivateGlobal(VD) &&
+               !IsConst && VD->hasGlobalStorage() && !isa<ParmVarDecl>(VD))
         SYCLDiagIfDeviceCode(*Locs.begin(), diag::err_sycl_restrict)
             << Sema::KernelGlobalVariable;
       // Disallow const statics and globals that are not zero-initialized
       // or constant-initialized.
-      else if (IsConst && VD->hasGlobalStorage() && !VD->isConstexpr() &&
+      else if (IsRuntimeEvaluated && IsConst && VD->hasGlobalStorage() &&
+               !VD->isConstexpr() &&
                !checkAllowedSYCLInitializer(VD, /*CheckValueDependent =*/true))
         SYCLDiagIfDeviceCode(*Locs.begin(), diag::err_sycl_restrict)
             << Sema::KernelConstStaticVariable;

clang/test/SemaSYCL/sycl-restrict.cpp

@@ -384,6 +384,10 @@ int moar_globals = 5;
 }
 }
 
+template<const auto &T>
+int uses_global(){}
+
+
 int addInt(int n, int m) {
   return n + m;
 }
@@ -401,6 +405,9 @@ int use2(a_type ab, a_type *abp) {
   if (ab.fm()) // expected-note {{called by 'use2'}}
     return 0;
 
+  // No error, as this is not in an evaluated context.
+  (void)(uses_global<another_global>() + uses_global<ns::glob>());
+
   return another_global; // expected-error {{SYCL kernel cannot use a non-const global variable}}
 
   return ns::glob +               // expected-error {{SYCL kernel cannot use a non-const global variable}}