[MLIR][SPIRV] Added storage class constraint on global variable

[georgemitenkov]

Summary

Added a check for 'Function' storage class in spv.globalVariable
verifier since it only can be used with spv.Variable.

mlir/lib/Dialect/SPIRV/SPIRVOps.cpp

static void print(spirv::GlobalVariableOp varOp, OpAsmPrinter &printer) {
   printVariableDecorations(op, printer, elidedAttrs);
   printer << " : " << varOp.type();
 }
 
 static LogicalResult verify(spirv::GlobalVariableOp varOp) {
   // SPIR-V spec: "Storage Class is the Storage Class of the memory holding the
   // object. It cannot be Generic. It must be the same as the Storage Class
   // operand of the Result Type."
-  if (varOp.storageClass() == spirv::StorageClass::Generic)
-    return varOp.emitOpError("storage class cannot be 'Generic'");
+  // Also, Function storage class is reserved by spv.Variable.
+  auto storageClass = varOp.storageClass();
+  if (storageClass == spirv::StorageClass::Generic ||
+      storageClass == spirv::StorageClass::Function) {
+    return varOp.emitOpError("storage class cannot be '")
+           << stringifyStorageClass(storageClass) << "'";
+  }
 
   if (auto init =
           varOp.getAttrOfType<FlatSymbolRefAttr>(kInitializerAttrName)) {
     Operation *initOp = SymbolTable::lookupNearestSymbolFrom(
         varOp.getParentOp(), init.getValue());
     // TODO: Currently only variable initialization with specialization
     // constants and other variables is supported. They could be normal
     // constants in the module scope as well.

mlir/test/Dialect/SPIRV/structure-ops.mlir

 spv.module Logical GLSL450 {
   // expected-error @+1 {{storage class cannot be 'Generic'}}
   spv.globalVariable @var0 : !spv.ptr<f32, Generic>
 }
 
 // -----
 
 spv.module Logical GLSL450 {
+  // expected-error @+1 {{storage class cannot be 'Function'}}
+  spv.globalVariable @var0 : !spv.ptr<f32, Function>
+}
+
+// -----
+
+spv.module Logical GLSL450 {
   spv.func @foo() "None" {
     // expected-error @+1 {{op must appear in a module-like op's block}}
     spv.globalVariable @var0 : !spv.ptr<f32, Input>
     spv.Return
   }
 }
 
 // -----