[Driver][SYCL] Add diagnostic for bad argument with -fsycl-device-obj (#15381)

When using -fsycl-device-obj, we would effectively ignore any bad
arguments and default to 'llvmir'. Add a diagnostic to inform the user
of the bad argument and what we are doing with our default behavior.

Author: mdtoguchi

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -522,6 +522,9 @@ def warn_drv_no_floating_point_registers: Warning<
   InGroup<UnsupportedABI>;
 def warn_ignoring_ftabstop_value : Warning<
   "ignoring invalid -ftabstop value '%0', using default value %1">;
+def warn_ignoring_value_using_default : Warning<
+  "ignoring invalid '%0' value '%1', using default value '%2'">,
+  InGroup<UnusedCommandLineArgument>;
 def warn_drv_overriding_option : Warning<
   "overriding '%0' option with '%1'">,
   InGroup<DiagGroup<"overriding-option">>;

clang/lib/Driver/Driver.cpp

@@ -1159,6 +1159,19 @@ void Driver::CreateOffloadingDeviceToolChains(Compilation &C,
       C.getInputArgs().getLastArg(options::OPT_fsycl_range_rounding_EQ);
   checkSingleArgValidity(RangeRoundingPreference, {"disable", "force", "on"});
 
+  // Evaluation of -fsycl-device-obj is slightly different, we will emit
+  // a warning and inform the user of the default behavior used.
+  // TODO: General usage of this option is to check for 'spirv' and fallthrough
+  // to using llvmir.  This can be improved to be more obvious in usage.
+  if (Arg *DeviceObj = C.getInputArgs().getLastArgNoClaim(
+          options::OPT_fsycl_device_obj_EQ)) {
+    StringRef ArgValue(DeviceObj->getValue());
+    SmallVector<StringRef, 2> DeviceObjValues = {"spirv", "llvmir"};
+    if (llvm::find(DeviceObjValues, ArgValue) == DeviceObjValues.end())
+      Diag(clang::diag::warn_ignoring_value_using_default)
+          << DeviceObj->getSpelling().split('=').first << ArgValue << "llvmir";
+  }
+
   Arg *SYCLForceTarget =
       getArgRequiringSYCLRuntime(options::OPT_fsycl_force_target_EQ);
   if (SYCLForceTarget) {

clang/test/Driver/sycl-offload.c

@@ -69,6 +69,13 @@
 // RUN:   | FileCheck -check-prefix=CHK-SYCL-TARGET %s
 // CHK-SYCL-TARGET-NOT: error: SYCL target is invalid
 
+/// -fsycl-device-obj argument check
+// RUN: %clang -### -fsycl-device-obj=test -fsycl %s 2>&1 \
+// RUN: | FileCheck -check-prefix=DEVICE_OBJ_WARN %s
+// RUN: %clang_cl -### -fsycl-device-obj=test -fsycl %s 2>&1 \
+// RUN: | FileCheck -check-prefix=DEVICE_OBJ_WARN %s
+// DEVICE_OBJ_WARN: warning: ignoring invalid '-fsycl-device-obj' value 'test', using default value 'llvmir' [-Wunused-command-line-argument]
+
 /// ###########################################################################
 
 /// Check warning for duplicate offloading targets.