[Driver][SYCL] Make /MD the default for -fsycl

When using -fsycl on Windows, make /MD the default behavior.  Any usage
of /MT will not be allowed and the driver will error upon usage.

Author: mdtoguchi

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -132,6 +132,7 @@ def err_drv_invalid_Xsycl_frontend_with_args : Error<
   "invalid -Xsycl-target-frontend argument: '%0', options requiring arguments are unsupported">;
 def err_drv_bad_fpga_device_count : Error<
   "More than one FPGA specific device binary found in input objects">;
+def err_drv_unsupported_opt_dpcpp : Error<"option '%0' unsupported with DPC++">;
 def err_drv_argument_only_allowed_with : Error<
   "invalid argument '%0' only allowed with '%1'">;
 def err_drv_argument_not_allowed_with : Error<

clang/lib/Driver/ToolChains/Clang.cpp

@@ -6769,14 +6769,25 @@ void Clang::AddClangCLArgs(const ArgList &Args, types::ID InputType,
                            bool *EmitCodeView) const {
   unsigned RTOptionID = options::OPT__SLASH_MT;
   bool isNVPTX = getToolChain().getTriple().isNVPTX();
+  bool isSYCL = Args.hasArg(options::OPT_fsycl) ||
+      getToolChain().getTriple().getEnvironment() == llvm::Triple::SYCLDevice;
+  // For SYCL Windows, /MD is the default.
+  if (isSYCL)
+    RTOptionID = options::OPT__SLASH_MD;
 
   if (Args.hasArg(options::OPT__SLASH_LDd))
-    // The /LDd option implies /MTd. The dependent lib part can be overridden,
-    // but defining _DEBUG is sticky.
-    RTOptionID = options::OPT__SLASH_MTd;
+    // The /LDd option implies /MTd (/MDd for SYCL). The dependent lib part
+    // can be overridden but defining _DEBUG is sticky.
+    RTOptionID = isSYCL ? options::OPT__SLASH_MDd : options::OPT__SLASH_MTd;
 
-  if (Arg *A = Args.getLastArg(options::OPT__SLASH_M_Group))
+  if (Arg *A = Args.getLastArg(options::OPT__SLASH_M_Group)) {
     RTOptionID = A->getOption().getID();
+    if (isSYCL && (RTOptionID == options::OPT__SLASH_MT ||
+                   RTOptionID == options::OPT__SLASH_MTd))
+      // Use of /MT or /MTd is not supported for SYCL.
+      getToolChain().getDriver().Diag(diag::err_drv_unsupported_opt_dpcpp)
+          << A->getOption().getName();
+  }
 
   StringRef FlagForCRT;
   switch (RTOptionID) {

clang/test/Driver/sycl-MD-default.cpp

@@ -0,0 +1,16 @@
+// REQUIRES: clang-driver
+
+// RUN: %clang_cl -### -fsycl -c %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHK-DEFAULT %s
+// RUN: %clang_cl -### -MD -fsycl -c %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHK-DEFAULT %s
+// RUN: %clang_cl -### -MDd -fsycl -c %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHK-DEFAULT %s
+// CHK-DEFAULT: "-D_MT" "-D_DLL"
+// CHK-DEFAULT: "--dependent-lib=msvcrt{{d*}}"
+
+// RUN: %clang_cl -### -MT -fsycl -c %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHK-ERROR %s
+// RUN: %clang_cl -### -MTd -fsycl -c %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=CHK-ERROR %s
+// CHK-ERROR: option 'MT{{d*}}' unsupported with DPC++