[clang] [flang] [flang][windows] Add option to link against specific MSVC CRT (PR #70833)

[Brad King via cfe-commits]

================
@@ -976,12 +976,46 @@ bool tools::addOpenMPRuntime(ArgStringList &CmdArgs, const ToolChain &TC,
   return true;
 }
 
-void tools::addFortranRuntimeLibs(const ToolChain &TC,
+void tools::addFortranRuntimeLibs(const ToolChain &TC, const ArgList &Args,
                                   llvm::opt::ArgStringList &CmdArgs) {
   if (TC.getTriple().isKnownWindowsMSVCEnvironment()) {
-    CmdArgs.push_back("Fortran_main.lib");
-    CmdArgs.push_back("FortranRuntime.lib");
-    CmdArgs.push_back("FortranDecimal.lib");
+    CmdArgs.push_back(Args.MakeArgString(
+        "/DEFAULTLIB:" + TC.getCompilerRTBasename(Args, "builtins")));
+    unsigned RTOptionID = options::OPT__SLASH_MT;
+    if (auto *rtl = Args.getLastArg(options::OPT_fms_runtime_lib_EQ)) {
+      RTOptionID = llvm::StringSwitch<unsigned>(rtl->getValue())
+                       .Case("static", options::OPT__SLASH_MT)
+                       .Case("static_dbg", options::OPT__SLASH_MTd)
+                       .Case("dll", options::OPT__SLASH_MD)
+                       .Case("dll_dbg", options::OPT__SLASH_MDd)
+                       .Default(options::OPT__SLASH_MT);
+    }
+    switch (RTOptionID) {
+    case options::OPT__SLASH_MT:
+      CmdArgs.push_back("/DEFAULTLIB:libcmt");
+      CmdArgs.push_back("Fortran_main.static.lib");
+      CmdArgs.push_back("FortranRuntime.static.lib");
+      CmdArgs.push_back("FortranDecimal.static.lib");
+      break;
+    case options::OPT__SLASH_MTd:
+      CmdArgs.push_back("/DEFAULTLIB:libcmtd");
+      CmdArgs.push_back("Fortran_main.static_debug.lib");
+      CmdArgs.push_back("FortranRuntime.static_debug.lib");
+      CmdArgs.push_back("FortranDecimal.static_debug.lib");
+      break;
+    case options::OPT__SLASH_MD:
+      CmdArgs.push_back("/DEFAULTLIB:msvcrt");
+      CmdArgs.push_back("Fortran_main.dynamic.lib");
+      CmdArgs.push_back("FortranRuntime.dynamic.lib");
+      CmdArgs.push_back("FortranDecimal.dynamic.lib");
+      break;
----------------
bradking wrote:

I'm not aware of any easy way to add defaultlib directives after-the-fact.

I think it's okay to merge this PR's approach as a temporary solution.  It does fix the empty-`program` example in #68017 work, and provide the Fortran runtime library variants.  However, I'm not sure how well CMake will be able to support this until the defaultlib part is added.

BTW, the `Fixes #68017` note in the PR description is no longer accurate.  It's not fully fixed yet.


https://github.com/llvm/llvm-project/pull/70833