[clang] Deprecate order file instrumentation (PR #121514)

[Ellis Hoag via cfe-commits]

================
@@ -8010,15 +8010,19 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     }
   }
 
-  if (Args.hasArg(options::OPT_forder_file_instrumentation)) {
-     CmdArgs.push_back("-forder-file-instrumentation");
-     // Enable order file instrumentation when ThinLTO is not on. When ThinLTO is
-     // on, we need to pass these flags as linker flags and that will be handled
-     // outside of the compiler.
-     if (!IsUsingLTO) {
-       CmdArgs.push_back("-mllvm");
-       CmdArgs.push_back("-enable-order-file-instrumentation");
-     }
+  if (const Arg *A =
+          Args.getLastArg(options::OPT_forder_file_instrumentation)) {
+    D.Diag(diag::warn_drv_deprecated_arg)
+        << A->getAsString(Args) << /*hasReplacement=*/true
+        << "-mllvm -pgo-temporal-instrumentation";
----------------
ellishg wrote:

I believe this shows the importance of deprecating this flag as using IRPGO has better support and will give better performance gains. I wasn't sure how many people are using this flag, so I'm glad you reached out.

I can add a section to describe how to use this flag in the user manual (https://clang.llvm.org/docs/UsersManual.html#profiling-with-instrumentation). Would that be sufficient? 

https://github.com/llvm/llvm-project/pull/121514