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

[Chris B via cfe-commits]

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@@ -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";
----------------
llvm-beanz wrote:

`-forder-file-instrumentation` was not only used by Meta. At a minimum I’ve been using it, and my former team at Apple was too. The instrumentation-based approaches to order file generation are significantly more robust than the dtrace-based solution (https://github.com/llvm/llvm-project/blob/main/clang/utils/perf-training/perf-helper.py#L128), that I upstreamed from Apple’s internal tooling in 2016 (https://github.com/llvm/llvm-project/commit/d8b5bde5d6300f15b6f798eed73b712378261314).

I don’t know if Apple ever updated their Clang builds to use the instrumentation-based approach, but it is vastly superior. Notably dtrace is not deterministic, so using it to drive part of a build process is not ideal.

I worry about having a deprecation policy allowing deprecating a publicly documented option for a non-public option. If the new feature isn’t stable, and the old feature still works, why are we deprecating it?

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