[clang] [clang][Driver] Fix triple config loading for clang-cl (PR #111397)

[Bo Anderson via cfe-commits]

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@@ -1286,6 +1299,16 @@ Compilation *Driver::BuildCompilation(ArrayRef<const char *> ArgList) {
           appendOneArg(Args, Opt, nullptr);
         }
     }
+
+    // The config file may have changed the architecture so apply it.
+    if (HasConfigFile && Args.hasArg(options::OPT__SLASH_arm64EC)) {
+      llvm::Triple T(TargetTriple);
+      if (T.getArch() != llvm::Triple::aarch64 ||
+          T.getSubArch() != llvm::Triple::AArch64SubArch_arm64ec) {
+        T.setArch(llvm::Triple::aarch64, llvm::Triple::AArch64SubArch_arm64ec);
+        TargetTriple = T.str();
----------------
Bo98 wrote:

There's two triple systems in the driver: `TargetTriple` and the triple fed to `ToolChain` (computed by `computeTargetTriple`). The former is feeds into the latter but is otherwise separate.

`-m32` and `-m64` is already handled in `computeTargetTriple` and so never affect `TargetTriple`. The code here was to keep the existing behaviour of `/arm64EC` affecting `TargetTriple`.

To be honest: I have no idea why there is two triples. It would be simpler if `TargetTriple` became `DefaultTriple` (and remains immutable), `Driver.getTargetTriple()` was removed and we just moved all this to `computeTargetTriple` but I was hesitant to change something I don't understand the history of.

https://github.com/llvm/llvm-project/pull/111397