[Clang][LTO][GISel] Propagate -fglobal-siel to LTO (#69747)

Translate `-fglobal-isel` to `-plugin-opt=-global-isel=1`.

Author: mshockwave

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -694,6 +694,16 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
     CmdArgs.push_back(Args.MakeArgString(Twine(PluginOptPrefix) +
                                          ParallelismOpt + Parallelism));
 
+  // Pass down GlobalISel options.
+  if (Arg *A = Args.getLastArg(options::OPT_fglobal_isel,
+                               options::OPT_fno_global_isel)) {
+    // Parsing -fno-global-isel explicitly gives architectures that enable GISel
+    // by default a chance to disable it.
+    CmdArgs.push_back(Args.MakeArgString(
+        Twine(PluginOptPrefix) + "-global-isel=" +
+        (A->getOption().matches(options::OPT_fglobal_isel) ? "1" : "0")));
+  }
+
   // If an explicit debugger tuning argument appeared, pass it along.
   if (Arg *A =
           Args.getLastArg(options::OPT_gTune_Group, options::OPT_ggdbN_Group)) {

clang/test/Driver/lto.c

@@ -105,3 +105,12 @@
 // FLTO-THIN: -flto=thin
 // FLTO-THIN-NOT: "-flto"
 // FLTO-THIN-NOT: -flto=full
+
+// -flto passes along an explicit GlobalISel flag.
+// RUN: %clang --target=riscv64-linux-gnu -### %s -flto -fglobal-isel 2> %t
+// RUN: FileCheck --check-prefix=CHECK-GISEL < %t %s
+// RUN: %clang --target=aarch64-linux-gnu -### %s -flto -fno-global-isel 2> %t
+// RUN: FileCheck --check-prefix=CHECK-DISABLE-GISEL < %t %s
+//
+// CHECK-GISEL:         "-plugin-opt=-global-isel=1"
+// CHECK-DISABLE-GISEL: "-plugin-opt=-global-isel=0"