[darwin][driver] Pass through -global-isel LLVM flags to ld.

[aemerson]

Summary

GlobalISel is currently not enabled when using -flto since the front-end -mvllm flags don't get passed through. This change fixes this for Darwin platforms. We have to do this in the driver because the code generator choice isn't embedded into the bitcode file.

[paquette]

LGTM

clang/lib/Driver/ToolChains/Darwin.cpp

if (MachOTC.SupportsEmbeddedBitcode()) {
       if (C.getDriver().embedBitcodeMarkerOnly() && Version[0] >= 278) {
         CmdArgs.push_back("-bitcode_process_mode");
         CmdArgs.push_back("marker");
       }
     } else
       D.Diag(diag::err_drv_bitcode_unsupported_on_toolchain);
   }
 
+  // If GlobalISel is enabled, pass it through to LLVM.
+  if (Arg *A = Args.getLastArg(options::OPT_fglobal_isel,
+                               options::OPT_fno_global_isel)) {
+    if (A->getOption().matches(options::OPT_fglobal_isel)) {
+      CmdArgs.push_back("-mllvm");
+      CmdArgs.push_back("-global-isel");
+      // Disable abort and fall back to SDAG silently.
+      CmdArgs.push_back("-mllvm");
+      CmdArgs.push_back("-global-isel-abort=0");
+    }
+  }
+
   Args.AddLastArg(CmdArgs, options::OPT_prebind);
   Args.AddLastArg(CmdArgs, options::OPT_noprebind);
   Args.AddLastArg(CmdArgs, options::OPT_nofixprebinding);
   Args.AddLastArg(CmdArgs, options::OPT_prebind__all__twolevel__modules);
   Args.AddLastArg(CmdArgs, options::OPT_read__only__relocs);
   Args.AddAllArgs(CmdArgs, options::OPT_sectcreate);
   Args.AddAllArgs(CmdArgs, options::OPT_sectorder);
   Args.AddAllArgs(CmdArgs, options::OPT_seg1addr);

clang/test/Driver/darwin-ld-lto.c

 // FULL_LTO_OBJECT_PATH: {{ld(.exe)?"}}
 // FULL_LTO_OBJECT_PATH-SAME: "-object_path_lto"
 // FULL_LTO_OBJECT_PATH-SAME: {{cc\-[a-zA-Z0-9_]+.o}}"
 // RUN: %clang -target x86_64-apple-darwin10 %s -flto=thin -### 2>&1 | \
 // RUN:   FileCheck -check-prefix=THIN_LTO_OBJECT_PATH %s
 // THIN_LTO_OBJECT_PATH: {{ld(.exe)?"}}
 // THIN_LTO_OBJECT_PATH-SAME: "-object_path_lto"
 // THIN_LTO_OBJECT_PATH-SAME: {{thinlto\-[a-zA-Z0-9_]+}}
+
+
+// Check that we pass through -fglobal-isel flags to libLTO.
+// RUN: %clang -target arm64-apple-darwin %s -flto -fglobal-isel -### 2>&1 | \
+// RUN:   FileCheck --check-prefix=GISEL %s
+// GISEL: {{ld(.exe)?"}}
+// GISEL: "-mllvm" "-global-isel"
+// GISEL: "-mllvm" "-global-isel-abort=0"