[PATCH 1/5] x86: Add -mindirect-branch=

[Martin Liška]

On 01/07/2018 11:59 PM, H.J. Lu wrote:
> +static void
> +output_indirect_thunk_function (bool need_bnd_p, int regno)
> +{
> +  char name[32];
> +  tree decl;
> +
> +  /* Create __x86_indirect_thunk/__x86_indirect_thunk_bnd.  */
> +  indirect_thunk_name (name, regno, need_bnd_p);
> +  decl = build_decl (BUILTINS_LOCATION, FUNCTION_DECL,
> +		     get_identifier (name),
> +		     build_function_type_list (void_type_node, NULL_TREE));
> +  DECL_RESULT (decl) = build_decl (BUILTINS_LOCATION, RESULT_DECL,
> +				   NULL_TREE, void_type_node);
> +  TREE_PUBLIC (decl) = 1;
> +  TREE_STATIC (decl) = 1;
> +  DECL_IGNORED_P (decl) = 1;
> +
> +#if TARGET_MACHO
> +  if (TARGET_MACHO)
> +    {
> +      switch_to_section (darwin_sections[picbase_thunk_section]);
> +      fputs ("\t.weak_definition\t", asm_out_file);
> +      assemble_name (asm_out_file, name);
> +      fputs ("\n\t.private_extern\t", asm_out_file);
> +      assemble_name (asm_out_file, name);
> +      putc ('\n', asm_out_file);
> +      ASM_OUTPUT_LABEL (asm_out_file, name);
> +      DECL_WEAK (decl) = 1;
> +    }
> +  else
> +#endif
> +    if (USE_HIDDEN_LINKONCE)
> +      {
> +	cgraph_node::create (decl)->set_comdat_group (DECL_ASSEMBLER_NAME (decl));
> +
> +	targetm.asm_out.unique_section (decl, 0);
> +	switch_to_section (get_named_section (decl, NULL, 0));
> +
> +	targetm.asm_out.globalize_label (asm_out_file, name);
> +	fputs ("\t.hidden\t", asm_out_file);
> +	assemble_name (asm_out_file, name);
> +	putc ('\n', asm_out_file);
> +	ASM_DECLARE_FUNCTION_NAME (asm_out_file, name, decl);
> +      }
> +    else
> +      {
> +	switch_to_section (text_section);
> +	ASM_OUTPUT_LABEL (asm_out_file, name);
> +      }
> +
> +  DECL_INITIAL (decl) = make_node (BLOCK);
> +  current_function_decl = decl;
> +  allocate_struct_function (decl, false);
> +  init_function_start (decl);
> +  /* We're about to hide the function body from callees of final_* by
> +     emitting it directly; tell them we're a thunk, if they care.  */
> +  cfun->is_thunk = true;
> +  first_function_block_is_cold = false;
> +  /* Make sure unwind info is emitted for the thunk if needed.  */
> +  final_start_function (emit_barrier (), asm_out_file, 1);
> +
> +  output_indirect_thunk (need_bnd_p, regno);
> +
> +  final_end_function ();
> +  init_insn_lengths ();
> +  free_after_compilation (cfun);
> +  set_cfun (NULL);
> +  current_function_decl = NULL;
> +}
> +

I'm wondering whether thunk creation can be a good target-independent generalization? I guess
we can emit the function declaration without direct writes to asm_out_file? And the emission
of function body can be potentially a target hook?

What about emitting body of the function with RTL instructions instead of direct assembly write?
My knowledge of RTL is quite small, but maybe it can bring some generalization and reusability
for other targets?

Thank you,
Martin