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

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Jan 8, 2018 at 2:10 AM, Martin Liška <mliska@suse.cz> wrote:
> 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?

Thunks are x86 specific and they are created the same way as 32-bit PIC thunks.
I don't see how a target hook is used.

-- 
H.J.