Re: Enable EBX for x86 in 32bits PIC code

[Uros Bizjak <ubizjak at gmail dot com>]

On Thu, Aug 28, 2014 at 3:29 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:

>>>>>>> diff --git a/gcc/calls.c b/gcc/calls.c
>>>>>>> index 4285ec1..85dae6b 100644
>>>>>>> --- a/gcc/calls.c
>>>>>>> +++ b/gcc/calls.c
>>>>>>> @@ -1122,6 +1122,14 @@ initialize_argument_information (int num_actuals
>>>>>>> ATTRIBUTE_UNUSED,
>>>>>>>       call_expr_arg_iterator iter;
>>>>>>>       tree arg;
>>>>>>>
>>>>>>> +    if (targetm.calls.implicit_pic_arg (fndecl ? fndecl : fntype))
>>>>>>> +      {
>>>>>>> +       gcc_assert (pic_offset_table_rtx);
>>>>>>> +       args[j].tree_value = make_tree (ptr_type_node,
>>>>>>> +                                       pic_offset_table_rtx);
>>>>>>> +       j--;
>>>>>>> +      }
>>>>>>> +
>>>>>>>       if (struct_value_addr_value)
>>>>>>>         {
>>>>>>>         args[j].tree_value = struct_value_addr_value;
>>>>>>
>>>>>> So why do you need this?  Can't this be handled in the call/call_value
>>>>>> expanders or what about attaching the use to CALL_INSN_FUNCTION_USAGE from
>>>>>> inside ix86_expand_call?  Basically I'm not seeing the need for another
>>>>>> target hook here.  I think that would significantly simply the patch as
>>>>>> well.
>>>>>
>>>>> GOT base address become an additional implicit arg with EBX relaxed
>>>>> and I handled it as all other args. I can move EBX initialization into
>>>>> ix86_expand_call. Would still need some hint from target to init
>>>>> pic_offset_table_rtx with proper value in the beginning of function
>>>>> expand.
>>>>
>>>> Maybe you can you use get_hard_reg_initial_val for this?
>>>
>>> Actually there is no input hard reg holding GOT address.  Currently I
>>> use initialization with ebx with following ebx initialization in
>>> prolog_epilog pass.  But this is a temporary workaround.  It is
>>> inefficient because always uses callee save reg to get GOT address.  I
>>> suppose we should generate pseudo reg for pic_offset_table_rtx and
>>> also set_got with this register as a destination in expand pass.
>>> After register allocation set_got may be transformed into get_pc_thunk
>>> call with proper hard reg.  But some target hook has to be used for
>>> this.
>>
>> Let me expand my idea a bit. IIRC, get_hard_reg_initial_val and
>> friends will automatically emit intialization of a pseudo from
>> pic_offset_table_rtx hard reg. After reload, real initialization of
>> pic_offset_table_rtx hard reg is emitted in pro_and_epilogue pass. I
>> don't know if this works with current implementation of dynamic
>> pic_offset_table_rtx selection, though.
>
> That means you should choose some hard reg early before register
> allocation to be used for PIC reg initialization.  I do not like we
> have to do this and want to just generate set_got with pseudo reg and
> do not involve any additional hard reg. That would look like
>
> (insn/f 168 167 169 2 (parallel [
>             (set (reg:SI 127)
>                 (unspec:SI [
>                         (const_int 0 [0])
>                     ] UNSPEC_SET_GOT))
>             (clobber (reg:CC 17 flags))
>         ]) test.cc:42 -1
>      (expr_list:REG_CFA_FLUSH_QUEUE (nil)
>         (nil)))
>
> after expand pass.  r127 is pic_offset_table_rtx here. And after
> reload it would become:
>
> (insn/f 168 167 169 2 (parallel [
>             (set (reg:SI 3 bx)
>                 (unspec:SI [
>                         (const_int 0 [0])
>                     ] UNSPEC_SET_GOT))
>             (clobber (reg:CC 17 flags))
>         ]) test.cc:42 -1
>      (expr_list:REG_CFA_FLUSH_QUEUE (nil)
>         (nil)))
>
> And no additional actions are required on pro_and_epilogue.  Also it
> simplifies analysis whether we should generate set_got at all.
> Current we check hard reg is ever live which is wrong with not fixed
> ebx because any usage of hard reg used to init GOT doesn't mean GOT
> usage.  And with my proposed scheme unused GOT would mean DCE just
> removes useless set_got.

Yes this is better. I was under impression you want to retain current
initialization insertion in expand_prologue.

Uros.