Re: [RFC][PATCH][X86_64] Eliminate PLT stubs for specified external functions via -fno-plt=

[Sriraman Tallam <tmsriram at google dot com>]

On Wed, Jun 3, 2015 at 1:09 PM, Richard Henderson <rth@redhat.com> wrote:
> On 06/03/2015 11:38 AM, Sriraman Tallam wrote:
>> +  { "no_plt",                   0, 0, true,  false, false,
>> +                           handle_no_plt_attribute, false },
>
> Call it noplt.  We don't add the underscore for noinline, noclone, etc.

Done.

>
>
>
>> Index: config/i386/i386.c
>> ===================================================================
>> --- config/i386/i386.c        (revision 223720)
>> +++ config/i386/i386.c        (working copy)
>> @@ -5479,7 +5479,10 @@ ix86_function_ok_for_sibcall (tree decl, tree exp)
>>        && !TARGET_64BIT
>>        && flag_pic
>>        && flag_plt
>> -      && decl && !targetm.binds_local_p (decl))
>> +      && decl
>> +      && (TREE_CODE (decl) != FUNCTION_DECL
>> +       || !lookup_attribute ("no_plt", DECL_ATTRIBUTES (decl)))
>> +      && !targetm.binds_local_p (decl))
>>      return false;
>>
>>    /* If we need to align the outgoing stack, then sibcalling would
>
> Is this really necessary?  I'd expect DECL to be NULL in this case,
> since the non-use of the PLT will mean that the (sib)call is indirect.

Removed.

>
>
>> @@ -25497,13 +25500,19 @@ ix86_expand_call (rtx retval, rtx fnaddr, rtx call
>>      }
>>    else
>>      {
>> -      /* Static functions and indirect calls don't need the pic register.  */
>> +      /* Static functions and indirect calls don't need the pic register.  Also,
>> +      check if PLT was explicitly avoided via no-plt or "no_plt" attribute, making
>> +      it an indirect call.  */
>>        if (flag_pic
>>         && (!TARGET_64BIT
>>             || (ix86_cmodel == CM_LARGE_PIC
>>                 && DEFAULT_ABI != MS_ABI))
>>         && GET_CODE (XEXP (fnaddr, 0)) == SYMBOL_REF
>> -       && ! SYMBOL_REF_LOCAL_P (XEXP (fnaddr, 0)))
>> +       && !SYMBOL_REF_LOCAL_P (XEXP (fnaddr, 0))
>> +       && flag_plt
>> +       && (TREE_CODE (SYMBOL_REF_DECL (XEXP(fnaddr, 0))) != FUNCTION_DECL
>> +           || !lookup_attribute ("no_plt",
>> +                  DECL_ATTRIBUTES (SYMBOL_REF_DECL (XEXP(fnaddr, 0))))))
>>       {
>>         use_reg (&use, gen_rtx_REG (Pmode, REAL_PIC_OFFSET_TABLE_REGNUM));
>>         if (ix86_use_pseudo_pic_reg ())
>
> Why are you testing FUNCTION_DECL?  Even if, somehow, the user were producing a
> function call to a data symbol, why do you think that lookup_attribute would
> produce incorrect results?
>
> Similarly in ix86_nopic_no_plt_attribute_p.

Fixed.

Patch attached with those changes.

Thanks
Sri

Attachment: noplt_attrib_patch_new.txt
Description: Text document