Re: [PATCH] Expand PIC calls without PLT with -fno-plt

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

On Mon, May 11, 2015 at 1:36 PM, Jeff Law <law@redhat.com> wrote:
> On 05/10/2015 10:59 AM, Jan Hubicka wrote:
>>>
>>> This patch introduces option -fno-plt that allows to expand calls that
>>> would
>>> go via PLT to load the address of the function immediately at call site
>>> (which
>>> introduces a GOT load).  Cover letter explains the motivation for this
>>> patch.
>>>
>>> New option documentation for invoke.texi is missing from the patch; if
>>> this is
>>> accepted I'll be happy to send a v2 with documentation added.
>>>
>>>         * calls.c (prepare_call_address): Transform PLT call to GOT
>>> lookup and
>>>         indirect call by forcing address into a pseudo with -fno-plt.
>>>         * common.opt (flag_plt): New option.
>>>
>>> diff --git a/gcc/common.opt b/gcc/common.opt
>>> index b49ac46..cd8b256 100644
>>> --- a/gcc/common.opt
>>> +++ b/gcc/common.opt
>>> @@ -1773,12 +1773,16 @@ Common Report Var(flag_pic,1) Negative(fpie)
>>>   Generate position-independent code if possible (small mode)
>>>
>>>   fpie
>>>   Common Report Var(flag_pie,1) Negative(fPIC)
>>>   Generate position-independent code for executables if possible (small
>>> mode)
>>>
>>> +fplt
>>> +Common Report Var(flag_plt) Init(1)
>>> +Use PLT for PIC calls (-fno-plt: load the address from GOT at call site)
>>> +
>>
>>
>> This won't play well with LTO since fplt will become another global flag
>> while
>> it affects codegen.
>
> I know Richi explained this to me in the past, but I can't remember the
> details of why this is bad.  Can you walk me through it again?
>

I have proposed a different approach:

https://gcc.gnu.org/ml/gcc/2015-05/msg00086.html


-- 
H.J.