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

[Jeff Law]

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?

jeff