Re: [PATCH ] PR 844422 Fix FCTID, FCTIW with -mcpu=power7

[Bill Schmidt <wschmidt at linux dot vnet dot ibm dot com>]

On Mar 16, 2018, at 5:51 PM, Segher Boessenkool <segher@kernel.crashing.org> wrote:
> 
> Hi Carl,
> 
> On Wed, Mar 14, 2018 at 08:27:08AM -0700, Carl Love wrote:
>> The following patch fixes an ICE when compiling the test case
>> 
>>  gcc -mcpu=power7 builtin-fctid-fctiw-runnable.c
>> 
>> The GCC compiler now gives a message 
>> 
>>    "error: builtin function ‘__builtin_fctiw’ requires the ‘-mpower8-vector’ option" 
>> 
>> and exits without generating an internal error.
> 
> This is an improvement over the ICE, for sure.
> 
> But fctiw is an ISA 1.xx instruction already (and fctid is as well,
> but explicitly undefined for 32-bit implementations until some 2.xx,
> I forgot which, 2.02 I think?)
> 
> Requiring power8 for it is weird and surprising.  power8-vector doubly so.
> 
> It does not seem to be a good idea to only enable the builtin for cases
> where the current implementation is not broken.
> 
> (The issue is that pre-power8 we do not allow SImode in FPR registers.
> Which makes the current fctiw implementation fail).
> 
> I think we have two good options:
> 
> 1) Remove these builtins;
> or
> 2) Make them work.
> 
I don't think we can remove them, as for a while they were the only way to
access these instructions.  We now have some vec_* intrinsics that are
the preferred interfaces, but for backward compatibility I think we have to
keep the old ones.

Thanks,
Bill

> 
> Segher
>