Re: [PATCH v2] rs6000: Add buildin for mffscrn instructions

[Peter Bergner <bergner@linux.ibm.com>]

On 5/23/23 12:24 AM, Kewen.Lin wrote:
> on 2023/5/23 01:31, Carl Love wrote:
>> The builtins were requested for use in GLibC.  As of version 2.31 they
>> were added as inline asm.  They requested a builtin so the asm could be
>> removed.
>
> So IMHO we also want the similar support for mffscrn, that is to make
> use of mffscrn and mffscrni on Power9 and later, but falls back to 
> __builtin_set_fpscr_rn + mffs similar on older platforms.

So __builtin_set_fpscr_rn everything we want (sets the RN bits) and
uses mffscrn/mffscrni on P9 and later and uses older insns on pre-P9.
The only problem is we don't return the current FPSCR bits, as the bif
is defined to return void.  Crazy idea, but could we extend the built-in
with an overload that returns the FPSCR bits?  To be honest, I like
the __builtin_set_fpscr_rn name better than __builtin_mffscrn[i].
The built-in machinery can see that the usage is expecting a return value
or not and for the pre-P9 code, can skip generating the ending mffs if
we don't want the return value.

Peter