Re: [rs6000] Add documentation for __builtin_mtfsf

[Segher Boessenkool <segher at kernel dot crashing dot org>]

On Sun, Jul 21, 2019 at 02:50:41PM -0500, Paul Clarke wrote:
> On 7/21/19 1:13 PM, Segher Boessenkool wrote:
> > On Sun, Jul 21, 2019 at 04:06:32AM -0500, Paul Clarke wrote:
> >> +@code{__builtin_mtfsf} takes a constant 8-bit integer field mask and a
> >> +representation of the new value of the FPSCR and generates the @code{mtfsf}
> >> +instruction to copy the supplied value into the FPSCR, subject to the field
> >> +mask, each bit of which represents a nibble of the FPSCR.  The

("nybble" fwiw.  Well the spelling list wisely avoids this one, so maybe
so should we ;-) )

> > "A representation of the new value"?  I guess you want to say that it
> > sits in an FPR?
> 
> It's the 2nd parameter to the builtin, so a "double".

Yeah, it's an integer that sits in the low bits of a double.  It's
probably best to not even try to describe it, just refer to the machine
instruction?

> It may or may not be in an FPR, but the user of the builtin doesn't
> really know or care.  (It'll eventually be in an FPR, of course, but
> the user has it in a variable.)  It's a "representation" because it's
> not actually the new value, because it gets written under mask.

> > Before we document __builtin_mtfsf, maybe we should make it work with
> > the W and/or L fields first, or at least, decide how we want that?
> 
> It's been available but undocumented for ages.  Do you want me to thus
> document how it is currently implemented by including something like
> "...generates the mtfsf (extended mnemonic) instruction ..." ?

That's a good idea yes.  L=W=0.

> If you think the basic mnemonic form needs to be supported, that's a
> whole other piece of work ;-)

Sure :-)  My thinking was, if it wasn't documented and we want to
define new variants, we may have a bit more leeway.  But since this
is such and old builtin, that's not going to happen anyway.

So can you reword a little bit and resend please?  It really doesn't
need to say more than "this does what the mtfsf insn does", and saying
more is Hard :-)

Thanks,


Segher