Re: [PATCH V2] rs6000: Support to build constants by li/lis+oris/xoris

[Segher Boessenkool <segher@kernel.crashing.org>]

On Mon, Nov 28, 2022 at 11:37:34AM +0800, Jiufu Guo wrote:
> Segher Boessenkool <segher@kernel.crashing.org> writes:
> > On Fri, Nov 25, 2022 at 04:11:49PM +0800, Kewen.Lin wrote:
> >> on 2022/10/26 19:40, Jiufu Guo wrote:
> >> for "li/lis + oris/xoris", I interpreted it into four combinations:
> >> 
> >>    li + oris, lis + oris, li + xoris, lis + xoris.
> >> 
> >> not sure just me interpreting like that, but the actual combinations
> >> which this patch adopts are:
> >> 
> >>    li + oris, li + xoris, lis + xoris.
> >> 
> >> It's a bit off, but not a big deal, up to you to reword it or not.  :)
> >
> > The first two are obvious, but the last one is almost never a good idea,
> > there usually are better ways to do the same.  I cannot even think of
> > any case where this is best?  A lis;rl* is always prefered (it can
> > optimise better, be combined with other insns).
> I understant your point here.  The first two: 'li' for lowest 16bits,
> 'oris/xoris' for next 16bits.
> 
> While for 'lis + xoris', it may not obvious, because both 'lis' and
> 'xoris' operates on 17-31bits.
> 'lis + xoris' is for case "32(1) || 1(0) || 15(x) || 16(0)". xoris is
> used to clean bit31.  This case seems hard to be supported by 'rlxx'.

Please put that in a separate patch?  First do a patch with just
lis;x?oris.  They are unrelated and different in almost every way.

> I hit to find this case when I analyze what kind of constants can be
> build by two instructions. Checked the posssible combinations:
> "addi/addis" + "neg/ori/../xoris/rldX/rlwX/../sradi/extswsli"(those
> instructions which accept one register and one immediate).
> 
> I also drafted the patch to use "li/lis+rlxx" to build constant.
> https://gcc.gnu.org/pipermail/gcc-patches/2022-September/601276.html
> https://gcc.gnu.org/pipermail/gcc-patches/2022-September/601277.html

Those seem to do many things in one patch as well :-(  It is very hard
to review such things, it takes many hours each to do properly.


Segher