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

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi Segher!

Thanks a lot for your comments!

Segher Boessenkool <segher@kernel.crashing.org> writes:

> Hi guys,
>
> 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'.

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

>
>> > +  HOST_WIDE_INT orig_c = c;
>
> If you ever feel you need a variable to hold an "orig" value, that is a
> good hint that you should restructure the code a bit, perhaps even
> factor it.  That often is overdue (like here), not caused by you, but
> you could help solve it ;-)
>
> (This is what made this patch hard to review, btw).
You are right.  Thanks for point out this!
>
>> >  			gen_rtx_IOR (DImode, copy_rtx (temp),
>> >  				     GEN_INT (ud1)));
>> >      }
>> > +  else if ((ud4 == 0xffff && ud3 == 0xffff)
>> > +	   && ((ud1 & 0x8000) || (ud1 == 0 && !(ud2 & 0x8000))))
>> > +    {
>> > +      temp = !can_create_pseudo_p () ? dest : gen_reg_rtx (DImode);
>> > +
>> > +      HOST_WIDE_INT imm = (ud1 & 0x8000) ? ((ud1 ^ 0x8000) - 0x8000)
>> > +					 : ((ud2 << 16) - 0x80000000);
>
> We really should have some "hwi::sign_extend (ud1, 16)" helper function,
> heh.  Maybe there already is?  Ah, "sext_hwi".  Fixing that up
> everywhere in this function is preapproved.
Great, thanks! 
>
>> > +      else
>> > +	{
>> > +	  emit_move_insn (temp,
>> > +			  GEN_INT (((ud2 << 16) ^ 0x80000000) - 0x80000000));
>> > +	  if (ud1 != 0)
>> > +	    emit_move_insn (temp, gen_rtx_IOR (DImode, temp, GEN_INT (ud1)));
>> > +	  emit_move_insn (dest,
>> > +			  gen_rtx_ZERO_EXTEND (DImode,
>> > +					       gen_lowpart (SImode, temp)));
>> > +	}
>
> Why this?  Please just write it in DImode, do not go via SImode?
Thanks for catch this. Yes, gen_lowpart with DImode would be ok.
>
>> > --- /dev/null
>> > +++ b/gcc/testsuite/gcc.target/powerpc/pr106708.h
>> > @@ -0,0 +1,9 @@
>> > +/* Test constants which can be built by li/lis + oris/xoris */
>> > +void  __attribute__ ((__noinline__, __noclone__)) foo (long long *arg)
>> > +{
>> > +  *arg++ = 0x98765432ULL;
>> > +  *arg++ = 0xffffffff7cdeab55ULL;
>> > +  *arg++ = 0xffffffff65430000ULL;
>> > +}
>
> Use noipa please (it is shorter, simpler, and covers more :-) )
Thanks!
>
> Could you comment what exact instructions are expected?
> li;xoris and li;xoris and lis;xoris I guess?  It helps if you just tell
> the reader here.
Sure, thanks!
>
> The li;oris and li;xoris parts look good.
>

BR,
Jeff (Jiufu)
>
> Segher