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

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

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).

> > +  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).

> >  			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.

> > +      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?

> > --- /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 :-) )

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.

The li;oris and li;xoris parts look good.


Segher