Re: [PATCH V3 1/4] rs6000: build constant via li;rotldi

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

Hi!

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

> Hi!
>
> On Fri, Jun 16, 2023 at 04:34:12PM +0800, Jiufu Guo wrote:
>> +/* Check if value C can be built by 2 instructions: one is 'li', another is
>> +   rotldi.
>> +
>> +   If so, *SHIFT is set to the shift operand of rotldi(rldicl), and *MASK
>> +   is set to -1, and return true.  Return false otherwise.  */
>
> Don't say "is set to -1", the point of having this is so you say "is set
> to the "li" value".  Just like you describe what SHIFT is for.
Yes, thanks!
>
>> +static bool
>> +can_be_built_by_li_and_rotldi (HOST_WIDE_INT c, int *shift,
>> +				   HOST_WIDE_INT *mask)
>> +{
>> +  int n;
>
> Put shis later, like:
Thanks!
>
>> +  /* Check if C can be rotated to a positive or negative value
>> +      which 'li' instruction is able to load.  */
>   int n;
>> +  if (can_be_rotated_to_lowbits (c, 15, &n)
>> +      || can_be_rotated_to_lowbits (~c, 15, &n))
>> +    {
>> +      *mask = HOST_WIDE_INT_M1;
>> +      *shift = HOST_BITS_PER_WIDE_INT - n;
>> +      return true;
>> +    }
>
> It is tricky to see ~c will always work, since what is really done is -c
> instead.  Can you just use that here?

Some explanation: 
A negative value of 'li' is:
0b11..11xxx there are 49 leading '1's, and the other 15 tailing bits can
be 0 or 1. With the '~' operation, there are 49 '0's.
After the value is rotated,  there are still 49 '1's. (xxx may also be
at head/tail.) 
For the rotated value, with the '~' operation, there are still 49 '0's.

So, for a value, if there are 49 successive '1's (may cross head/tail).
It should be able to rotate to low 15 bits after the '~' operation.

It would not be enough if using the '-' operation, since '-x=~x+1' in
the bit aspect. As the below case 'li_rotldi_3': 0xffff8531ffffffffLL
(rotate left 0x8531 32bit).
The '~c' is 0x7ace00000000, this can be rotated from 0x7ace. (~0x8531).
But '-c' is 0x7ace00000001. this value is not good.

>
>> @@ -10266,15 +10291,14 @@ static void
>>  rs6000_emit_set_long_const (rtx dest, HOST_WIDE_INT c)
>>  {
>>    rtx temp;
>> +  int shift;
>> +  HOST_WIDE_INT mask;
>>    HOST_WIDE_INT ud1, ud2, ud3, ud4;
>>  
>>    ud1 = c & 0xffff;
>> -  c = c >> 16;
>> -  ud2 = c & 0xffff;
>> -  c = c >> 16;
>> -  ud3 = c & 0xffff;
>> -  c = c >> 16;
>> -  ud4 = c & 0xffff;
>> +  ud2 = (c >> 16) & 0xffff;
>> +  ud3 = (c >> 32) & 0xffff;
>> +  ud4 = (c >> 48) & 0xffff;
>>  
>>    if ((ud4 == 0xffff && ud3 == 0xffff && ud2 == 0xffff && (ud1 & 0x8000))
>>        || (ud4 == 0 && ud3 == 0 && ud2 == 0 && ! (ud1 & 0x8000)))
>> @@ -10305,6 +10329,17 @@ rs6000_emit_set_long_const (rtx dest, HOST_WIDE_INT c)
>>        emit_move_insn (dest, gen_rtx_XOR (DImode, temp,
>>  					 GEN_INT ((ud2 ^ 0xffff) << 16)));
>>      }
>> +  else if (can_be_built_by_li_and_rotldi (c, &shift, &mask))
>> +    {
>> +      temp = !can_create_pseudo_p () ? dest : gen_reg_rtx (DImode);
>> +      unsigned HOST_WIDE_INT imm = (c | ~mask);
>> +      imm = (imm >> shift) | (imm << (HOST_BITS_PER_WIDE_INT - shift));
>> +
>> +      emit_move_insn (temp, GEN_INT (imm));
>> +      if (shift != 0)
>> +	temp = gen_rtx_ROTATE (DImode, temp, GEN_INT (shift));
>> +      emit_move_insn (dest, temp);
>> +    }
>
> If you would rewrite so it isn't such a run-on thing with "else if",
> instead using early outs, or even some factoring, you could declare the
> variable used only in a tiny scope in that tiny scope instead.

Yes! Early returning is better for a lot of cases.  I would like
to have a refactor patch.

>
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.target/powerpc/const-build.c
>> @@ -0,0 +1,54 @@
>> +/* { dg-do run } */
>> +/* { dg-options "-O2 -save-temps" } */
>> +/* { dg-require-effective-target has_arch_ppc64 } */
>
> Please put a tiny comment here saying what this test is *for*?  The file
> name is a bit of hint already, but you can indicate much more in one or
> two lines :-)

Oh, yes, thanks for point out this!

>
> With those adjustments, okay for trunk.  Thanks!
>
> (If -c doesn't work, it needs more explanation).

Sure, some words as above.

BR,
Jeff (Jiufu Guo)

>
>
> Segher