Re: [PATCH V4] rs6000: Optimize cmp on rotated 16bits constant

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

Hi!

On Mon, Jul 25, 2022 at 09:29:22PM +0800, Jiufu Guo wrote:
> When checking eq/neq with a constant which has only 16bits, it can be
> optimized to check the rotated data.  By this, the constant building
> is optimized.

"ne", not "neq".

> gcc/ChangeLog:
> 
> 	* config/rs6000/rs6000-protos.h (rotate_from_leading_zeros_const):
> 	New decl.

"New declaration." or just "New".  Also, don't break lines early please,
especially if that means ending a line in a colon, which then looks as
if you forgot to write something there.

> 	* config/rs6000/rs6000.cc (rotate_from_leading_zeros_const): New
> 	define for checking simply rotated constant.

"New." or "New definition." or such.

> +/* Check if C can be rotated from an immediate which contains leading
> +   zeros at least CLZ.

"Which starts (as 64 bit integer) with at least CLZ bits zero" or such.

> +  /* xx0..0xx: rotate enough bits firstly, then check case a.  */
> +  const int rot_bits = HOST_BITS_PER_WIDE_INT - clz + 1;
> +  unsigned HOST_WIDE_INT rc = (c >> rot_bits) | (c << (clz - 1));
> +  tz = ctz_hwi (rc);
> +  if (clz_hwi (rc) + tz >= clz)
> +    return tz + rot_bits;

This could use some more explanation.

> +(define_code_iterator eqne [eq ne])


> +;; "i == C" ==> "rotl(i,N) == rotl(C,N)"
> +(define_insn_and_split "*rotate_on_cmpdi"
> +  [(set (pc)
> +     (if_then_else (eqne (match_operand:DI 1 "gpc_reg_operand" "r")

Wrong indentation.  The ( should be in the same column as the preceding (
it matches.

Setting "pc" to either 0 or 1 is never correct.

> +  "TARGET_POWERPC64 && !reload_completed && can_create_pseudo_p ()

reload_completed in splitters is almost always wrong.  It isn't any
better if it is in the insn condition of a define_insn_and_split :-)

> +   && num_insns_constant (operands[2], DImode) > 1
> +   && (rotate_from_leading_zeros_const (~UINTVAL (operands[2]), 49) > 0
> +       || rotate_from_leading_zeros_const (UINTVAL (operands[2]), 48) > 0)"

There must be a better way to describe this.

> +  if (rot < 0)
> +  {
> +     sgn = true;
> +     rot = rotate_from_leading_zeros_const (~C, 49);
> +  }

Bad indentation.

> +  rtx cmp = ne ? gen_rtx_NE (CCmode, cc, const0_rtx)
> +	       : gen_rtx_EQ (CCmode, cc, const0_rtx);

  rtx cmp = gen_rtx_<EQNE> (...);

(and define a code_attr EQNE to just output the uppercase EQ or NE).

Why is this doing a conditional branch at all?  Unpredictable
conditional branches are extremely costly.

> +/* { dg-require-effective-target lp64 } */

arch_ppc64

> +/* { dg-final { scan-assembler-times "cmpldi" 10  } } */
> +/* { dg-final { scan-assembler-times "cmpdi" 4  } } */
> +/* { dg-final { scan-assembler-times "rotldi" 14  } } */

Please use \m and \M .

Thanks,


Segher