Re: [PATCH] RISC-V: costs: support shift-and-add in strength-reduction

[Palmer Dabbelt <palmer@rivosinc.com>]

On Tue, 08 Nov 2022 11:54:34 PST (-0800), philipp.tomsich@vrull.eu wrote:
> The strength-reduction implementation in expmed.c will assess the
> profitability of using shift-and-add using a RTL expression that wraps
> a MULT (with a power-of-2) in a PLUS.  Unless the RISC-V rtx_costs
> function recognizes this as expressing a sh[123]add instruction, we
> will return an inflated cost---thus defeating the optimization.
>
> This change adds the necessary idiom recognition to provide an
> accurate cost for this for of expressing sh[123]add.
>
> Instead on expanding to
> 	li	a5,200
> 	mulw	a0,a5,a0
> with this change, the expression 'a * 200' is sythesized as:
> 	sh2add	a0,a0,a0   // *5 = a + 4 * a
> 	sh2add	a0,a0,a0   // *5 = a + 4 * a
> 	slli	a0,a0,3    // *8

That's more instructions, but multiplication is generally expensive.  At 
some point I remember the SiFive cores getting very fast integer 
multipliers, but I don't see that reflected in the cost model anywhere 
so maybe I'm just wrong?  Andrew or Kito might remember...

If the mul-based sequences are still faster on the SiFive cores then we 
should probably find a way to keep emitting them, which may just be a 
matter of adjusting those multiply costs.  Moving to the shift-based 
sequences seems reasonable for a generic target, though.

Either way, it probably warrants a test case to make sure we don't 
regress in the future.

>
> gcc/ChangeLog:
>
> 	* config/riscv/riscv.c (riscv_rtx_costs): Recognize shNadd,
> 	if expressed as a plus and multiplication with a power-of-2.
>
> ---
>
>  gcc/config/riscv/riscv.cc | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
> index ab6c745c722..0b2c4b3599d 100644
> --- a/gcc/config/riscv/riscv.cc
> +++ b/gcc/config/riscv/riscv.cc
> @@ -2451,6 +2451,19 @@ riscv_rtx_costs (rtx x, machine_mode mode, int outer_code, int opno ATTRIBUTE_UN
>  	  *total = COSTS_N_INSNS (1);
>  	  return true;
>  	}
> +      /* Before strength-reduction, the shNadd can be expressed as the addition
> +	 of a multiplication with a power-of-two.  If this case is not handled,
> +	 the strength-reduction in expmed.c will calculate an inflated cost. */
> +      if (TARGET_ZBA
> +	  && mode == word_mode
> +	  && GET_CODE (XEXP (x, 0)) == MULT
> +	  && REG_P (XEXP (XEXP (x, 0), 0))
> +	  && CONST_INT_P (XEXP (XEXP (x, 0), 1))
> +	  && IN_RANGE (pow2p_hwi (INTVAL (XEXP (XEXP (x, 0), 1))), 1, 3))

IIUC the fall-through is biting us here and this matches power-of-2 +1 
and power-of-2 -1.  That looks to be the case for the one below, though, 
so not sure if I'm just missing something?

> +	{
> +	  *total = COSTS_N_INSNS (1);
> +	  return true;
> +	}
>        /* shNadd.uw pattern for zba.
>  	 [(set (match_operand:DI 0 "register_operand" "=r")
>  	       (plus:DI