Re: [PATCH][AArch64][1/3] Expand signed mod by power of 2 using CSNEG

[James Greenhalgh <james dot greenhalgh at arm dot com>]

On Fri, Jul 24, 2015 at 11:55:33AM +0100, Kyrill Tkachov wrote:
> Hi all,
> 
> This patch implements an aarch64-specific expansion of the signed modulo by a power of 2.
> The proposed sequence makes use of the conditional negate instruction CSNEG.
> For a power of N, x % N can be calculated with:
> negs   x1, x0
> and    x0, x0, #(N - 1)
> and    x1, x1, #(N - 1)
> csneg  x0, x0, x1, mi
> 
> So, for N == 256 this would be:
> negs   x1, x0
> and    x0, x0, #255
> and    x1, x1, #255
> csneg  x0, x0, x1, mi
> 
> For comparison, the existing sequence emitted by expand_smod_pow2 in expmed.c is:
> asr     x1, x0, 63
> lsr     x1, x1, 56
> add     x0, x0, x1
> and     x0, x0, 255
> sub     x0, x0, x1
> 
> Note that the CSNEG sequence is one instruction shorter and that the two and operations
> are independent, compared to the existing sequence where all instructions are dependent
> on the preceeding instructions.
> 
> For the special case of N == 2 we can do even better:
> cmp     x0, xzr
> and     x0, x0, 1
> csneg   x0, x0, x0, ge
> 
> I first tried implementing this in the generic code in expmed.c but that didn't work
> out for a few reasons:
> 
> * This relies on having a conditional-negate instruction. We could gate it on
> HAVE_conditional_move and the combiner is capable of merging the final negate into
> the conditional move if a conditional negate is available (like on aarch64) but on
> targets without a conditional negate this would end up emitting a separate negate.
> 
> * The first negs has to be a negs for the sequence to be a win i.e. having a separate
> negate and compare makes the sequence slower than the existing one (at least in my
> microbenchmarking) and I couldn't get subsequent passes to combine the negate and combine
> into the negs (presumably due to the use of the negated result in one of the ands).
> Doing it in the aarch64 backend where I could just call the exact gen_* functions that
> I need worked much more cleanly.
> 
> The costing logic is updated to reflect this sequence during the intialisation of
> expmed.c where it calculates the smod_pow2_cheap metric.
> 
> The tests will come in patch 3 of the series which are partly shared with the equivalent
> arm implementation.
> 
> Bootstrapped and tested on aarch64.
> Ok for trunk?
> 
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index 9d88a60..7bb4a55 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -6639,8 +6639,26 @@ cost_plus:
>  	  if (VECTOR_MODE_P (mode))
>  	    *cost += extra_cost->vect.alu;
>  	  else if (GET_MODE_CLASS (mode) == MODE_INT)
> -	    *cost += (extra_cost->mult[mode == DImode].add
> -		      + extra_cost->mult[mode == DImode].idiv);
> +	    {
> +	      /* We can expand signed mod by power of 2 using a
> +		 NEGS, two parallel ANDs and a CSNEG.  Assume here
> +		 that CSNEG is COSTS_N_INSNS (1).  This case should
> +		 only ever be reached through the set_smod_pow2_cheap check
> +		 in expmed.c.  */
> +	      if (code == MOD
> +		  && CONST_INT_P (XEXP (x, 1))
> +		  && exact_log2 (INTVAL (XEXP (x, 1))) > 0
> +		  && (mode == SImode || mode == DImode))
> +		{
> +		  *cost += COSTS_N_INSNS (3)
> +			   + 2 * extra_cost->alu.logical
> +			   + extra_cost->alu.arith;
> +		  return true;
> +		}
> +
> +	      *cost += (extra_cost->mult[mode == DImode].add
> +			+ extra_cost->mult[mode == DImode].idiv);
> +	    }
>  	  else if (mode == DFmode)
>  	    *cost += (extra_cost->fp[1].mult
>  		      + extra_cost->fp[1].div);

This looks like it calculates the wrong cost for !speed. I think we will
still expand through mod<mode>3 when compiling for size, so we probably
still want to cost the multiple instructions.

Have I misunderstood?

Thanks,
James