Re: [PATCH] simplify-rtx: Implement constant folding of SS_TRUNCATE, US_TRUNCATE

[Richard Sandiford <richard.sandiford@arm.com>]

Kyrylo Tkachov via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> Hi all,
>
> This patch implements RTL constant-folding for the SS_TRUNCATE and US_TRUNCATE codes.
> The semantics are a clamping operation on the argument with the min and max of the narrow mode,
> followed by a truncation. The signedness of the clamp and the min/max extrema is derived from
> the signedness of the saturating operation.
>
> We have a number of instructions in aarch64 that use SS_TRUNCATE and US_TRUNCATE to represent
> their operations and we have pretty thorough runtime tests in gcc.target/aarch64/advsimd-intrinsics/vqmovn*.c.
> With this patch the instructions are folded away at optimisation levels and the correctness checks still
> pass.
>
> Bootstrapped and tested on aarch64-none-linux-gnu and aarch64_be-none-elf.
> Ok for trunk?
>
> Thanks,
> Kyrill
>
> gcc/ChangeLog:
>
> 	* simplify-rtx.cc (simplify_const_unary_operation):
> 	Handle US_TRUNCATE, SS_TRUNCATE.
>
> diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
> index 276be67aa67247dd46361ab9badc46ab089d6df0..5983a06e5a8ca89c717e8648be410024147b16e6 100644
> --- a/gcc/simplify-rtx.cc
> +++ b/gcc/simplify-rtx.cc
> @@ -2131,6 +2131,22 @@ simplify_const_unary_operation (enum rtx_code code, machine_mode mode,
>  	  result = wide_int::from (op0, width, UNSIGNED);
>  	  break;
>  
> +	case US_TRUNCATE:
> +	case SS_TRUNCATE:
> +	  {
> +	    signop sgn = code == US_TRUNCATE ? UNSIGNED : SIGNED;
> +	    wide_int nmax
> +	      = wide_int::from (wi::max_value (width, sgn),
> +				GET_MODE_PRECISION (imode), sgn);
> +	    wide_int nmin
> +	      = wide_int::from (wi::min_value (width, sgn),
> +				GET_MODE_PRECISION (imode), sgn);
> +	    result
> +	      = wide_int::from (op0, GET_MODE_PRECISION (imode), sgn);
> +	    result = wi::min (wi::max (result, nmin, sgn), nmax, sgn);

FWIW, it looks like this could be:

	    result = wi::min (wi::max (op0, nmin, sgn), nmax, sgn);

without the first assignment to result.  That feels more natural IMO,
since no conversion is being done on op0.

Thanks,
Richard

> +	    result = wide_int::from (result, width, sgn);
> +	    break;
> +	  }
>  	case SIGN_EXTEND:
>  	  result = wide_int::from (op0, width, SIGNED);
>  	  break;