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

[Richard Sandiford via Gcc-patches]

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;