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

[Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>]

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Hi Richard,

> -----Original Message-----
> From: Richard Sandiford <richard.sandiford@arm.com>
> Sent: Friday, June 9, 2023 7:08 PM
> To: Kyrylo Tkachov via Gcc-patches <gcc-patches@gcc.gnu.org>
> Cc: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
> Subject: Re: [PATCH] simplify-rtx: Implement constant folding of
> SS_TRUNCATE, US_TRUNCATE
> 
> 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..5983a06e5a8ca89c717e864
> 8be410024147b16e6 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, that works indeed.
I'll push the attached patch to trunk once bootstrap and testing completes.
Kyrill

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

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diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index 276be67aa67247dd46361ab9badc46ab089d6df0..21b7eb484d05818bb563e086e07a6152a3a3c6b7 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -2131,6 +2131,20 @@ 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 = wi::min (wi::max (op0, nmin, sgn), nmax, sgn);
+	    result = wide_int::from (result, width, sgn);
+	    break;
+	  }
 	case SIGN_EXTEND:
 	  result = wide_int::from (op0, width, SIGNED);
 	  break;