Re: [PATCH] c++: Optimize away NULLPTR_TYPE comparisons [PR101443]

[Jason Merrill <jason@redhat.com>]

On 7/15/21 3:53 AM, Jakub Jelinek wrote:
> Hi!
> 
> Comparisons of NULLPTR_TYPE operands cause all kinds of problems in the
> middle-end and in fold-const.c, various optimizations assume that if they
> see e.g. a non-equality comparison with one of the operands being
> INTEGER_CST and it is not INTEGRAL_TYPE_P (which has TYPE_{MIN,MAX}_VALUE),
> they can build_int_cst (type, 1) to find a successor.
> 
> The following patch fixes it by making sure they don't appear in the IL,
> optimize them away at cp_fold time as all can be folded.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> Though, I've just noticed that clang++ rejects the non-equality comparisons
> instead, foo () > 0 with
> invalid operands to binary expression ('decltype(nullptr)' (aka 'nullptr_t') and 'int')
> and foo () > nullptr with
> invalid operands to binary expression ('decltype(nullptr)' (aka 'nullptr_t') and 'nullptr_t')
> 
> Shall we reject those too, in addition or instead of parts of this patch?

Yes.

> If so, wouldn't this patch be still useful for backports, I bet we don't
> want to start reject it on the release branches when we used to accept it.

Sounds good.

> 2021-07-15  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/101443
> 	* cp-gimplify.c (cp_fold): For comparisons with NULLPTR_TYPE
> 	operands, fold them right away to true or false.
> 
> 	* g++.dg/cpp0x/nullptr46.C: New test.
> 
> --- gcc/cp/cp-gimplify.c.jj	2021-06-25 10:36:22.141020337 +0200
> +++ gcc/cp/cp-gimplify.c	2021-07-14 12:04:24.221860756 +0200
> @@ -2424,6 +2424,32 @@ cp_fold (tree x)
>         op0 = cp_fold_maybe_rvalue (TREE_OPERAND (x, 0), rval_ops);
>         op1 = cp_fold_rvalue (TREE_OPERAND (x, 1));
>   
> +      /* decltype(nullptr) has only one value, so optimize away all comparisons
> +	 with that type right away, keeping them in the IL causes troubles for
> +	 various optimizations.  */
> +      if (COMPARISON_CLASS_P (org_x)
> +	  && TREE_CODE (TREE_TYPE (op0)) == NULLPTR_TYPE
> +	  && TREE_CODE (TREE_TYPE (op1)) == NULLPTR_TYPE)
> +	{
> +	  switch (code)
> +	    {
> +	    case EQ_EXPR:
> +	    case LE_EXPR:
> +	    case GE_EXPR:
> +	      x = constant_boolean_node (true, TREE_TYPE (x));
> +	      break;
> +	    case NE_EXPR:
> +	    case LT_EXPR:
> +	    case GT_EXPR:
> +	      x = constant_boolean_node (false, TREE_TYPE (x));
> +	      break;
> +	    default:
> +	      gcc_unreachable ();
> +	    }
> +	  return omit_two_operands_loc (loc, TREE_TYPE (x), x,
> +					op0, op1);
> +	}
> +
>         if (op0 != TREE_OPERAND (x, 0) || op1 != TREE_OPERAND (x, 1))
>   	{
>   	  if (op0 == error_mark_node || op1 == error_mark_node)
> --- gcc/testsuite/g++.dg/cpp0x/nullptr46.C.jj	2021-07-14 11:48:03.917122727 +0200
> +++ gcc/testsuite/g++.dg/cpp0x/nullptr46.C	2021-07-14 11:46:52.261092097 +0200
> @@ -0,0 +1,11 @@
> +// PR c++/101443
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-O2" }
> +
> +decltype(nullptr) foo ();
> +
> +bool
> +bar ()
> +{
> +  return foo () > nullptr || foo () < nullptr;
> +}
> 
> 	Jakub
>