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c++: Optimize away NULLPTR_TYPE comparisons [PR101443]

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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.

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?
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.

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.
This commit is contained in:
Jakub Jelinek 2021-07-15 18:53:20 +02:00
parent b25edf6e6f
commit 7094a69bd6
2 changed files with 37 additions and 0 deletions
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26
gcc/cp/cp-gimplify.c
View File

@ -2423,6 +2423,32 @@ 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)

11
gcc/testsuite/g++.dg/cpp0x/nullptr46.C Normal file
View File

@ -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;
}