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c++: Add support for __real__/__imag__ modifications in constant expressions [PR88174] 

We claim we support P0415R1 (constexpr complex), but e.g.
 #include <complex>

constexpr bool
foo ()
{
  std::complex<double> a (1.0, 2.0);
  a += 3.0;
  a.real (6.0);
  return a.real () == 6.0 && a.imag () == 2.0;
}

static_assert (foo ());

fails with
test.C:12:20: error: non-constant condition for static assertion
   12 | static_assert (foo ());
      |                ~~~~^~
test.C:12:20:   in ‘constexpr’ expansion of ‘foo()’
test.C:8:10:   in ‘constexpr’ expansion of ‘a.std::complex<double>::real(6.0e+0)’
test.C:12:20: error: modification of ‘__real__ a.std::complex<double>::_M_value’ is not a constant expression

The problem is we don't handle REALPART_EXPR and IMAGPART_EXPR
in cxx_eval_store_expression.
The following patch attempts to support it (with a requirement
that those are the outermost expressions, ARRAY_REF/COMPONENT_REF
etc. are just not possible on the result of these, BIT_FIELD_REF
would be theoretically possible if trying to extract some bits
from one part of a complex int, but I don't see how it could appear
in the FE trees.

For these references, the code handles value being COMPLEX_CST,
COMPLEX_EXPR or CONSTRUCTOR_NO_CLEARING empty CONSTRUCTOR (what we use
to represent uninitialized values for C++20 and later) and the
code starts by rewriting it to COMPLEX_EXPR, so that we can freely
adjust the individual parts and later on possibly optimize it back
to COMPLEX_CST if both halves are constant.

2022-08-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/88174
	* constexpr.cc (cxx_eval_store_expression): Handle REALPART_EXPR
	and IMAGPART_EXPR.  Change ctors from releasing_vec to
	auto_vec<tree *>, adjust all uses.  For !preeval, update ctors
	vector.

	* g++.dg/cpp1y/constexpr-complex1.C: New test.
This commit is contained in:
Jakub Jelinek 2022-08-07 10:07:38 +02:00
parent a46bca36b7
commit 1907767735
2 changed files with 104 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

94
gcc/cp/constexpr.cc
View File

@ -5732,6 +5732,20 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
}
break;
case REALPART_EXPR:
gcc_assert (probe == target);
vec_safe_push (refs, probe);
vec_safe_push (refs, TREE_TYPE (probe));
probe = TREE_OPERAND (probe, 0);
break;
case IMAGPART_EXPR:
gcc_assert (probe == target);
vec_safe_push (refs, probe);
vec_safe_push (refs, TREE_TYPE (probe));
probe = TREE_OPERAND (probe, 0);
break;
default:
if (evaluated)
object = probe;
@ -5770,7 +5784,8 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
type = TREE_TYPE (object);
bool no_zero_init = true;
releasing_vec ctors, indexes;
auto_vec<tree *> ctors;
releasing_vec indexes;
auto_vec<int> index_pos_hints;
bool activated_union_member_p = false;
bool empty_base = false;
@ -5810,14 +5825,36 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
*valp = ary_ctor;
}
/* If the value of object is already zero-initialized, any new ctors for
subobjects will also be zero-initialized. */
no_zero_init = CONSTRUCTOR_NO_CLEARING (*valp);
enum tree_code code = TREE_CODE (type);
tree reftype = refs->pop();
tree index = refs->pop();
if (code == COMPLEX_TYPE)
{
if (TREE_CODE (*valp) == COMPLEX_CST)
*valp = build2 (COMPLEX_EXPR, type, TREE_REALPART (*valp),
TREE_IMAGPART (*valp));
else if (TREE_CODE (*valp) == CONSTRUCTOR
&& CONSTRUCTOR_NELTS (*valp) == 0
&& CONSTRUCTOR_NO_CLEARING (*valp))
{
tree r = build_constructor (reftype, NULL);
CONSTRUCTOR_NO_CLEARING (r) = 1;
*valp = build2 (COMPLEX_EXPR, type, r, r);
}
gcc_assert (TREE_CODE (*valp) == COMPLEX_EXPR);
ctors.safe_push (valp);
vec_safe_push (indexes, index);
valp = &TREE_OPERAND (*valp, TREE_CODE (index) == IMAGPART_EXPR);
gcc_checking_assert (refs->is_empty ());
type = reftype;
break;
}
/* If the value of object is already zero-initialized, any new ctors for
subobjects will also be zero-initialized. */
no_zero_init = CONSTRUCTOR_NO_CLEARING (*valp);
if (code == RECORD_TYPE && is_empty_field (index))
/* Don't build a sub-CONSTRUCTOR for an empty base or field, as they
have no data and might have an offset lower than previously declared
@ -5860,7 +5897,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
no_zero_init = true;
}
vec_safe_push (ctors, *valp);
ctors.safe_push (valp);
vec_safe_push (indexes, index);
constructor_elt *cep
@ -5922,11 +5959,11 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
semantics are not applied on an object under construction.
They come into effect when the constructor for the most
derived object ends." */
for (tree elt : *ctors)
for (tree *elt : ctors)
if (same_type_ignoring_top_level_qualifiers_p
(TREE_TYPE (const_object_being_modified), TREE_TYPE (elt)))
(TREE_TYPE (const_object_being_modified), TREE_TYPE (*elt)))
{
fail = TREE_READONLY (elt);
fail = TREE_READONLY (*elt);
break;
}
}
@ -5967,6 +6004,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
valp = ctx->global->values.get (object);
for (unsigned i = 0; i < vec_safe_length (indexes); i++)
{
ctors[i] = valp;
constructor_elt *cep
= get_or_insert_ctor_field (*valp, indexes[i], index_pos_hints[i]);
valp = &cep->value;
@ -6029,17 +6067,45 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
CONSTRUCTORs, if any. */
bool c = TREE_CONSTANT (init);
bool s = TREE_SIDE_EFFECTS (init);
if (!indexes->is_empty ())
{
tree last = indexes->last ();
if (TREE_CODE (last) == REALPART_EXPR
|| TREE_CODE (last) == IMAGPART_EXPR)
{
/* And canonicalize COMPLEX_EXPR into COMPLEX_CST if
possible. */
tree *cexpr = ctors.last ();
if (tree c = const_binop (COMPLEX_EXPR, TREE_TYPE (*cexpr),
TREE_OPERAND (*cexpr, 0),
TREE_OPERAND (*cexpr, 1)))
*cexpr = c;
else
{
TREE_CONSTANT (*cexpr)
= (TREE_CONSTANT (TREE_OPERAND (*cexpr, 0))
& TREE_CONSTANT (TREE_OPERAND (*cexpr, 1)));
TREE_SIDE_EFFECTS (*cexpr)
= (TREE_SIDE_EFFECTS (TREE_OPERAND (*cexpr, 0))
| TREE_SIDE_EFFECTS (TREE_OPERAND (*cexpr, 1)));
}
c = TREE_CONSTANT (*cexpr);
s = TREE_SIDE_EFFECTS (*cexpr);
}
}
if (!c || s || activated_union_member_p)
for (tree elt : *ctors)
for (tree *elt : ctors)
{
if (TREE_CODE (*elt) != CONSTRUCTOR)
continue;
if (!c)
TREE_CONSTANT (elt) = false;
TREE_CONSTANT (*elt) = false;
if (s)
TREE_SIDE_EFFECTS (elt) = true;
TREE_SIDE_EFFECTS (*elt) = true;
/* Clear CONSTRUCTOR_NO_CLEARING since we've activated a member of
this union. */
if (TREE_CODE (TREE_TYPE (elt)) == UNION_TYPE)
CONSTRUCTOR_NO_CLEARING (elt) = false;
if (TREE_CODE (TREE_TYPE (*elt)) == UNION_TYPE)
CONSTRUCTOR_NO_CLEARING (*elt) = false;
}
if (lval)

24
gcc/testsuite/g++.dg/cpp1y/constexpr-complex1.C Normal file
View File

@ -0,0 +1,24 @@
// PR c++/88174
// { dg-do compile { target c++14 } }
constexpr bool
foo (double x, double y, double z, double w)
{
__complex__ double a = 0;
__real__ a = x;
__imag__ a = y;
#if __cpp_constexpr >= 201907L
__complex__ double b;
__real__ b = z;
#else
__complex__ double b = z;
#endif
__imag__ b = w;
a += b;
a -= b;
a *= b;
a /= b;
return __real__ a == x && __imag__ a == y;
}
static_assert (foo (1.0, 2.0, 3.0, 4.0), "");