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* [Bug c++/108158] New: modification of '...' is not a constant expression
@ 2022-12-17 21:00 serge.guelton@telecom-bretagne.eu
2022-12-17 21:35 ` [Bug c++/108158] " pinskia at gcc dot gnu.org
` (9 more replies)
0 siblings, 10 replies; 11+ messages in thread
From: serge.guelton@telecom-bretagne.eu @ 2022-12-17 21:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
Bug ID: 108158
Summary: modification of '...' is not a constant expression
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: serge.guelton@telecom-bretagne.eu
Target Milestone: ---
Since version 11.3, gcc fails on compiling a (complex) constant expression from
the frozen[0] library. gcc 11.2 is fine, and so are clang and msvc.
I've reduced the code to the following: https://godbolt.org/z/e5bje7v35
It's still a relatively large piece of code, I failed to reduce it more
significantly, sorry about that. The error message is
```
<source>:458:39: in 'constexpr' expansion of
'ze_set.frozen::unordered_set<short unsigned int, 3>::lookup<int,
frozen::elsa<short unsigned int> >(4, frozen::elsa<short unsigned int>())'
<source>:453:32: in 'constexpr' expansion of '((const
frozen::unordered_set<short unsigned int,
3>*)this)->frozen::unordered_set<short unsigned int,
3>::tables_.frozen::bits::pmh_tables<8, frozen::elsa<short unsigned int>
>::lookup<int, frozen::elsa<short unsigned int> >((* & key), (* & hash))'
<source>:458:67: in 'constexpr' expansion of 'frozen::bits::seed_or_index()'
<source>:458:67: error: modification of 'ze_set' is not a constant expression
458 | constexpr auto nocount = ze_set.lookup(4, frozen::elsa<uint16_t>());
|
```
But I don't see where such a modification would happen, and the fact that older
version of gcc and clang and msvc correctly evaluate that expression makes me
think it could be a GCC bug. Maybe bisecting between 11.2 and 11.3 would help
understanding the origin of the regression?
[0] https://github.com/serge-sans-paille/frozen
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] modification of '...' is not a constant expression
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
@ 2022-12-17 21:35 ` pinskia at gcc dot gnu.org
2022-12-17 21:36 ` [Bug c++/108158] [11/12/13 Regression] " pinskia at gcc dot gnu.org
` (8 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-12-17 21:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 54119
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54119&action=edit
testcase
Next text attach the testcase rather than just doing a link to godbolt .
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
2022-12-17 21:35 ` [Bug c++/108158] " pinskia at gcc dot gnu.org
@ 2022-12-17 21:36 ` pinskia at gcc dot gnu.org
2022-12-20 16:58 ` [Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304 jakub at gcc dot gnu.org
` (7 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-12-17 21:36 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |rejects-valid
Known to work| |11.2.0
Summary|modification of '...' is |[11/12/13 Regression]
|not a constant expression |modification of '...' is
| |not a constant expression
Target Milestone|--- |11.4
Known to fail| |11.3.0, 12.1.0, 13.0
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
2022-12-17 21:35 ` [Bug c++/108158] " pinskia at gcc dot gnu.org
2022-12-17 21:36 ` [Bug c++/108158] [11/12/13 Regression] " pinskia at gcc dot gnu.org
@ 2022-12-20 16:58 ` jakub at gcc dot gnu.org
2022-12-20 17:12 ` mpolacek at gcc dot gnu.org
` (6 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: jakub at gcc dot gnu.org @ 2022-12-20 16:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2022-12-20
Status|UNCONFIRMED |NEW
Keywords|needs-bisection |
Ever confirmed|0 |1
Summary|[11/12/13 Regression] |[11/12/13 Regression]
|modification of '...' is |modification of '...' is
|not a constant expression |not a constant expression
| |since r12-2304
CC| |jakub at gcc dot gnu.org,
| |mpolacek at gcc dot gnu.org
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Started to be rejected with r12-2304-ga42f8120442cf3ba25d621bed857b5be19019d0c
aka PR101371 fix, which got later backported to 11 branch in r11-9710.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
` (2 preceding siblings ...)
2022-12-20 16:58 ` [Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304 jakub at gcc dot gnu.org
@ 2022-12-20 17:12 ` mpolacek at gcc dot gnu.org
2022-12-20 17:13 ` mpolacek at gcc dot gnu.org
` (5 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: mpolacek at gcc dot gnu.org @ 2022-12-20 17:12 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
Marek Polacek <mpolacek at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Assignee|unassigned at gcc dot gnu.org |mpolacek at gcc dot gnu.org
Status|NEW |ASSIGNED
--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Mine then :(
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
` (3 preceding siblings ...)
2022-12-20 17:12 ` mpolacek at gcc dot gnu.org
@ 2022-12-20 17:13 ` mpolacek at gcc dot gnu.org
2022-12-27 15:02 ` marxin at gcc dot gnu.org
` (4 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: mpolacek at gcc dot gnu.org @ 2022-12-20 17:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
Marek Polacek <mpolacek at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Priority|P3 |P2
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
` (4 preceding siblings ...)
2022-12-20 17:13 ` mpolacek at gcc dot gnu.org
@ 2022-12-27 15:02 ` marxin at gcc dot gnu.org
2023-01-27 23:17 ` mpolacek at gcc dot gnu.org
` (3 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: marxin at gcc dot gnu.org @ 2022-12-27 15:02 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
Martin Liška <marxin at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |marxin at gcc dot gnu.org
--- Comment #4 from Martin Liška <marxin at gcc dot gnu.org> ---
I've just reduced that into:
$ cat pr108158.ii
template <class T, int N> struct carray {
T data_[N]{};
constexpr T operator[](long index) const { return data_[index]; }
};
struct seed_or_index {
private:
long value_ = 0;
};
template <int M> struct pmh_tables {
carray<seed_or_index, M> first_table_;
template <typename KeyType, typename HasherType>
constexpr void lookup(KeyType, HasherType) const {
first_table_[0];
}
};
template <int N> struct unordered_set {
int equal_;
carray<int, N> keys_;
pmh_tables<N> tables_;
constexpr unordered_set() : equal_{} {}
template <class KeyType, class Hasher>
constexpr auto lookup(KeyType key, Hasher hash) const {
tables_.lookup(key, hash);
return keys_;
}
};
constexpr unordered_set<3> ze_set;
constexpr auto nocount = ze_set.lookup(4, int());
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
` (5 preceding siblings ...)
2022-12-27 15:02 ` marxin at gcc dot gnu.org
@ 2023-01-27 23:17 ` mpolacek at gcc dot gnu.org
2023-02-03 18:58 ` cvs-commit at gcc dot gnu.org
` (2 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: mpolacek at gcc dot gnu.org @ 2023-01-27 23:17 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Candidate fix:
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index be99bec17e7..6718c29b0cd 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4301,9 +4301,13 @@ cxx_eval_array_reference (const constexpr_ctx *ctx, tree
t,
if (!SCALAR_TYPE_P (elem_type))
{
new_ctx = *ctx;
- if (ctx->object)
+ if (ctx->object
+ && !same_type_ignoring_top_level_qualifiers_p
+ (TREE_TYPE (t), TREE_TYPE (ctx->object)))
/* If there was no object, don't add one: it could confuse us
- into thinking we're modifying a const object. */
+ into thinking we're modifying a const object. Similarly, if
+ the types are the same, replacing .object could lead to a
+ failure to evaluate it (c++/108158). */
new_ctx.object = t;
new_ctx.ctor = build_constructor (elem_type, NULL);
ctx = &new_ctx;
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
` (6 preceding siblings ...)
2023-01-27 23:17 ` mpolacek at gcc dot gnu.org
@ 2023-02-03 18:58 ` cvs-commit at gcc dot gnu.org
2023-02-03 18:59 ` [Bug c++/108158] [11/12 " mpolacek at gcc dot gnu.org
2023-05-29 10:07 ` jakub at gcc dot gnu.org
9 siblings, 0 replies; 11+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2023-02-03 18:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:
https://gcc.gnu.org/g:27ac6a707e7438c3cec79c24f5d53de79493e2f8
commit r13-5693-g27ac6a707e7438c3cec79c24f5d53de79493e2f8
Author: Marek Polacek <polacek@redhat.com>
Date: Mon Jan 30 09:02:00 2023 -0500
c++: wrong error with constexpr array and value-init [PR108158]
In this test case, we find ourselves evaluating 't' which is
((const struct carray *) this)->data_[VIEW_CONVERT_EXPR<long int>(index)]
in cxx_eval_array_reference. ctx->object is non-null, a RESULT_DECL, so
we replace it with 't':
new_ctx.object = t; // result_decl replaced
and then we go to cxx_eval_constant_expression to evaluate an
AGGR_INIT_EXPR, where we end up evaluating an INIT_EXPR (which is in the
body of the constructor for seed_or_index):
((struct seed_or_index *) this)->value_ = NON_LVALUE_EXPR <0>
whereupon in cxx_eval_store_expression we go to the probe loop
where the 'this' is evaluated to
ze_set.tables_.first_table_.data_[0]
so the 'object' is ze_set, but that isn't in ctx->global->get_value_ptr
so we fail with a bogus error. ze_set is not there because it comes
from a different constexpr context (it's not in cv_cache either).
The problem started with r12-2304 where I added the new_ctx.object
replacement. That was to prevent a type mismatch: the type of 't'
and ctx.object were different.
It seems clear that we shouldn't have replaced ctx.object here.
The cxx_eval_array_reference I mentioned earlier is called from
cxx_eval_store_expression:
6257 init = cxx_eval_constant_expression (&new_ctx, init,
vc_prvalue,
6258 non_constant_p,
overflow_p);
which already created a new context, whose .object we should be
using unless, for instance, INIT contained a.b and we're evaluating
the 'a' part, which I think was the case for r12-2304; in that case
ctx.object has to be something different.
It no longer seems necessary to replace new_ctx.object (likely due to
changes in empty class handling).
PR c++/108158
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_array_reference): Don't replace
new_ctx.object.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/constexpr-108158.C: New test.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] [11/12 Regression] modification of '...' is not a constant expression since r12-2304
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
` (7 preceding siblings ...)
2023-02-03 18:58 ` cvs-commit at gcc dot gnu.org
@ 2023-02-03 18:59 ` mpolacek at gcc dot gnu.org
2023-05-29 10:07 ` jakub at gcc dot gnu.org
9 siblings, 0 replies; 11+ messages in thread
From: mpolacek at gcc dot gnu.org @ 2023-02-03 18:59 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
Marek Polacek <mpolacek at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|[11/12/13 Regression] |[11/12 Regression]
|modification of '...' is |modification of '...' is
|not a constant expression |not a constant expression
|since r12-2304 |since r12-2304
--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed on trunk so far.
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug c++/108158] [11/12 Regression] modification of '...' is not a constant expression since r12-2304
2022-12-17 21:00 [Bug c++/108158] New: modification of '...' is not a constant expression serge.guelton@telecom-bretagne.eu
` (8 preceding siblings ...)
2023-02-03 18:59 ` [Bug c++/108158] [11/12 " mpolacek at gcc dot gnu.org
@ 2023-05-29 10:07 ` jakub at gcc dot gnu.org
9 siblings, 0 replies; 11+ messages in thread
From: jakub at gcc dot gnu.org @ 2023-05-29 10:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|11.4 |11.5
--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 11.4 is being released, retargeting bugs to GCC 11.5.
^ permalink raw reply [flat|nested] 11+ messages in thread
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2022-12-17 21:36 ` [Bug c++/108158] [11/12/13 Regression] " pinskia at gcc dot gnu.org
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