https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101436
Bug ID: 101436
Summary: Yet another bogus "array subscript is partly outside
array bounds"
Product: gcc
Version: 11.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: cassio.neri at gmail dot com
Target Milestone: ---
This bogus warning was reported at least twice recently: #98266 and #101374.
Below is a new case that, it seems, hasn't been addressed yet.
#include <typeinfo>
struct A {
virtual ~A();
};
template <typename T>
struct B : A {
T x;
};
struct C1 {
int i;
double j;
};
struct C2 {
int i;
};
void do_something(int);
B<C2> get_BC2();
void h(A& a) {
if (typeid(a) == typeid(B<C1>)) {
B<C1>& b = static_cast<B<C1>&>(a);
int i = b.x.i;
do_something(i);
}
}
void foo() {
B<C2> x = get_BC2();
h(x);
}
Compiled with '-O3 -Warray-bounds' yields:
<source>: In function 'void foo()':
<source>:27:9: warning: array subscript 'B<C1>[0]' is partly outside array
bounds of 'B<C2> [1]' [-Warray-bounds]
27 | int i = b.x.i;
| ^
<source>:33:9: note: while referencing 'x'
33 | B<C2> x = get_BC2();
FWIW:
1) This is a regression from GCC 10.3.
2) The warning goes away if any of the following changes are made:
* Remove C1::j.
* Change type of C1::j to any of int, char, bool, unsigned or float. (Perhaps
any type T such that sizeof(T) <= sizeof(int)).
* Compile with '-fPIC' (however, if h is marked inline then the warning comes
back).
3) If b is declared as B<C1> (as opposed to B<C1>&), then the warning points to
line 'struct B: A {'.
4) The test case could be simplified further by removing A's virtual destructor
and the typeid check. However, this would make the code to invoke UB and I hope
the code above doesn't.
5) #98266 regards virtual inheritance which does not appear here and a test
cases therein issues no warning when compiled with GCC 11.1.
6) IIUC the warning reported by #101374 happens in GCC's own code and was
caused by some recent change that is not part of GCC 11.1. Indeed a test case
reported therein compiles fine with GCC 11.1 whereas the one above doesn't.
See also:
Test case above: https://godbolt.org/z/n4obaohPs
Test case from #98266: https://godbolt.org/z/PEjfhs3T6
Test case from #101374: https://godbolt.org/z/Ebb8YszT5
