https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79648
Bug ID: 79648
Summary: error on x in constant expression when
__builtin_constant_p(x) == true
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
While looking into bug 79482 and bug 65656 I noticed that although G++
successfully compiles function f() below it gives an error on g(). The only
difference between the two functions is that in g() the local variable is
const-qualified. GCC folds the (i - i) into zero and propagates the constant
result into the argument of __builtin_constant_p() but then doesn't (cannot) do
the same thing in the initializer for b.
In C++ mode this error is independent of optimization. In C mode, it happens
only with optimization and not without.
$ cat t.c && gcc -S -Wall -Wunused -Wpedantic t.c
int f (int i)
{
int x = i - i;
enum { b = __builtin_constant_p (x) ? x : 0 };
return b;
}
int g (int i)
{
const int x = i - i;
enum { b = __builtin_constant_p (x) ? x : 0 };
return b;
}
t.c: In function ‘int g(int)’:
t.c:14:45: error: the value of ‘x’ is not usable in a constant expression
enum { b = __builtin_constant_p (x) ? x : 0 };
^
t.c:12:13: note: ‘x’ was not initialized with a constant expression
const int x = i - i;
^
t.c:14:45: error: the value of ‘x’ is not usable in a constant expression
enum { b = __builtin_constant_p (x) ? x : 0 };
^
t.c:12:13: note: ‘x’ was not initialized with a constant expression
const int x = i - i;
^
t.c:14:45: error: enumerator value for ‘b’ is not an integer constant
enum { b = __builtin_constant_p (x) ? x : 0 };
^
