https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68121
Bug ID: 68121
Summary: __builtin_constant_p should not warn about integer
overflow
Product: gcc
Version: 5.2.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: eggert at gnu dot org
Target Milestone: ---
Compile this with "gcc -S" (x86-64):
enum {
x = 2000000000,
y = 2000000000,
a = __builtin_constant_p (x + y) ? x : -1,
};
GCC warns "integer overflow in expression [-Woverflow]" and points to the "+",
even though the "+" is not evaluated. This sort of warning should be disabled
in the argument of __builtin_constant_p, just as it's disabled in the argument
of other expressions like sizeof that do not actually evaluate their arguments.
In the above case, __builtin_constant_p should return 1 without warning about
overflow.
I ran into this problem when using a macro definition like this:
#define INT_ADD_OVERFLOW(a, b) \
(__builtin_constant_p ((a) + (b)) \
? _GL_INT_ADD_OVERFLOW (a, b) \
: __builtin_add_overflow (a, b, &(__typeof__ ((a) + (b))) {0}))
where _GL_INT_ADD_OVERFLOW (a, b) expands to a complicated expression that does
not overflow, and that yields 1 if A+B would overflow. GCC incorrectly warned
about (a)+(b) overflowing in the argument of __builtin_constant_p. I worked
around the bug by changing the first "+" to "==" in the macro body, but that's
kinda weird.
