https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66963
Bug ID: 66963
Summary: __builtin_constant_p and __builtin_choose_expr do not
agree on what is a constexpr with -O2
Product: gcc
Version: 5.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: nicstange at gmail dot com
Target Milestone: ---
The first argument to __builtin_choose_expr() must be a constant expression.
However, at -O2, the following does not work:
int foo (void)
{
const int a = 0;
return __builtin_choose_expr(__builtin_choose_expr(__builtin_constant_p(a +
1), a + 1, 0),
0, 0);
}
test.c: In function ‘foo’:
test.c:4:10: error: first argument to ‘__builtin_choose_expr’ not a constant
return __builtin_choose_expr(__builtin_choose_expr(__builtin_constant_p(a +
1), a + 1, 0),
^
Obviously, __builtin_constant_p() recognizes 'a+1' as being constant-foldable
and returns nonzero. This makes the inner __builtin_choose_expr() select its
second argument, i.e. 'a+1'. The outer __builtin_choose_expr() in turn refuses
to accept that 'a+1' as a constant expression for its condition argument
because it does not recognize it as a constexpr.
As a workaround, you can wrap the 'a+1' in a __builtin_expect() which seems to
apply constant-folding in the same way the __builtin_constant_p() does:
__builtin_choose_expr(__builtin_choose_expr(__builtin_constant_p(a + 1),
__builtin_expect(a + 1, 1), 0),
0, 0);
Thus, it looks like constant folding is not applied to
__builtin_choose_expr()'s first argument.
