https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83484
Bug ID: 83484
Summary: constexpr not evaluated at compile time
Product: gcc
Version: 7.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: gjl at gcc dot gnu.org
Target Milestone: ---
The following test case triggers a link failure because the constexpr are not
evaluated at compile time:
$ g++-8 tmp.cpp -Os -save-temps
tmp.o: In function `Port<7>::set(Port<7>::Value)':
tmp.cpp:(.text._ZN4PortILi7EE3setENS0_5ValueE[_ZN4PortILi7EE3setENS0_5ValueE]+0x2):
undefined reference to `Port<7>::High'
tmp.o: In function `main':
tmp.cpp:(.text.startup+0x3): undefined reference to `Port<7>::Low'
tmp.cpp:(.text.startup+0x1a): undefined reference to `Port<7>::High'
collect2: error: ld returned 1 exit status
== Test case ==
unsigned volatile PORTB;
template<int p>
struct Port
{
enum class Value : unsigned { High, Low };
static inline void set (Value);
static constexpr Value High = Value::High;
static constexpr Value Low = Value::Low;
};
template<> constexpr Port<7>::Value Port<7>::High;
template<> constexpr Port<7>::Value Port<7>::Low;
template<>
inline void Port<7>::set (Port<7>::Value value)
{
if (High == value)
PORTB |= 2;
else
PORTB &= ~2;
}
int main()
{
Port<7>::set (Port<7>::Low);
Port<7>::set (PORTB ? Port<7>::Low : Port<7>::High);
}
Apart from the link failure you see that even with optimizations turned on and
for an almost trivial function call like Port<7>::set (Port<7>::Low), the
compiler generates a call. Fot some targets the compiler even generates calls
with just 2 instructions (one of which is a return statement).
So far I see this for v7 and v8 on x86_64 and avr.
