https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85747
Bug ID: 85747
Summary: suboptimal code without constexpr
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Consider the following code snippet:
// Bubble-like sort. Anything complex enough will work
template <class It>
constexpr void sort(It first, It last) {
for (;first != last; ++first) {
auto it = first;
++it;
for (; it != last; ++it) {
if (*it < *first) {
auto tmp = *it;
*it = *first;
*first = tmp;
}
}
}
}
static int generate() {
int a[7] = {3, 7, 4, 2, 8, 0, 1};
sort(a + 0, a + 7);
return a[0] + a[6];
}
int no_constexpr() {
return generate();
}
Above code generates ~30 assembly instructions instead of just generating:
no_constexpr():
mov eax, 8
ret
But if we change `static` to `constexpr` then the compiler will optimize the
code correctly.
Could the compiler detect that `a[7]` holds values known at compile time and
force the constexpr on `sort(a + 0, a + 7);`? Could the compiler detect that
the function `generate()` is an `__attribute__((const))` function without
arguments and fully evaluate it's body?
