https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82280
Bug ID: 82280
Summary: Missed optimization opportunity constexpr/inline data
copy not elided
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: matt at godbolt dot org
Target Milestone: ---
Consider the C++ code:
```
#include <array>
#include <algorithm>
constexpr auto get_tokens()
{
return std::array<char, 53>{"This is my string of possible tokens to
match."};
}
struct S {
#ifndef LOCAL
static constexpr auto m_tokens = get_tokens();
#endif
constexpr bool has_token(const char c) const {
#ifdef LOCAL
const constexpr auto m_tokens = get_tokens();
#endif
for (const auto val : m_tokens) {
if (val == c) { return true; }
}
return false;
}
};
int main(const int argc, const char *[]) {
constexpr S s;
return s.has_token(char(argc));
}
```
Compiled with `-O3 -std=c++1z`, the `m_tokens` is a struct-level static
constexpr data member. Adding `-DLOCAL` makes it a const constexpr local
variable within the `has_token` routine.
When compiled with local, the result of the `get_tokens()` call is copied to
stack, and is used from there. The copy introduces 10 instructions. Given that
the source is immutable, the copy can be elided and the original data referred
to instead. See, for example: https://godbolt.org/g/ZVZMi6
Clang avoids the copy (see https://godbolt.org/g/j5P8Uv - NB this is clang 5.0
with `-O2` to prevent some of its other optimizations from confusing the issue
further).
