https://bugs.llvm.org/show_bug.cgi?id=39109
Bug ID: 39109
Summary: constexpr fibonacci is not computed at compile time
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: C++
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
constexpr uint32_t fib(const uint32_t n) {
if (n <= 1) return 1;
return fib(n - 1) + fib(n - 2);
}
int main() {
return fib(7);
}
int main2() {
const uint32_t res = fib(7);
return res;
}
Clang is unable to compute fib in main (but it can in main2) at compile time,
GCC can do it in both functions.
main: # @main
mov edi, 7
jmp fib(unsigned int)
While main2 is interesting.. Clang computes it as 21 but then it still calls
fib uselessly.
main2(): # @main2()
push rax
mov edi, 7
call fib(unsigned int)
mov eax, 21
pop rcx
ret
Latest GCC:
main:
mov eax, 21
ret
main2():
mov eax, 21
ret
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