Pass-by-reference hinders gcc from tail call elimination - c++

Question

See BlendingTable::create and BlendingTable::print. Both have the same form of tail recursion, but while create will be optimized as a loop, print will not and cause a stack overflow.
Go down to see a fix, which I got from a hint from one of the gcc devs on my bug report of this problem.
#include <cstdlib>
#include <iostream>
#include <memory>
#include <array>
#include <limits>
class System {
public:
template<typename T, typename... Ts>
static void print(const T& t, const Ts&... ts) {
std::cout << t << std::flush;
print(ts...);
}
static void print() {}
template<typename... Ts>
static void printLine(const Ts&... ts) {
print(ts..., '\n');
}
};
template<typename T, int dimension = 1>
class Array {
private:
std::unique_ptr<T[]> pointer;
std::array<int, dimension> sizes;
int realSize;
public:
Array() {}
template<typename... Ns>
Array(Ns... ns):
realSize(1) {
checkArguments(ns...);
create(1, ns...);
}
private:
template<typename... Ns>
static void checkArguments(Ns...) {
static_assert(sizeof...(Ns) == dimension, "dimension mismatch");
}
template<typename... Ns>
void create(int d, int n, Ns... ns) {
realSize *= n;
sizes[d - 1] = n;
create(d + 1, ns...);
}
void create(int) {
pointer = std::unique_ptr<T[]>(new T[realSize]);
}
int computeSubSize(int d) const {
if (d == dimension) {
return 1;
}
return sizes[d] * computeSubSize(d + 1);
}
template<typename... Ns>
int getIndex(int d, int n, Ns... ns) const {
return n * computeSubSize(d) + getIndex(d + 1, ns...);
}
int getIndex(int) const {
return 0;
}
public:
template<typename... Ns>
T& operator()(Ns... ns) const {
checkArguments(ns...);
return pointer[getIndex(1, ns...)];
}
int getSize(int d = 1) const {
return sizes[d - 1];
}
};
class BlendingTable : public Array<unsigned char, 3> {
private:
enum {
SIZE = 0x100,
FF = SIZE - 1,
};
public:
BlendingTable():
Array<unsigned char, 3>(SIZE, SIZE, SIZE) {
static_assert(std::numeric_limits<unsigned char>::max() == FF, "unsupported byte format");
create(FF, FF, FF);
}
private:
void create(int dst, int src, int a) {
(*this)(dst, src, a) = (src * a + dst * (FF - a)) / FF;
if (a > 0) {
create(dst, src, a - 1);
} else if (src > 0) {
create(dst, src - 1, FF);
} else if (dst > 0) {
create(dst - 1, FF, FF);
} else {
return;
}
}
void print(int dst, int src, int a) const {
System::print(static_cast<int>((*this)(FF - dst, FF - src, FF - a)), ' ');
if (a > 0) {
print(dst, src, a - 1);
} else if (src > 0) {
print(dst, src - 1, FF);
} else if (dst > 0) {
print(dst - 1, FF, FF);
} else {
System::printLine();
return;
}
}
public:
void print() const {
print(FF, FF, FF);
}
};
int main() {
BlendingTable().print();
return EXIT_SUCCESS;
}
Changing the class definition of System from
class System {
public:
template<typename T, typename... Ts>
static void print(const T& t, const Ts&... ts) {
std::cout << t << std::flush;
print(ts...);
}
static void print() {}
template<typename... Ts>
static void printLine(const Ts&... ts) {
print(ts..., '\n');
}
};
to
class System {
public:
template<typename T, typename... Ts>
static void print(T t, Ts... ts) {
std::cout << t << std::flush;
print(ts...);
}
static void print() {}
template<typename... Ts>
static void printLine(Ts... ts) {
print(ts..., '\n');
}
};
magically allows gcc to eliminate the tail calls.
Why does 'whether or not passing function arguments by reference' make such a big difference in gcc's behaviour? Semantically they both look the same to me in this case.

Answer 1

As it is noted by #jxh the cast static_cast<int>() creates a temporary whose reference is passed to the print function. Without such cast the tail recursion is optimized correctly.
The issue is very similar to the old case Why isn't g++ tail call optimizing while gcc is? and the workaround may be similar to https://stackoverflow.com/a/31793391/4023446.
It is still possible to use System with the arguments passed by reference if call to System::print will be moved to a separate private helper function SystemPrint:
class BlendingTable : public Array<unsigned char, 3> {
//...
private:
void SystemPrint(int dst, int src, int a) const
{
System::print(static_cast<int>((*this)(FF - dst, FF - src, FF - a)), ' ');
}
void print(int dst, int src, int a) const {
SystemPrint(dst, src, a);
if (a > 0) {
print(dst, src, a - 1);
} else if (src > 0) {
print(dst, src - 1, FF);
} else if (dst > 0) {
print(dst - 1, FF, FF);
} else {
System::printLine();
return;
}
}
// ...
}
Now the tail call optimization works (g++ (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2 with the optimization option -O2) and the print does not cause a stack overflow.
Update
I verified it with other compilers:
the original code without any change is perfectly optimized by clang++ Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn) with -O1 optimization
g++ (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4 fails to perform TCO even without the cast or with the wrapping function SystemPrint workaround; here only the workaround with System::print arguments by values works.
So, the issue is very specific to compiler versions.