Why is ARM using a link register (LR)?

It’s been used for a better performance, but only works for leaf functions.

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Working szenario (Leaf function)

void foo() { return; }
void bar() { return; }

int main()
{
  foo(); // Write PC+1 (foo()+1) into LR, goto foo and come back via LR
  bar(); // Write PC+1 (bar()+1) into LR, goto bar and come back via LR
  return 0;
}

Nice, it’s fast, cause we only cache the return address in a register (LR) and not on the stack.

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Not working szenario (Non-leaf function)

void bar();
void foo() {
  bar(); // Write PC+1 (bar()+1) into LR, goto bar and come back via LR
  return; // You can't get back to main, cause LR points to bar()+1!
}
void bar() { return; }

int main()
{
  foo(); // Write PC+1 (foo()+1) into LR, goto foo and come back via LR?
  return 0;
}

Shit, for non-leaf functions we have to operate like Intel/AMD and have to store the LR on the stack.