5 ms·

Would you not have to use a jump instead of call for it to be a tail call at all- ie otherwise a new frame is created on each call

nagaiaida 2y ago
the call is still in tail position whether or not it reuses the stack frame. there are also more involved ways to do tail call optimization than a direct single-jump compilation when you leave ret behind entirely, such as in forth-style threaded interpreters
- frogsRnice 2y ago
  I guess were talking about optimising tail recursion. Would there be any reason to refer to a tail call other than that optimisation? I’ll do some reading on the latter part of your post, thank you!
  - nagaiaida 2y ago
    i only meant that "optimized/eliminated tail call" is more useful terminology than an uneliminated tail call not counting as "a tail call". i find this distinction useful when discussing clojure, for instance, where you have to explicitly trampoline recursive tail calls and there is a difference between an eliminated tail call and a call in tail position which is eligible for TCO i'm not sure how commonly tail calls are eliminated in other forthlikes at the ~runtime level since you can just do it at call time when you really need it by dropping from the return stack, but i find it nice to be able to not just pop the stack doing things naively. basically since exit is itself a threaded word you can simply¹ check if the current instruction precedes a call to exit and drop a return address in case it's helpful this is the relevant bit from mine (which started off as a toy 64-bit port of jonesforth): .macro STEP lodsq jmp *(%rax) .endm INTERPRET: mov (%rsi), %rcx mov $EXIT, %rdx lea 8(%rbp), %rbx cmp %rcx, %rdx # tail call? cmovz (%rbp), %rsi # if so, we cmovz %rbx, %rbp # can reuse RPUSH %rsi # ret stack add $8, %rax mov %rax, %rsi STEP ¹ provided you're willing to point the footguns over at the return stack manipulation side of things instead
  - biodniggnj 2y ago
    You don’t need recursion to make use of tail call elimination. In Scheme and SML all tail calls are eliminated. GCC also does it, but less often. Still, it’s not recursion that triggers it.

nagaiaida 2y ago

the call is still in tail position whether or not it reuses the stack frame. there are also more involved ways to do tail call optimization than a direct single-jump compilation when you leave ret behind entirely, such as in forth-style threaded interpreters

frogsRnice 2y ago
I guess were talking about optimising tail recursion. Would there be any reason to refer to a tail call other than that optimisation? I’ll do some reading on the latter part of your post, thank you!
- nagaiaida 2y ago
  i only meant that "optimized/eliminated tail call" is more useful terminology than an uneliminated tail call not counting as "a tail call". i find this distinction useful when discussing clojure, for instance, where you have to explicitly trampoline recursive tail calls and there is a difference between an eliminated tail call and a call in tail position which is eligible for TCO i'm not sure how commonly tail calls are eliminated in other forthlikes at the ~runtime level since you can just do it at call time when you really need it by dropping from the return stack, but i find it nice to be able to not just pop the stack doing things naively. basically since exit is itself a threaded word you can simply¹ check if the current instruction precedes a call to exit and drop a return address in case it's helpful this is the relevant bit from mine (which started off as a toy 64-bit port of jonesforth): .macro STEP lodsq jmp *(%rax) .endm INTERPRET: mov (%rsi), %rcx mov $EXIT, %rdx lea 8(%rbp), %rbx cmp %rcx, %rdx # tail call? cmovz (%rbp), %rsi # if so, we cmovz %rbx, %rbp # can reuse RPUSH %rsi # ret stack add $8, %rax mov %rax, %rsi STEP ¹ provided you're willing to point the footguns over at the return stack manipulation side of things instead
- biodniggnj 2y ago
  You don’t need recursion to make use of tail call elimination. In Scheme and SML all tail calls are eliminated. GCC also does it, but less often. Still, it’s not recursion that triggers it.