5 ms·

How do functions that not end in ret work?

to11mtm 2y ago
My gut (been a while since I've been that low level) is various forms of inlining and/or flow continuation (which is kinda inlining, except when we talk about obfuscation/protection schemes where you might inline but then do fun stuff on the inlined version.)
duskwuff 2y ago
The return is somewhere before the end of the function, e.g. loop: do stuff if some condition: return do more stuff goto loop Alternatively, the function might end with a tail-call to another function, written as an unconditional branch.
jcranmer 2y ago
There are things like compiling a tail call as JMP func_addr.
- frogsRnice 2y ago
  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.
- russdill 2y ago
  Yes, I think the most common is a tail call. There also of course can be several ret's from a single function.
DSMan195276 2y ago
In addition to what others said, I'd simply point out that all 'ret' does on x86 is pop an address off the top of the stack and jump to it. It's more of a "helper" than a special instruction and it's use is never required as long as you ensure the stack will be kept correct (such as with a tail-call situation).
- dcrazy 2y ago
  `ret` also updates the branch predictor’s shadow stack. Failing to balance `call` and `ret` can seriously impact performance.
  - dkersten 2y ago
    If anyone else is looking for more information on this, like I was, this stack is called the “return stack buffer”.
  - DSMan195276 2y ago
    Right, I didn't want to get into it but definitely using 'ret' "properly" has big performance benefits. My point was just that it won't prevent your code from running, it's not like x86 will trigger an exception if they don't match up.
- ack_complete 2y ago
  RET does more these days. If Intel CET is enabled then it also updates the hardware shadow stack, and the program will crash if RET is bypassed unless the SSP is adjusted. IIRC Windows x64 also has pertinent requirements on how the function epilog restores registers and returns since it will trace portions of the instruction stream during stack unwinding.
ngneer 2y ago
If compilation uses jmp2ret mitigation, a trailing ret instruction will be replaced by a jmp to a return thunk. It is up to the return thunk to do as it pleases with program state.
mananaysiempre 2y ago
A function with an unlikely slowpath can easily end up arranged as top part jxx slow fast middle part end: bottom part ret slow: slow middle part jmp end There may be more than one slow part, the slow parts might actually be exiled from inside a loop and not a simple linear code path and can themselves contain loops, etc. Play with __builtin_expect and objdump --visualize-jumps a bit and you’ll encounter many variations.

My gut (been a while since I've been that low level) is various forms of inlining and/or flow continuation (which is kinda inlining, except when we talk about obfuscation/protection schemes where you might inline but then do fun stuff on the inlined version.)

duskwuff 2y ago

The return is somewhere before the end of the function, e.g. loop: do stuff if some condition: return do more stuff goto loop Alternatively, the function might end with a tail-call to another function, written as an unconditional branch.

jcranmer 2y ago

There are things like compiling a tail call as JMP func_addr.

frogsRnice 2y ago
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.
russdill 2y ago
Yes, I think the most common is a tail call. There also of course can be several ret's from a single function.

DSMan195276 2y ago

In addition to what others said, I'd simply point out that all 'ret' does on x86 is pop an address off the top of the stack and jump to it. It's more of a "helper" than a special instruction and it's use is never required as long as you ensure the stack will be kept correct (such as with a tail-call situation).

dcrazy 2y ago
`ret` also updates the branch predictor’s shadow stack. Failing to balance `call` and `ret` can seriously impact performance.
- dkersten 2y ago
  If anyone else is looking for more information on this, like I was, this stack is called the “return stack buffer”.
- DSMan195276 2y ago
  Right, I didn't want to get into it but definitely using 'ret' "properly" has big performance benefits. My point was just that it won't prevent your code from running, it's not like x86 will trigger an exception if they don't match up.
ack_complete 2y ago
RET does more these days. If Intel CET is enabled then it also updates the hardware shadow stack, and the program will crash if RET is bypassed unless the SSP is adjusted. IIRC Windows x64 also has pertinent requirements on how the function epilog restores registers and returns since it will trace portions of the instruction stream during stack unwinding.

ngneer 2y ago

If compilation uses jmp2ret mitigation, a trailing ret instruction will be replaced by a jmp to a return thunk. It is up to the return thunk to do as it pleases with program state.

mananaysiempre 2y ago

A function with an unlikely slowpath can easily end up arranged as top part jxx slow fast middle part end: bottom part ret slow: slow middle part jmp end There may be more than one slow part, the slow parts might actually be exiled from inside a loop and not a simple linear code path and can themselves contain loops, etc. Play with __builtin_expect and objdump --visualize-jumps a bit and you’ll encounter many variations.