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You make some very good points, and I agree with your conclusion, viz it's a trade-off. I'm still wondering if the OP (@duped) meant the same thing as you did,

You make some very good points, and I agree with your conclusion, viz it's a trade-off. I'm still wondering if the OP (@duped) meant the same thing as you did, or if he had another use-case in mind when he said "calling conventions". FWIW ... > But the Ruby code needs the arity of the code that's being called to be passed along. Putting it in a register that's not used for arguments in the standard calling convention means that when calling out to e.g. glibc the C code just won't be able to access those values. In general, when a call returns you can't trust the register contents for those registers that are unused in the calling convention, because the callee might clobber it (for example by calling back into your language before returning). What I did[1] when doing the ffi to the external C functions was emit a `setjmp()`, call the function, and emit `longjmp()` when the callee returned. That allowed recursive calls no matter how many times that FFI boundary is crossed in a single function call: The language calls into C API, which calls something that invokes a callback in the language, which makes another call to a C function, etc. [1] IOW, this is not the usual way, as far as I know. All a bit hackier than when I (for a different toy language) used `libffi` when emitting the code to call external C functions, which (IIRC) makes certain gaurantees about some of the registers.

vidarh 3y ago
> In general, when a call returns you can't trust the register contents for those registers that are unused in the calling convention, because the callee might clobber it (for example by calling back into your language before returning). Depends on the register and the calling convention. E.g. for i386 the Intel ABI requires that eax, edx, and ecx are caller-saved, and so can be clobbered by the called function, but the rest are callee-saved, and so you can rely on them surviving just fine if you're calling any code generated by a compiler conforming to the Intel ABI. So calling out to C-code is fine. (Calling back into the Ruby code requires extra effort, because the C code can't know how to set the arity; I might change this to store the arity of a method so that it can be looked up by the caller, in which case it might be doable to not require setting it at the time of the call, but that increases the amount of code at each call site as you'd need to resolve the method address and look it up relative to the actual method address instead of an indirect call, in order to avoid race conditions if someone redefines a method (we do love redefining methods at runtime in Ruby; it doesn't actually happen that often, but it can in theory happen at any time, and proving it won't for a given piece of code is a pain); it might be worth it at some point, even optionally, as when you can safely cache the method address you'd be able to jump straight past the arity check if you can guarantee the right number of arguments, but it's not simple).

vidarh 3y ago

> In general, when a call returns you can't trust the register contents for those registers that are unused in the calling convention, because the callee might clobber it (for example by calling back into your language before returning). Depends on the register and the calling convention. E.g. for i386 the Intel ABI requires that eax, edx, and ecx are caller-saved, and so can be clobbered by the called function, but the rest are callee-saved, and so you can rely on them surviving just fine if you're calling any code generated by a compiler conforming to the Intel ABI. So calling out to C-code is fine. (Calling back into the Ruby code requires extra effort, because the C code can't know how to set the arity; I might change this to store the arity of a method so that it can be looked up by the caller, in which case it might be doable to not require setting it at the time of the call, but that increases the amount of code at each call site as you'd need to resolve the method address and look it up relative to the actual method address instead of an indirect call, in order to avoid race conditions if someone redefines a method (we do love redefining methods at runtime in Ruby; it doesn't actually happen that often, but it can in theory happen at any time, and proving it won't for a given piece of code is a pain); it might be worth it at some point, even optionally, as when you can safely cache the method address you'd be able to jump straight past the arity check if you can guarantee the right number of arguments, but it's not simple).