4 ms·

Can someone ELI5 what exactly happens when the external method call is "JIT"ed in a language like LuaJIT -- what does that mean? I understand calling dlopen()

Can someone ELI5 what exactly happens when the external method call is "JIT"ed in a language like LuaJIT -- what does that mean? I understand calling dlopen() and dlsym() And I understand this idea of a PLT and it's indirection But this idea of something external to the JIT'ed program being JIT'ed I do not understand. Does it mean it inlined the instructions of the external function into the JIT'ed code?

deleted 5y ago
[deleted]
deathanatos 5y ago
No, I don't think Lua (or the author) is inlining the external function. What's being JIT'd is the Lua code into machine code, and where Lua would need to call a C function, it (apparently) is just emitting a `call the_external_fn` into the JIT's resulting assembly. That's a direct function call, so it's about as fast as you're going to get, but somewhat counter-intuitively, it'll be faster than C, as we don't do anything with the PLT or indirection. Just call the function.
- gavinray 5y ago
  Ahh got it, that makes sense -- thanks!
- dataflow 5y ago
  I think you need a layer of indirection there - you can't just directly call a function in another library in x64, whereas the library itself can. So in general the library should have a speed advantage when calling itself in x64.