> On Dec 9, 2021, at 6:36 PM, Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx> wrote: > >> >> The code generated by the Gambit compiler is sufficiently complex that it would be impossible (in the halting problem sense) to check that the tail-calling constraints are not violated. However, I know the constraints are not violated because the code was designed that way. And if I’m mistaken in this belief, then I am the one that will bare the blame of the ensuing problems. > > On most architectures you can simply check if there are any "return" > instructions generated, no? Or do you generate any actual calls (to > user code) as well? > You mean a “return” at the assembly code level? Seems very brittle… on x86 a “pop %eax; jmp *%eax” will do the same as a “ret” and gcc might generate that sequence (from my point of view). In any case, as I said previously, I don’t want to check if gcc did not optimize the tail calls… it is too late to do anything other than essentially tell the programmer “sorry I couldn’t compile that program correctly”. >> So having a way to check during or after the compilation that tail-calls were optimized is not sufficient. A Scheme compiler that sometimes fails to compile is not very useful. What is needed is a guarantee that specific C calls are optimized. > > Then you need to write code that works as you want on all configurations > you support, state what those configs are, and test that it compiles on > all such configs, before every release. This does not solve my problem either. No testing will cover all possible situations because for a particular architecture the tail call optimization may work on all the tests I can come up with, but some programmer using Gambit may write a program for which the tail calls in the C code generated by the Gambit compiler are not optimized. I’m not looking for a guarantee that tail calls are supported transparently on all architectures. Currently Gambit uses a trampoline to implement Scheme tail calls, and this can be used as a fallback for architectures where C TCO is not guaranteed. So what I need is some way to test at compile time if for the target architecture and compile options there is a guarantee that tail calls will be optimized (if the C code obeys the usual set of constraints). Something like #if __has_attribute(tail_call_supported_on_this_architecture) /* use tail calls for jumping around... */ #else /* use trampoline fallback... */ #endif Marc
