Support more arguments to tail calls by passing them through the domain state #10595

…in state In 2004, commit af9b98f, the calling conventions for the i386 port of ocamlopt were changed: the first 6 integer arguments go into registers, like before, but the next 16 arguments go into a global array `caml_extra_params`, instead of being passed on stack like before. The reason for this hack is that passing arguments in global memory does not preclude tail call optimization, unlike passing arguments on stack. Parameters passed via `caml_extra_params` are immediately copied on stack or in registers on function entry, before another function call, a GC, or a context switch can take place, so everything is safe in OCaml, and in Multicore OCaml as long as there is only one execution domain. This hack was justified by the paucity of registers provided by the i386 architecture. It was believed that other architectures provide enough registers for parameter passing that most if not all reasonable tail calls can be accommodated. Now it's 2021 and users want tail calls with more arguments than available registers on all the architectures we support. So, biting the bullet and swallowing some pride, this commit extends the 2004 i386 hack to all the architectures supported by OCaml. Once the registers available for passing function arguments are exhausted, the next 64 arguments are passed in a memory area that is part of the domain state. This argument passing is compatible with tail calls, so we get guaranteed tail calls up to 70 arguments (in the worst case). The domain state is used instead of a global array so that (1) this is compatible with Multicore OCaml and concurrent execution of multiple domains, and (2) we benefit from efficient addressing from the domain state register. For i386, we don't have a domain state register, and Multicore OCaml will support only one domain on this architecture, so we keep using a global `caml_extra_params` array; only, its size was increased to support 64 arguments.

Works both in 32 and 64 bits.

Integer registers 0 to 15 are now available for returning results, not 0 to 7 as before.

- Test tail calls to other functions, not just to self - Test up to 32 arguments.

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Support more arguments to tail calls by passing them through the domain state #10595

Support more arguments to tail calls by passing them through the domain state #10595

Commits on Sep 9, 2021