Trampolines twice in unix stack bootstrapping

[pchickey]

Hi - I was single-stepping through this code to make sure I understood how it worked, and I discovered that, at least on the x64 unix code (https://github.com/Xudong-Huang/generator-rs/blob/master/src/detail/x86_64_unix.rs), the trampoline ends up running twice.

The first swap context sets stack pointer to sp[-4] (using the sp defined in initialize_call_frame), and then returns into bootstrap_green_task. that function then puts fptr from R14 into sp[-2], returns again into bootstrap_green_task which is at sp[-3]. the copies from r12, r13, and r14 run again, harmlessly, and then finally it returns into fptr.

I believe the only necessary correction is to change mov %r14, 8(%rsp) to mov %r14, (%rsp) in bootstrap_green_task. The assignment of sp[-3] to bootstrap_green_task in initialize_call_frame can then be eliminated as well.

Alternately, you can not put fptr in regs.gpr[r14] at all, and put it directly into sp[-3], and then eliminate the mov %r14 from the bootstrap entirely.

However, it is also possible I have no idea what I'm talking about, because I've only learned Intel assembly programming very recently.

[Xudong-Huang]

You are right. The trampoline may run twice. When I was developing this project, I found that debug and release versions have different stack layout that generated by the compiler. we need to make sure on both versions that we can trap into bootstrap_green_task.

But this version is not as sophisticated as the libfringe version. In next release I will switch to the libfringe branch. It's much faster than the method currently used in master branch. please ref #4.

[pchickey]

That makes sense. Thanks for the explanation!