On 9/23/20 2:51 PM, Arvind Sankar wrote: > On Wed, Sep 23, 2020 at 02:17:30PM -0500, Madhavan T. Venkataraman wrote: >> >> >> On 9/23/20 4:11 AM, Arvind Sankar wrote: >>> For libffi, I think the proposed standard trampoline won't actually >>> work, because not all ABIs have two scratch registers available to use >>> as code_reg and data_reg. Eg i386 fastcall only has one, and register >>> has zero scratch registers. I believe 32-bit ARM only has one scratch >>> register as well. >> >> The trampoline is invoked as a function call in the libffi case. Any >> caller saved register can be used as code_reg, can it not? And the >> scratch register is needed only to jump to the code. After that, it >> can be reused for any other purpose. >> >> However, for ARM, you are quite correct. There is only one scratch >> register. This means that I have to provide two types of trampolines: >> >> - If an architecture has enough scratch registers, use the currently >> defined trampoline. >> >> - If the architecture has only one scratch register, but has PC-relative >> data references, then embed the code address at the bottom of the >> trampoline and access it using PC-relative addressing. >> >> Thanks for pointing this out. >> >> Madhavan > > libffi is trying to provide closures with non-standard ABIs as well: the > actual user function is standard ABI, but the closure can be called with > a different ABI. If the closure was created with FFI_REGISTER abi, there > are no registers available for the trampoline to use: EAX, EDX and ECX > contain the first three arguments of the function, and every other > register is callee-save. > > I provided a sample of the kind of trampoline that would be needed in > this case -- it's position-independent and doesn't clobber any registers > at all, and you get 255 trampolines per page. If I take another 16-byte > slot out of the page for the end trampoline that does the actual work, > I'm sure I could even come up with one that can just call a normal C > function, only the return might need special handling depending on the > return type. > > And again, do you actually have any example of an architecture that > cannot run position-independent code? PC-relative addressing is an > implementation detail: the fact that it's available for x86_64 but not > for i386 just makes position-independent code more cumbersome on i386, > but it doesn't make it impossible. For the tiny trampolines here, it > makes almost no difference. > Hi Arvind, I am preparing a response for all of your comments. I will send it out tomorrow. Sorry for the delay. Madhavan
