Re: Avoid stack references in inline assembly

[Segher Boessenkool <segher at kernel dot crashing dot org>]

Hi!

On Mon, Jun 17, 2019 at 10:28:37PM +0200, Vincent Rivière wrote:
> My goal is to create optimal C bindings for Atari ST system calls, using 
> m68k-elf-gcc (tested with version 7.1.0). Basically, system calls are 
> similar to function calls: parameters are stacked in the reverse order, 
> last one being a function number. But there are 2 differences from standard 
> C calling convention:
> - actual jump to subroutine is replaced by trap #1
> - clobbered registers include d2/a2 in addition to standard d0-d1/a0-a1
> 
> Here is the problem. If I use "g" as parameter constraint, everything looks 
> fine, but GCC may replace the parameter token with a reference to a stack 
> location (specially when compiling with -fomit-frame-pointer). If another 
> parameter has already been pushed on the stack, the reference to the second 
> one is not properly adjusted, so the pushed address is wrong.

You could make a builtin for this, so that you can say something like

  retval = __builtin_syscall_m68k_st (syscall_nr, arg0, arg1);

(which can be varargs, take any number of args you support).  We could
have a generic builtin for this, hrm...  Like

  retval = __builtin_syscall (STYLE, syscall_nr, arg0, arg1);

with some target hook that checks if the STYLE is something it supports,
and then if the number and type of remaining args are okay.

I've never seen a target that could define their syscalls perfectly with
just existing facilities, inline asm is such a blunt intrument in some
ways.


Segher