Re: [PATCH] Fix for call feature having nine parameters or more in AIX

[Ulrich Weigand via Gdb-patches <gdb-patches@sourceware.org>]

Aditya Kamath1 <Aditya.Kamath1@ibm.com> wrote:

>So, debugging further I realized that the parameters of function in AIX
>are stored in registers 3 to 10. More about this fact can be read in this
>document {https://www.ibm.com/docs/en/aix/7.2?topic=overview-register-usage-conventions}.
>If the function has more than 8 parameters then the 9th one onwards,  we store
>the function parameters in the stack. This can be seen in the rs6000-aix-tdep.c
>file in the dummy_call function from line 700 and beyond. Over here we have
>this line below.
>
>write_memory (sp + 24 + (ii * 4), arg->contents ().data (), len);
> 
>This the root cause of this issue.

I agree you've identified a problem, but I think your patch isn't quite complete.

For example, immediately after the code you changed follows:
          ii += ((len + 3) & -4) / 4;
The intent is to always uses full stack slots even for arguments of odd sizes.

But I understand in the 64-bit ABI the stack slot size is 8 bytes, so this
should round up to the next multiple of 8.

Similarly, you need to make sure that the first loop that computes the *size*
of the stack that will be used for arguments performs the same calculations
as the code that actually fills in the arguments - or else you can overwrite
unrelated areas:

      if (argbytes)
        {
          space += ((len - argbytes + 3) & -4);
          jj = argno + 1;
        }
      else
        jj = argno;


      for (; jj < nargs; ++jj)
        {
          struct value *val = args[jj];
          space += ((val->type ()->length ()) + 3) & -4;
        }

All of this should round up to wordsize instead of 4, I guess.

Bye,
Ulrich