gdb: LoongArch: Handle special struct in dummy call

gdb: LoongArch: Handle special struct in dummy call

When execute the following command on LoongArch:

  make check-gdb TESTS="gdb.base/infcall-nested-structs-c++.exp"

there exist some failed testcases:

  === gdb Summary ===

  # of expected passes		5533
  # of unexpected failures	367

The root cause is related with a struct containing floating-point
members as function argument or return value for a dummy call.

(1) Structure consists of one floating-point member within FRLEN bits
    wide, it is passed in an FAR if available.
(2) Structure consists of two floating-point members both within FRLEN
    bits wide, it is passed in two FARs if available.
(3) Structure consists of one integer member within GRLEN bits wide and
    one floating-point member within FRLEN bits wide, it is passed in a
    GAR and an FAR if available.

Note that in the above cases, empty structure or union members are also
ignored even in C++.

Here is a simple test on LoongArch:

  loongson@bogon:~$ cat test.c

  #include<stdio.h>

  struct test {
	  long   a;
	  double b __attribute__((aligned(16)));
  };
  struct test val = { 88, 99.99 };
  int check_arg_struct (struct test arg)
    {
      printf("arg.a = %ld\n", arg.a);
      printf("arg.b = %f\n", arg.b);
      printf("sizeof(val) = %d\n", sizeof(val));
      return 1;
    }
  int main()
  {
     check_arg_struct (val);
     return 0;
  }
  loongson@bogon:~$ gcc -g test.c -o test
  loongson@bogon:~$ ./test
  arg.a = 88
  arg.b = 99.990000
  sizeof(val) = 32

Before:

loongson@bogon:~$ gdb test
...
(gdb) start
...
Temporary breakpoint 1, main () at test.c:19
19	   check_arg_struct (val);
(gdb) p check_arg_struct (val)
arg.a = 140737488286128
arg.b = -nan
sizeof(val) = 32
$1 = 1
...

After:

loongson@bogon:~$ gdb test
...
(gdb) start
...
Temporary breakpoint 1, main () at test.c:19
19	   check_arg_struct (val);
(gdb) p check_arg_struct (val)
arg.a = 88
arg.b = 99.990000
sizeof(val) = 32
$1 = 1
...

With this patch, there are no failed testcases:

  make check-gdb TESTS="gdb.base/infcall-nested-structs-c++.exp"

   === gdb Summary ===

   # of expected passes		5900

Signed-off-by: Hui Li <lihui@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>