Respected GDB community members,

Hi,

Please find attached a patch. {See: 0001-Fix-to-step-instruction-due-to-P10-prefix-instructio.patch}

This is a patch to fix the SIGILL instruction coming in AIX running on a power 10 machine.

Consider a program p10-test.c

cat ~/p10_test.c
#include <stdio.h>
int global_variable = 2;
unsigned long
add (unsigned long a)
{
  printf ("%ld is the num \n", a + 0x12345U);
  return a + 0x12345U;
}
int main(){
  unsigned long local_variable = 1;
  add (local_variable);
  printf ("Simple print statement \n");
  printf ("Hello Bengaluru \n");
  return 0;
}

The disassembly equivalent of the a + 0x12345U has a paddi instruction. This instruction is 8 bytes as shown in the GDB output of the disassemble of add () in the above code.

Breakpoint 2, add (a=1) at /home/buildusr/p10_test.c:6
6         printf ("%ld is the num \n", a + 0x12345U);
(gdb) disassemble
Dump of assembler code for function add:
   0x10000538 <+0>:       mflr    r0
   0x1000053c <+4>:       stw     r0,8(r1)
   0x10000540 <+8>:       stw     r31,-4(r1)
   0x10000544 <+12>:      stwu    r1,-80(r1)
   0x10000548 <+16>:      mr      r31,r1
   0x1000054c <+20>:      stw     r3,104(r31)
=> 0x10000550 <+24>:      lwz     r9,104(r31)
   0x10000554 <+28>:      paddi   r9,r9,74565
   0x1000055c <+36>:      mr      r4,r9
   0x10000560 <+40>:      lwz     r3,64(r2)
   0x10000564 <+44>:      bl      0x10000648 <printf>
   0x10000568 <+48>:      lwz     r2,20(r1)
   0x1000056c <+52>:      lwz     r9,104(r31)
   0x10000570 <+56>:      paddi   r9,r9,74565
   0x10000578 <+64>:      mr      r3,r9
   0x1000057c <+68>:      addi    r1,r31,80
   0x10000580 <+72>:      lwz     r0,8(r1)

Since during a stepi we are stepping 4 bytes and once we press stepi after we execute instruction at 0x10000554 we get

(gdb) si
0x10000554        6         printf ("%ld is the num \n", a + 0x12345U);
(gdb)

Program received signal SIGILL, Illegal instruction.
0x10000554 in add (a=1) at /home/buildusr/p10_test.c:6
6         printf ("%ld is the num \n", a + 0x12345U);

The reason being we stepped into an illegal instruction.

In this patch we use (insn & 0xfc000000) == 1 << 26 to check for the p10 instruction and then calculate the location at which stepi needs to be done.

The output after we apply this patch is.

In 32 bit mode:- stepi jumps 8 bytes when required.
Breakpoint 2, add (a=1) at /home/buildusr/p10_test.c:6
6         printf ("%ld is the num \n", a + 0x12345U);
(gdb) disassemble
Dump of assembler code for function add:
   0x10000538 <+0>:       mflr    r0
   0x1000053c <+4>:       stw     r0,8(r1)
   0x10000540 <+8>:       stw     r31,-4(r1)
   0x10000544 <+12>:      stwu    r1,-80(r1)
   0x10000548 <+16>:      mr      r31,r1
   0x1000054c <+20>:      stw     r3,104(r31)
=> 0x10000550 <+24>:      lwz     r9,104(r31)
   0x10000554 <+28>:      paddi   r9,r9,74565
   0x1000055c <+36>:      mr      r4,r9
   0x10000560 <+40>:      lwz     r3,64(r2)
   0x10000564 <+44>:      bl      0x10000648 <printf>
   0x10000568 <+48>:      lwz     r2,20(r1)
   0x1000056c <+52>:      lwz     r9,104(r31)
   0x10000570 <+56>:      paddi   r9,r9,74565
   0x10000578 <+64>:      mr      r3,r9
   0x1000057c <+68>:      addi    r1,r31,80
   0x10000580 <+72>:      lwz     r0,8(r1)
   0x10000584 <+76>:      mtlr    r0
   0x10000588 <+80>:      lwz     r31,-4(r1)
   0x1000058c <+84>:      blr
   0x10000590 <+88>:      .long 0x0
   0x10000594 <+92>:      .long 0x2061
   0x10000598 <+96>:      lwz     r0,257(r1)
   0x1000059c <+100>:     .long 0x0
   0x100005a0 <+104>:     .long 0x58
   0x100005a4 <+108>:     .long 0x36164
   0x100005a8 <+112>:     oris    r31,r0,0
End of assembler dump.
(gdb) si
0x10000554        6         printf ("%ld is the num \n", a + 0x12345U);
(gdb)
0x1000055c        6         printf ("%ld is the num \n", a + 0x12345U);
(gdb)
0x10000560        6         printf ("%ld is the num \n", a + 0x12345U);

In 64 bit mode:- stepi jumps 8 bytes when required.

Breakpoint 1, add (a=1) at /home/buildusr/p10_test.c:6
6         printf ("%ld is the num \n", a + 0x12345U);
(gdb) disassemble
Dump of assembler code for function add:
   0x00000001000006c0 <+0>:       mflr    r0
   0x00000001000006c4 <+4>:       std     r0,16(r1)
   0x00000001000006c8 <+8>:       std     r31,-8(r1)
   0x00000001000006cc <+12>:      stdu    r1,-128(r1)
   0x00000001000006d0 <+16>:      mr      r31,r1
   0x00000001000006d4 <+20>:      std     r3,176(r31)
=> 0x00000001000006d8 <+24>:      ld      r9,176(r31)
   0x00000001000006dc <+28>:      paddi   r9,r9,74565
   0x00000001000006e4 <+36>:      mr      r4,r9
   0x00000001000006e8 <+40>:      ld      r3,120(r2)
   0x00000001000006ec <+44>:      bl      0x1000007a4 <printf>
   0x00000001000006f0 <+48>:      ld      r2,40(r1)
   0x00000001000006f4 <+52>:      ld      r9,176(r31)
   0x00000001000006f8 <+56>:      paddi   r9,r9,74565
   0x0000000100000700 <+64>:      mr      r3,r9
   0x0000000100000704 <+68>:      addi    r1,r31,128
   0x0000000100000708 <+72>:      ld      r0,16(r1)
   0x000000010000070c <+76>:      mtlr    r0
   0x0000000100000710 <+80>:      ld      r31,-8(r1)
   0x0000000100000714 <+84>:      blr
   0x0000000100000718 <+88>:      .long 0x0
   0x000000010000071c <+92>:      .long 0x2061
   0x0000000100000720 <+96>:      lwz     r0,257(r1)
   0x0000000100000724 <+100>:     .long 0x0
   0x0000000100000728 <+104>:     .long 0x58
   0x000000010000072c <+108>:     .long 0x36164
   0x0000000100000730 <+112>:     oris    r31,r0,0
End of assembler dump.
(gdb) si
0x00000001000006dc        6         printf ("%ld is the num \n", a + 0x12345U);
(gdb)
0x00000001000006e4        6         printf ("%ld is the num \n", a + 0x12345U);
(gdb) q

Setting a breakpoint at that instruction

(gdb) b add
Breakpoint 1 at 0x100006d8: file /home/buildusr/p10_test.c, line 6.
(gdb) r
Starting program: /home/buildusr/p10_test_64

Breakpoint 1, add (a=1) at /home/buildusr/p10_test.c:6
6         printf ("%ld is the num \n", a + 0x12345U);
(gdb) disassemble
Dump of assembler code for function add:
   0x00000001000006c0 <+0>:       mflr    r0
   0x00000001000006c4 <+4>:       std     r0,16(r1)
   0x00000001000006c8 <+8>:       std     r31,-8(r1)
   0x00000001000006cc <+12>:      stdu    r1,-128(r1)
   0x00000001000006d0 <+16>:      mr      r31,r1
   0x00000001000006d4 <+20>:      std     r3,176(r31)
=> 0x00000001000006d8 <+24>:      ld      r9,176(r31)
   0x00000001000006dc <+28>:      paddi   r9,r9,74565
   0x00000001000006e4 <+36>:      mr      r4,r9
   0x00000001000006e8 <+40>:      ld      r3,120(r2)
   0x00000001000006ec <+44>:      bl      0x1000007a4 <printf>
   0x00000001000006f0 <+48>:      ld      r2,40(r1)
   0x00000001000006f4 <+52>:      ld      r9,176(r31)
   0x00000001000006f8 <+56>:      paddi   r9,r9,74565
   0x0000000100000700 <+64>:      mr      r3,r9
   0x0000000100000704 <+68>:      addi    r1,r31,128
   0x0000000100000708 <+72>:      ld      r0,16(r1)
   0x000000010000070c <+76>:      mtlr    r0
   0x0000000100000710 <+80>:      ld      r31,-8(r1)
   0x0000000100000714 <+84>:      blr
   0x0000000100000718 <+88>:      .long 0x0
   0x000000010000071c <+92>:      .long 0x2061
   0x0000000100000720 <+96>:      lwz     r0,257(r1)
   0x0000000100000724 <+100>:     .long 0x0
   0x0000000100000728 <+104>:     .long 0x58
   0x000000010000072c <+108>:     .long 0x36164
   0x0000000100000730 <+112>:     oris    r31,r0,0
End of assembler dump.
(gdb) b *0x00000001000006dc
Breakpoint 2 at 0x1000006dc: file /home/buildusr/p10_test.c, line 6.
(gdb) b *0x00000001000006e4
Breakpoint 3 at 0x1000006e4: file /home/buildusr/p10_test.c, line 6.
(gdb) c
Continuing.

Breakpoint 2, 0x00000001000006dc in add (a=1) at /home/buildusr/p10_test.c:6
6         printf ("%ld is the num \n", a + 0x12345U);
(gdb) c
Continuing.

Breakpoint 3, 0x00000001000006e4 in add (a=1) at /home/buildusr/p10_test.c:6
6         printf ("%ld is the num \n", a + 0x12345U);
(gdb) c
Continuing.
74566 is the num
Simple print statement
Hello Bengaluru
[Inferior 1 (process 6685154) exited normally]
(gdb)

So, I would like your views and guidance on this. Though this patch fixes things in this example we might need to take care many other things which I might not be aware of since this involves step and breakpoint.

Kindly let me know what you all think and fix this issue for GDB and AIX.

Awaiting a reply,

Have a nice day ahead.

Thanks and regards,
Aditya.