Re: writing a jump table

[Nicolas Bock <nicolasbock@gmail.com>]

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I think I figured it out now. I used gcc to compile PIC for the C switch
statement and checked what it does. I don't fully understand it to be
honest, but it seems to do the job also for non PIC code. For posterity,
here is the code:

  .text
  .global jump_table
  .type jump_table, @function

jump_table:
  # Push stack pointer so we can make room for local storage.
  push %rax

  mov $0x02, %rax # Move index into rax; 2 is supposed to end up at
label_02.

  lea 0(,%rax, 4), %rdx
  lea table(%rip), %rax
  mov (%rdx, %rax), %edx
  movslq %edx, %rdx
  lea table(%rip), %rax
  lea (%rdx, %rax), %rax
  jmp *%rax

  .section .rodata
  .align 4
table:
  .long label_00-table
  .long label_01-table
  .long label_02-table
  .long label_03-table

  .text
label_00:
  jmp done

label_01:
  jmp done

label_02:
  jmp done

label_03:
  jmp done

done:
  pop %rax
  ret

  .size jump_table, .-jump_table


On 03/15/11 18:25, Brian Raiter wrote:
>> I am trying to write a jump table, but unfortunately with limited
>> success. When I compile the code and disassemble it, the offset of
>> "table" is 0, which I guess means that something didn't work out.
> 
> Actually, table appears immediately following your indirect jump
> instruction, so I would assume that the offset would be zero. Did you
> actually try this code to verify that it doesn't do what you expect?
> 
> b
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