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objdump very long run time when using -D -z flags

Fangrui Song i@maskray.me
Fri May 1 01:22:11 GMT 2020 
On Thu, Apr 30, 2020 at 7:26 AM Haim Shimonovich via Binutils
<binutils@sourceware.org> wrote:
>
> Hi,
> This problem occurs when defining a large uninitialized array or section.
> A simple example can recreate this bug:
>
> #include "stdio.h"
> int arr[1000000];
> int  main(){
>                 printf("hello world\n");
>                 return 0;
> }
>
> I traced the problem to the following highlighted code (objdump.c):
>
>   addr_offset = start_offset;
>   while (addr_offset < stop_offset)
>     {
>       bfd_vma z;
>       bfd_boolean need_nl = FALSE;
>       int previous_octets;
>
>       /* Remember the length of the previous instruction.  */
>       previous_octets = octets;
>       octets = 0;
>
>       /* Make sure we don't use relocs from previous instructions.  */
>       aux->reloc = NULL;
>
>       /* If we see more than SKIP_ZEROES octets of zeroes, we just
>        print `...'.  */
>          for (z = addr_offset * opb; z < stop_offset * opb; z++)
>                 if (data[z] != 0)
>                     break;
>
> I suggest the following fix (performing the loop only when -z is not used):
>
>
> addr_offset = start_offset;
>   while (addr_offset < stop_offset)
>     {
>       bfd_vma z;
>       bfd_boolean need_nl = FALSE;
>       int previous_octets;
>
>       /* Remember the length of the previous instruction.  */
>       previous_octets = octets;
>       octets = 0;
>
>       /* Make sure we don't use relocs from previous instructions.  */
>       aux->reloc = NULL;
>
>       /* If we see more than SKIP_ZEROES octets of zeroes, we just
>        print `...'.  */
>        if (! disassemble_zeroes){
>          for (z = addr_offset * opb; z < stop_offset * opb; z++)
>                 if (data[z] != 0)
>                     break;
> }
>
> I would like to hear any comments regarding my suggestion.
>
> Thanks,
> Haim Shimonovich
>

This just hides the problem. -z is still going to be slow. To avoid
the quadratic behavior, we should increase the address by at least `z`
if we have scanned `z` zeros.
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