Darryl Gove's blog

Identifying misaligned loads in 32-bit code using dtrace

A previous blog entry talks about handling and detecting misaligned memory accesses. For 64-bit code this is easy to achieve using the Performance Analyzer, for 32-bit code the analysis is a bit more tricky. Fortunately it is possible to do the 32-bit analysis with dtrace

Consider the following program which has a misaligned memory access. The default mode of the compiler (since Sun Studio 9) will compile the binary to trap to fix the misalignment and continue

% more align.c
void main()
{
  volatile char a[10];
  int i;
  for (i=0; i<100000000; i++) {(*(int*)(&a[1]))++;}
}

The following dtrace script will instrument the misaligned data access trap handler and report all the pids that trigger this

% more tr.d
fbt::do_unaligned:entry
{
  @p[pid]=count();
}

It can be run with

% sudo dtrace -s tr.d
dtrace: script 'tr.d' matched 1 probe
^C


    14873           260932

The script returns the pid which is having misalignment issues. This information is useful, in that it is trivial to recompile the binary with a different setting for -xmemalign and avoid the behaviour. But it would be very useful to know where the traps are occuring in the binary - perhaps most of the traps only happen in one place, and that place can be fixed in the source.

% more tr.d
fbt::do_unaligned:entry
{
  @[ustack()]=count();
}

This script produces output that identifies the locations in the binary where the traps are being generated. For the simple test code there are two locations - the load and the store.

sudo dtrace -s tr.d
dtrace: script 'tr.d' matched 1 probe
^C
              align`main+0x10
              align`_start+0x108
           130466

              align`main+0x18
              align`_start+0x108
           130466

The disassembly for the loop is as follows

main()
        10b80:  9d e3 bf 90  save       %sp, -112, %sp
...
        10b90:  d0 07 bf f7  ld         [%fp - 9], %o0  <<<<<< misaligned
        10b94:  90 02 20 01  inc        %o0
        10b98:  d0 27 bf f7  st         %o0, [%fp - 9]  <<<<<< misaligned
        10b9c:  ba 07 60 01  inc        %i5
        10ba0:  80 a7 40 09  cmp        %i5, %o1
        10ba4:  06 bf ff fb  bl         main+0x10       ! 0x10b90
        10ba8:  01 00 00 00  nop

Posted at 01:00PM Jun 12, 2007 by Darryl Gove in Sun  |