Re: [PATCH] Speed-up use-after-scope (re-writing to SSA) (version 2)

[Martin Liška]

On 01/20/2017 12:49 PM, Martin Liška wrote:
> Great, thanks a lot. I'm going to re-trigger asan-bootstrap with your patch.
> I'm also adding gcc/testsuite/gcc.dg/asan/use-after-scope-10.c that is a valid
> test-case for this issue.

Hi.

Unfortunately this way would not work as clobber marks content of the memory as 
uninitialize
is different behavior that just marking a memory can be used (and maybe already 
contains a value).

This shows the problem:

#include <string.h>

char cc;
char ptr[] = "sparta2";

void get(char **x)
{
  *x = ptr;
}
  
int main()
{
  char *here = &cc;

  for (;;)
    {
    next_line:
        if (here == NULL)
          __builtin_abort();
        get (&here);
        if (strcmp (here, "sparta") == 0)
            goto next_line;
        else if (strcmp (here, "sparta2") == 0)
          break;
    }
}

With the patch, DSE would optimize out '*here = &cc;' and thus aborts. The 
problem is definitely
related to goto magic, where we are more defensive in placement of 
ASAN_MARK(UNPOISON,...).
Hope your optimization is still valid for situations w/o artificial 
ASAN_MARK(UNPOISON,...) placed due
to goto magic.

Do we still want to do it now, or postponing to GCC 8 would be better option?

Thanks,
Martin