Re: [Valgrind-developers] We should use -Wextra, or at least -Wtype-limits

[Konstantin S. <kon...@gm...>]

On Mon, Feb 23, 2009 at 9:30 AM, Nicholas Nethercote
<n.n...@gm...> wrote:
> Hi,
>
> This code is from coregrind/m_libcbase.c:
>
> void* VG_(memmove)(void *dest, const void *src, SizeT sz)
> {
>   SizeT i;
>   if (sz == 0)
>      return dest;
>   if (dest < src) {
>      for (i = 0; i < sz; i++) {
>         ((UChar*)dest)[i] = ((UChar*)src)[i];
>      }
>   }
>   else if (dest > src) {
>      for (i = sz - 1; i >= 0; i--) {
>         ((UChar*)dest)[i] = ((UChar*)src)[i];
>      }
>   }
>   return dest;
> }
>
> It has a 50% chance of crashing or looping infinitely.  Why?  The
> condition of the for-loop in the else-branch never fails, because i is
> unsigned.  Fortunately this function is not actually used anywhere.

Anywhere in the trunk -- yes.
But I've hit this crash in Dec when I started using STL in valgrind
(STL uses memmove).

The patch I use is this:
  else if (dest > src) {
     UChar *d = (UChar*)dest + sz - 1;
     UChar *s = (UChar*)src + sz - 1;
     for (; d >= (UChar*)dest; d--, s--) {
        *d = *s;
     }
  }


>
> The warning -Wtype-limits finds this problem, and 185 other ones like
> it (ie. comparisons that are always true or false) in the core and
> tools, and some more in the tests.  I've looked at a few of these,
> some of them are just redundant assertions that an unsigned value is
> greater than zero, but even those are worrying.
>
> I'd like to turn this warning on.  Possibly via -Wextra, which brings
> some other warnings into the mix, but if so then -Wno-sign-compare and
> -Wno-unused-parameter would also be needed to avoid lots of IMHO
> unimportant warnings.  Thoughts?

--kcc



>
> Nick
>
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