[patch] libstdc++/66327 don't pass null pointers to memcmp

[Jonathan Wakely]

std::equal((int*)0, (int*)0, p) and std::equal(p, p, (int*)0) are
valid for any input iterator p, and must not pass a null pointer to
memcpy.

Similarly, std::lexicographical_compare((int*)0, (int*)0, p, q) and
std::lexicographical_compare(p, q, (int*)0, (int*)0) are valid for any
input iterators p and q, and must not pass a null pointer to memcpy.

This is a rather brute force fix, but if noone has any better ideas
I'll commit this to the trunk and 4.9 and 5 branches tomorrow. (I
think it should go on the branches, because 4.9 is known to optimise
away null pointer checks after invalid calls to memcmp like this).

I am not adding tests for these as we have no way to use ubsan in the
testsuite at present. I plan to make that possible, and will go back
and add tests using -fsanitize=undefined for all the recent fixes I've
made for ubsan errors.

Tested powerpc64le-linux.

commit 323133a6f623e827c7e70fa6918c7149e8932443
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Thu May 28 20:23:22 2015 +0100

    	PR libstdc++/66327
    	* include/bits/stl_algobase.h (__equal<true>::equal): Do not call
    	memcmp with null pointers.
    	(__lexicographical_compare<true>::__lc): Do not call memcmp for empty
    	ranges.

diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index 409ef36..db065e2 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -812,6 +812,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         static bool
         equal(const _Tp* __first1, const _Tp* __last1, const _Tp* __first2)
         {
+	  if (__first1 == 0 || __first2 == 0)
+	    return __first1 == __last1;
+
 	  return !__builtin_memcmp(__first1, __first2, sizeof(_Tp)
 				   * (__last1 - __first1));
 	}
@@ -917,9 +920,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	{
 	  const size_t __len1 = __last1 - __first1;
 	  const size_t __len2 = __last2 - __first2;
-	  const int __result = __builtin_memcmp(__first1, __first2,
-						std::min(__len1, __len2));
-	  return __result != 0 ? __result < 0 : __len1 < __len2;
+	  if (__len1 && __len2)
+	    {
+	      if (int __result = __builtin_memcmp(__first1, __first2,
+						  std::min(__len1, __len2)))
+		{
+		  return __result < 0;
+		}
+	    }
+	  return __len1 < __len2;
 	}
     };