Monday, May 23, 2016 [Tweets] [Favorites]

C Compilers Break for Objects Larger Than PTRDIFF_MAX Bytes

Pascal Cuoq (via Jean-Francois Roy):

The malloc call in the program below succeeds (the program would return 0 if the malloc invocation failed), despite Clang having the same half-the-address-space limitation for 64-bit as for 32-bit.

[…]

Unfortunately, an optimizing compiler’s code appears to be littered with optimizations that assume object sizes are less than PTRDIFF_MAX. As an example, we all know that X86 Clang and GCC implement pointer comparison as an assembly unsigned comparison (“cmpl” is an agnostic comparison instruction, but here it is followed by “cmoval”, where the “a”, for “above”, indicates conditional move based on an unsigned comparison). This means that it is allowed to compare pointers inside a putative 2.5GiB array on a 32-bit platform, right?

Not all pointer comparisons.

Because of this optimization.

Comments

Stay up-to-date by subscribing to the Comments RSS Feed for this post.

Leave a Comment