[Bug target/81801] [PATCH] Difference of two pointers generates signed overflow

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81801

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Fri, 11 Aug 2017, oss at malat dot biz wrote:

> One could solve it by dividing both pointers by the size before the
> subtraction, but that would make the operation more expensive.

See also what I said in bug 67999 comment 24.

The combination (subtract to produce a ptrdiff_t result, with modulo 
arithmetic on overflow, then divide in ptrdiff_t) isn't correct for 
objects too large in bytes for ptrdiff_t.  But the following should be 
correct: right shift (logical) both pointers for the power-of-2 factor in 
what you are dividing by, subtract (modulo), multiply (modulo) by the (mod 
2^n) reciprocal of the odd part of what you are dividing by, interpret the 
result as signed.

It's less efficient than code sequences that only work for objects less 
than half the address space, and such large objects can never work if you 
do pointer subtraction of pointers to char that are too far apart to be 
represented in ptrdiff_t.  I think it makes most sense to disallow such 
large objects properly (including in malloc and mmap), but an option to 
support pointer subtraction in them would not be ridiculous.