On Thu, Aug 11, 2011 at 12:15 PM, Richard Guenther <richard.guent...@gmail.com> wrote: > On Thu, Aug 11, 2011 at 7:13 PM, Joe Buck <joe.b...@synopsys.com> wrote: >> On Thu, Aug 11, 2011 at 09:05:19AM -0700, Florian Merz wrote: >>> If I remember the standard correctly, pointer subtraction is valid if both >>> pointers point to elements of the same array or to one past the last element >>> of the array. According to this 0x80000000 - 0x7FFFFFFF should be a valid >>> pointer subtraction with the result 0x00000001. >>> >>> But if the subtraction is treated as a signed, this would be an signed >>> integer >>> overflow, as we subtract INT_MAX from INT_MIN, which surely must overflow, >>> and >>> the result therefore would be undefined. >> >> It is true that the C and C++ languages make signed integer overflow >> undefined, but that's for actual integer types as declared by the user. >> For pointers, though the subtraction has to be signed (because, for two >> pointers, either can can come later in the address space), this signed >> subtraction has to be defined to work in a two's complement fashion (so >> the wraparound in your example case works reliably). > > Of course GCC can't (yet) do both at the same time.
yes, but GCC should mark its internal artifacts so that it surely distinguishes user-provided abstractions (which may be subjected to harsh treatments) from its own blessed babies. > Thus we have to > use unsigned arithmetic when we want two's complement arithmetic.
