Incorrect result from pointer subtraction if result is >2Gbytes

[Dominic Plunkett]

32bit gcc 8.2 -o2

If you have an area of memory which is greater than 2Gbytes with a start and end pointers. Then if you do end -start
the compiler generates a signed shift right ASR and thus bit 31 stays set. The compiler needs to check for a carry from the subtraction and either do ASR #2 or LSR #2

See below for an example.

https://godbolt.org/z/iVE4J5

[Ramana Radhakrishnan (ramana)]

Firstly, this is undefined behaviour in as much as pointer arithmetic on 2 unrelated values. When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object

What exactly is your concern ?

[Dominic Plunkett (dp111)]

I'm sorry I over simplified the example. I think the compiler should be using LSR instead of ASR as pointers are unsigned.

https://godbolt.org/z/ehLsMq