Why does gcc compiler behave differently in these two cases?

Question

I have two similar while loops and am curious why C compiler behaves differently in each case.

Details: Compiler: x86-64 gcc 8.2

Options: -O3

void use() 
{ 
    int* ptr;
    while (*ptr == 6);
}

Assembly:

use:
        cmp     DWORD PTR ds:0, 6
        jne     .L1
.L3:
        jmp     .L3
.L1:
        ret

As we see, the compiler still generates an infinite loop. However, if I use this:

void use() 
{ 
    int p;
    while (p == 6);
} 

Assembly:

use:
        ret

I expected both of them to generate no while loop. My guess is that, for 1st version, since we cannot possibly know what ptr points to, we cannot assume anything about *ptr. But could we make a similar argument for the 2nd version, where p is just a local variable, ie p could contain anything?

Answer 1

Both versions have Undefined Behavior because you access and read uninitialized variables. As such the compiler can generate any code.