Is this compiler transformation allowed?

Question

Consider this code, where x and y are integers:

if (x)
    y = 42;

Is the following compiler transformation allowed ?

int tmp = y;
y = 42;

if (!x)
    y = tmp;

context:

This is from Bjarne Stroustrup's FAQ:

// start with x==0 and y==0

if (x) y = 1;   // Thread 1 

if (y) x = 1;   // Thread 2

The FAQ states this is data race free; with x and y both 0, none of the vars should be written to.
But what if the transformation is allowed ?

Answer 1

Unlike I wrote in my incorrect comment, this transformation is actually not allowed if y is potentially shared between threads and the compiler cannot prove any existing UB in the original code.

The standard explicitly says:

Compiler transformations that introduce assignments to a potentially shared memory location that would not be modified by the abstract machine are generally precluded by this standard, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race.

[intro.multithread] (1.10/22) in N3337, (1.10/25) in N4141.

So if x is always 0, the original code would be race-free, while the transformed one wouldn't. Thus the transformation is not legal.