bug#41357: 28.0.50; GC may miss to mark calle safe register content

[Paul Eggert <eggert@cs.ucla.edu>]

On 5/17/20 10:24 AM, Eli Zaretskii wrote:
> I struggle to see
> how a Lisp object could appear in a register without leaving any trace
> on the stack

Quite easily. It happens all the time. If I do something like this:

    Lisp_Object a = Fcons (b, c);
    f (x, y);
    return a;

The compiler might put 'a' into a callee-save register R, which means that while
f is running there's no trace of 'a' on the stack (unless f's code itself
decides to use R for whatever reason, but let's suppose it doesn't). This
situation can persist even if f calls g which calls h which calls the garbage
collector, and the garbage collector will then think the cons is garbage even
though it's not.

The proposed fix is harmless except it may execute a handful more instructions
per GC. So the cost of applying the fix is tiny, whereas the potential
reliability benefit is large.