Only gcc doesn't do any of that. If you have more than one module (a.c and b.c in this case) which reserve different variables for the same register, gcc happily overwrites all of the variables with the last assigned one. (1/m)
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register int foo asm ("rbx");
int g(int a, int b) {
int t = rand()*100;
foo = t;
printf("foo inited to %d\n", t);
return foo;
}
void i() {
printf("foo = %d\n", foo);
} (2/m)
register int bar asm ("rbx");
int h(int a, int b) {
int t = rand()*2000;
bar = t;
printf("bar inited to %d\n", t);
return bar;
}
(3/m)
void i();
int main(void) {
int a = 10;
int b = 20;
int result = g(a, b);
printf("%d\n", result);
result = h(a, b);
printf("%d\n", result);
i();
return 0;
} (4/m)
foo inited to 40311868
40311868
bar inited to 1644657376
1644657376
foo = 1644657376
foo is overwritten because of the assignment to bar (which can very well be in an dynamically linked library whose source you can't modify). (5/m)