Should ALL global variables be volatile-qualified?

Question

In this example, does correctness require global_value to be declared volatile?

int global_value = 0;

void foo () {
    ++ global_v         


        

Answer 1

Volatile isn't needed in this example. If, for instance, some_function() outputs something, the asm listing seems changes observable behaviour of the c++ machine and violates the standard.

I guess it is a compiler bug Here is GCC assembler output:

.cfi_def_cfa_register 5
subl    $24, %esp
.loc 1 67 0
movl    global_value, %eax
addl    $1, %eax
movl    %eax, global_value
movl    global_value, %eax
movl    %eax, (%esp)
call    _Z13some_functioni
.loc 1 68 0
call    _Z3foov
.loc 1 69 0
movl    global_value, %eax
addl    $1, %eax
movl    %eax, global_value
movl    global_value, %eax
movl    %eax, (%esp)
call    _Z13some_functioni
.loc 1 70 0
leave
.cfi_restore 5

global_value is reloaded, as expected, between function calls

Also volatiles are for thread-safety too, simply v-qualifier is not sufficient for thread safety in all cases (you sometimes need additional care about atomicity and memory barriers, but interthread communication variables should be volatile...

[EDITED]: ... if they are repeatedly read and may be changed by another thread between reads. This, however, is not the case if any syncronization lock (mutex, etc) is used, since lock guarantees the variables can not be changed by any concurrent activity) (thanks to R..)