how do the -fvolatile- switches work?

[Marcus Binny]

I could not figure out how does the -fvolatile or
-fvolatile-global switches work? 

for code like

extern int flag;

void foo ()
{
	while (!flag) ;
}

-O2 keeps flag into a register only. I tried
-fvolatile switches but no help. I wanted -O2 to load
globals from memory only. Is that possible from
command line? without declaring globals volatile.

M



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