On Sun, Nov 28, 2021 at 4:11 PM Jan Hubicka <hubi...@kam.mff.cuni.cz> wrote: > > > Hi, > > In function ref_maybe_used_by_call_p_1, there is below code snippet > > /* The following builtins do not read from memory. */ > > case BUILT_IN_FREE: > > ... > > return false; > > > > I am confused because free function does read from (and even write to) > > memory pointed to by passed argument? > > Free is a black box and makes the memory pointed to disappear without > actually worrying what values it holds. We rely on fact that we do not > see free imlementation and does not worry about the details of its > implementation (whcih probably has sort of linked list before address > ptr points to) > > I am thinking DSE optimizations like: > > *ptr = value; > > free(ptr); > > *ptr = undef; > > Does GCC take advantage of UB to eliminate the first store to ptr if > > free is considered not reading memory? > > The aim here is to optimize out *ptr = value. > > Honza Thanks very much for explaining.
Thanks, bin
