optimization - Why doesn't my C++ compiler optimize these memory writes away? -

i created program. nothing of interest use processing power.

looking @ output objdump -d, can see 3 rand calls , corresponding mov instructions near end when compiling o3 .

why doesn't compiler realize memory isn't going used , replace bottom half while(1){}? i'm using gcc, i'm interested in required standard.

/*  * create program nothing except slow down computer.  */ #include <cstdlib> #include <unistd.h>  int getrand(int max) {   return rand() % max; }  int main() {   (int thread = 0; thread < 5; thread++) {     fork();   }   int len = 1000;   int *garbage = (int*)malloc(sizeof(int)*len);   (int x = 0; x < len; x++) {     garbage[x] = x;   }   while (true) {     garbage[getrand(len)] = garbage[getrand(len)] - garbage[getrand(len)];   } } 

because gcc isn't smart enough perform optimization on dynamically allocated memory. however, if change garbageto local array instead, gcc compiles loop this:

.l4:     call    rand     call    rand     call    rand     jmp .l4 

this calls rand repeatedly (which needed because call has side effects), optimizes out reads , writes.

if gcc smarter, optimize out randcalls, because side effects affect later randcalls, , in case there aren't any. however, sort of optimization waste of compiler writers' time.