gcc -O3，数据指针似乎丢失了

Question

Depending on optimization level the output differ as follows:

With unexpected output:

$ gcc -Wall -O3  otest.c -o otest                   
$ otest                                             
*x: 0 
y: 2048.899902 
y: 0.000000 

With expected output:

$ gcc -Wall -O2  otest.c -o otest 
$ otest                           
*x: 45000e66 
y: 0.000000 
y: 2048.899902

source code :

#include <stdio.h>
int main(void)
{
   float y = 2048.9;
   void *p = &y;
   unsigned int *x = p;
   printf(" *x: %x \n",*x);
   *x = 0; 
   printf(" y: %f \n",y);
   *x = 0x45000e66;
   printf(" y: %f \n",y);  
   return 0;
 }

gcc version is 4.2.1.

Am I missing any important directive ?

Answer 1

Yes. Your code is violating the strict aliasing rule (when you have a float , but you access it through a pointer to unsigned int , which is an incompatible type), invoking undefined behavior, so the compiler is allowed to do anything it pleases with your code, including entirely eliminating parts of it.