Re: strict aliasing benefit examples

[Paolo Bonzini]

for(i = 0;i<*t;i++)
  *f += 1.0;

This one is pretty realistic, especially if you consider C++ and inlining:

        struct s {
          int size;
          float *data;
        };

        void f(struct s *d, struct s *s)
        {
          int i;
          for (i = 0; i < s->size; i++)
            d->data[i] += s->data[i];
        }

can be optimized by GCC to this:

        lwz r0,0(r4)
        cmpwi cr7,r0,0
        blelr- cr7
        lwz r3,4(r3)
        mtctr r0
        lwz r4,4(r4)
        li r9,0
L4:
        slwi r2,r9,2
        addi r9,r9,1
        lfsx f0,r3,r2
        lfsx f13,r4,r2
        fadds f0,f0,f13
        stfsx f0,r3,r2
        bdnz L4
        blr

Note that the only load in the loop is for vec->data[i]. vec->size is 
known not to change during the loop.

With an "int *data" it is impossible to disambiguate.  However, 
base+offset alias analysis can disambiguate the accesses to s->size and 
d->data[i] if struct s is defined as follows:

struct s {
  int size;
  int data[1];
};

Paolo