Re: [GCC 4.3] Strange -O2 optimization issue

Mateusz Loskot wrote:

> Why the first value printed is different (136623933) in the 3rd
> test case.

Your suspicion is correct, as this violates the ISO C aliasing rules:

> static unsigned long HashDouble(double* pdfVal)
> {
>      unsigned int* pnValue = (unsigned int*)pdfVal;

You're accessing a variable of type double through a pointer of type
unsigned int.  For the purposes of optimization the compiler is allowed
to assume that values of type double will only be accessed through
variables of type double, and thus it can assume that pdfVal and pnValue
can't refer to the same thing.  It may seem nonsensical in this instance
that it would assume that, but it's still legal for the compiler to do
so.  And being able to make this assumption allows for interesting
optimizations, for example consider something like:

typedef struct { int size; float *data; } foo;

void bar (foo *src, foo *dest)
{
  for (int i = 0; i < src->size; i++)
    dest->data[i] += src->data[i];
}

In this example all the stuff happening in the loop with the data arrays
involves floats so the compiler can prove to itself that src->size (an