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Re: help interpreting gcc 4.1.1 optimisation bug


On 6/12/06, andrew@walrond.org <andrew@walrond.org> wrote:
On Mon, Jun 12, 2006 at 01:01:47PM +0100, Andrew Haley wrote:
>
> I'm starting to be a little suspicious about host2little().  I wonder
> if that's well-defined.
>
> I'm just guessing here becasue I can't see the code.  Is it possible

    template<typename T> T swap_endian(T x)
    {
        char* a = reinterpret_cast<char*>(&x);
        char* b = a + sizeof(T) - 1;
        while (a<b) {
            char tmp = *a;
            *a++ = *b;
            *b-- = tmp;
        }
        return x;
    }

    template<typename T> T host2little(T x)
    {
        unsigned u = 1; return (*reinterpret_cast<char*>(&u) ? x : swap_endian(x));
    }

You are violating aliasing rules. Try -fno-strict-aliasing, which should make the code work, or fix it.

Richard.


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