[Scummvm-devel] Broken Sword 2 problem

[Willem Jan Palenstijn]

On Tue, Aug 31, 2010 at 06:26:41PM +0300, yotam barnoy wrote:
> For the (hopefully) last episode in this bug saga, see commit 52473.

Wow, very nice catch! Well done!

If I understand your commit message correctly, the issue is that some of the
READ_*_UINT* functions break strict aliasing rules? It would probably be good
to try to fix that instead of using -fno-strict-aliasing, if possible.


The subtleties of these strict aliasing rules still elude me, but it might be
the case that the issue is that the two Unaligned32 structs are local to their
functions, and might be considered different.

I'm trying with this code:

inline __attribute__((__always_inline__)) int read_value(const void* ptr)
{
        struct Unaligned32 { int val; } __attribute__ ((__packed__));
        return ((const Unaligned32 *)ptr)->val;
}

inline __attribute__((__always_inline__)) void write_value(void* ptr, int value)
{
        struct Unaligned32 { int val; } __attribute__ ((__packed__));
        ((Unaligned32 *)ptr)->val = value;
}

int test(char* x) {
        int y = read_value(x);
        write_value(x, y+1);
        return read_value(x);
}

When compiled with g++ 4.5.1 on x86_64 with -O3 and strict aliasing, it returns
y, and not y+1.

When I move the definition of Unaligned32 out of the functions, it returns y+1:

struct Unaligned32 { int val; } __attribute__ ((__packed__));
inline __attribute__((__always_inline__)) int read_value(const void* ptr)
{
        return ((const Unaligned32 *)ptr)->val;
}
inline __attribute__((__always_inline__)) void write_value(void* ptr, int value)
{
        ((Unaligned32 *)ptr)->val = value;
}

int test(char* x) {
        int y = read_value(x);
        write_value(x, y+1);
        return read_value(x);
}


Does it behave the same for you on your platform?

-Willem Jan