Re: libalias patch for review / testing

Ruslan Ermilov <ru_at_freebsd.org> writes:
> : `-fstrict-aliasing'
> :      Allows the compiler to assume the strictest aliasing rules
> :      applicable to the language being compiled.  For C (and C++), this
> :      activates optimizations based on the type of expressions.  In
> :      particular, an object of one type is assumed never to reside at
> :      the same address as an object of a different type, unless the
> :      types are almost the same.  For example, an `unsigned int' can
> :      alias an `int', but not a `void*' or a `double'.  A character type
> :      may alias any other type.
>
> And asking myself a question: should those (void *)'s in your patch
> be (char *)'s instead, e.g., in twowords() and DifferentialChecksum(),
> or am I misreading the above?

You're misreading, we're doing u_short * <-> void * (both pointers)
but the man page speaks about int <-> void * (scalar vs pointer)

Also, I doubt DifferentialChecksum() is a problem, since it's a
function call.  I think the problem may be in the code I've replaced
with calls to twowords().

> Also, the easiest way to check if strict aliasing is guilty for not
> working libalias compiled with -O2, is to compile the original code
> with -O2 -fno-strict-aliasing.

True, I didn't think of that.

DES
-- 
Dag-Erling Smørgrav - des_at_des.no