[Project-managers] Re: [Coin-tlc] lots of things to test

[Laszlo Ladanyi]

Just a note...

There are quite a number of warnings of the kind:
"dereferencing type-punned pointer will break strict-aliasing rules".

These warnings are there, because we use -fstrict-alias when gcc is used to
allow gcc to optimize better. However, I have found examples on the web where
a code for which this warning was emitted had actually been miscompiled.

So what should we do? As a short term solution we can just remove
-fstrict-alias, but it'd be better to get rid of the warning. It comes up
when, e.g., a double* is assigned to an int* or any type of pointer is cast to
a void*. The way to get rid of the warning is to use unions.

For example the following code triggers the warning (when the first part is
taken):
int hash(double value)
{
#if sizeof(int)*2 == sizeof(double)
	int *xx = (int*)(&value)
	xx[0] += xx[1];
	return xx[0];
#else
	return (int) value;
#endif
}

But the equivalent code does not (and I don't think this is any slower, in
fact it may well be faster since no memory dereferencing is needed, hence
the whole thing can even be executed in registers):
int hash(double value)
{
#if sizeof(int)*2 == sizeof(double)
	union { double d; int i[2]; } xx;
	xx.d = value;
	xx.i[0] += xx.i[1];
	return xx.i[0];
#else
	return (int) value;
#endif
}


Any thoughts?

Cheers,
--Laci