On Thursday, 3 December 2015 at 13:02:24 UTC, Steven
Schveighoffer wrote:
First I will say, there is confusion on what is valid and what
is not. Misaligned pointers are pointers that are stored
misaligned. In other words, they are stored not on a 4-byte or
8-byte boundary for 32 bits or 64 bits arch respectively.
An interior pointer is a pointer that is *properly aligned* but
does not point at the first byte of a piece of memory.
taggedPointer and taggedClassRef create *interior pointers*,
not *misaligned pointers*. Andrei's proposal will create
*misaligned pointers*. There is a huge difference.
I can make an interior pointer without casts on any type:
SomeType *pointer = ...;
void[] p = pointer[0..1];
p = p[1..$];
If the GC does not support this being the only pointer to a
memory location, then the GC is not suitable for D. Period.
Code will break in subtle ways if you use such a GC.
I can't see how a language with void* and/or unions could allow
such a GC.
Indeed, I was talking about interior pointers. But you're right,
I missed the fact that void pointers (and some others) can be
valid interior pointers even to unstructured values. So the
optimization I had in mind is not applicable in D, anyway.
We should then just adjust the specification to specifically
allow changing the LSBs.
