Comment by bluGill
8 hours ago
The pointer might be something you forced. The compiler needs to do the right thing but if you set the pointer to an unaligned address because you have information on the hardware you can get this undefined situation with nothing the compiler can do about it.
Any reason the hardware pointer can't be accessed via the packed structure?
https://news.ycombinator.com/item?id=48205371
The same reason you probably aren’t adding manual alignment fixes to your code?
No reason at all, then. Because I am manually dealing with alignment in my code.
Wrote a lisp, its bytes type supports reading and writing integers at arbitrary locations within the buffer. Test suite exercises aligned and unaligned memory access for every C integer type. Also wrote my own mem* functions, dealing with alignment in those was certainly a fun exercise. It wasn't necessary, I just wanted the performance benefits.
however you certainly can do that. The point of unaligned is the hardware can't load it from a single memory location in one address. It needs two accesses. And in that time, the value of one of the two addresses that the hardware has to load can change.
I would hope you're not so stupid as to design hardware that relies on this, but the fact is it certainly is possible for someone to do that. And if you do that, there is nothing that the compiler or the standard can do. It can't be done correctly
Yeah, the unaligned accesses aren't going to be atomic unless the hardware supports it.
> And in that time, the value of one of the two addresses that the hardware has to load can change.
You mean volatile addresses that could spontaneously change in the middle of the reads? Like memory mapped I/O addresses?
I would expect these to have stricter access requirements than arbitrary general purpose memory locations.
> I would hope you're not so stupid as to design hardware that relies on this
You and me both.
> And if you do that, there is nothing that the compiler or the standard can do. It can't be done correctly
Anything that does that is broken and terrible anyway. It really shouldn't contaminate language design. It's the sort of thing that compilers should be adding attributes for, rather than constraining the language to the point nothing works correctly and making us use attributes on everything to restore some sane baseline behavior.
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