Comment by matheusmoreira
7 hours ago
If some architecture traps on unaligned access, then the compiler can and should simply generate the correct code so that it loads the integer piece by piece instead. Load multiple integers and shift and mask away the irrelevant bits, done. This is exactly what modern architectures already do in hardware. Works, it's just a little slower.
This is exactly what the compilers do if you use a packed structure to access unaligned data. Works everywhere, as expected. Compilers have always known what to do, they just weren't doing it. C standard says no.
The fact is the standard is garbage and the first thing every C programmer should learn is that they can and should ignore it. There is never any reason to wonder what the standard is supposed to do. The only thing that matters is what compilers actually do.
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?
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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
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But if it's a pointer, the compiler doesn't know the alignment at compile time. Should the compiler insert an alignment check of every pointer access?
Compilers could add support for an unaligned attribute that we can apply to pointers. I'd prefer that to wrapping everything in a packed structure which is quite unsightly.
Would have been better if correct behavior was the default while pointer alignment requirements were opt in, just like vector stuff. Nothing we can do about it now.
I would hope the compiler is smart enough to figure out which accesses are aligned and unaligned on its own.