Comment by IshKebab
10 hours ago
There are still modern CPUs that don't support misaligned access. It would be insane for C to mandate that misaligned accesses are supported.
However I do agree that just saying "the behaviour is undefined" is an unhelpful cop-out. They could easily say something like "non-atomic misaligned accesses either succeed or trap" or something like that.
> In the end it's the CPU and not the compiler which decides whether an unaligned access is a problem or not.
Not just the CPU - memory decides as well. MMIO devices often don't support misaligned accesses.
> They could easily say something like "non-atomic misaligned accesses either succeed or trap" or something like that.
That means that the compiler must emit the read, even if the value is already known or never used, as it might trap. There is a reason for the UB!
No it doesn't. Compilers are only required to emit the read for volatile types. If the type is non-volatile, misaligned, and can be optimised out then it would be perfectly fine to omit it (that would be the "succeed" option).
If a trap is observable behaviour, then the compiler either needs to add code, that checks for the condition and then traps explicitly or it needs to actually perform the read. Currently it can be optimized out, because it is UB.
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On hardware that doesn't support it, misaligned loads could be compiled to multiple loads and shifts. Probably not great for performance, and it doesn't work if you need it to be atomic, but it isn't impossible.
That still requires detecting when a misaligned load happens.
That is only really possible if you know the pointer is misaligned at compile time (which does happen, e.g. for packed structs). The examples in the article are for runtime misalignment. It would be crazy to generate code so that every function checked if every access was aligned at runtime.
(Note the normal way to handle that if the hardware doesn't actually support it is for the access to trap and then the OS or firmware emulates it.)
For x86 SSE there are aligned instructions that will trap on unaligned access.