[hedora]

But (other than floating point, which is surprising, if true for word and smaller values on machines that actually have floating point units), all of those examples are either non-portable assembly code (SIMD) or directly violate the C memory model (unaligned access).

You can't dereference a non-aligned pointer in portable C. It will bus error on certain architectures (including some arm variants).

(I've written plenty of code that relies on unaligned reads, and also that relies on non-torn reads/writes, just not at the same time, and never when portability was a concern.)

[Kranar]

We're jumping all over the place I'm afraid. If you're talking about writing standard/portable C then data races are undefined behavior and hence there is no portable manner in which a data race can be observed. A C compiler is free to produce any observable behavior whatsoever in the presence of a data race.

If you wish to discuss x86 or ARM, well a data race can occur in a C program through the use of SIMD instructions or writing through an unaligned pointer. If you want to pick an architecture that does not allow unaligned accesses, sure we can discuss the PowerPC 500 series where unaligned accesses are a bus error, but then reads and writes of 32 bit values are not atomic and hence can produce data races.

We can't mix properties of one architecture with properties of another architecture and also discuss portable C. Any consideration of data races or undefined behavior in general must be specific to a particular architecture and we must apply the rules of any given architecture consistently.