undefined | Better HN

0 pointscodeflo2y ago0 comments

Of course, that's technically incorrect. The way the standards are written, the compiler is free to replace the program with any other program that has the same (in a precisely defined sense) observable behavior (these are the famous "as if" formulations in language specs). Heating up the CPU is not considered observable behavior.

If someone really just wants a delay, it's easy to either (for programs running on normal OSs) call a sleep function, or (on tiny embedded systems) add an empty inline assembler statement that the compiler can't see through.

0 comments

carlmr2y ago

>Heating up the CPU is not considered observable behavior.

Neither is measuring delays of cached versus non-cached instructions. Yet it turns out to be very observable.

codefloOP2y ago

Of course these things are “observable” in the literal sense. And yet, they aren’t considered to be observable by the memory model of any language spec that I know of. Same as CPU power draw, which has been used as a side-channel to extract bits of crypto keys, and is very much influenced by common optimizations.

Practically, if you need to execute a specific sequence of machine instructions in order to prevent side-channel attacks, then you have to rely on assembler, compiler intrinsics and/or OS support. But that was true way before Spectre.

j / k navigate · click thread line to collapse