Volatiles Are Miscompiled, and What to Do about It [pdf]

Imagine you work on a production quality C compiler. The pedestrian pieces (e.g., lexer, parser, AST) have been stable forever. The piece that you're very likely going to be working on is the optimizer (or maybe the code generator) to make use of new techniques or new instructions. While you're going about your business, you're probably not thinking about that arcane corner of the C language spec that discusses volatile. What's more, when you finally complete your feature, all of the compiler's test suites pass because there's insufficient coverage for volatile.

The biggest contribution of this paper, besides the fact that it identified this issue across various compilers, is the notion of access summary testing and advocating for it to be included as part of the test suite for C compilers.

In particular, it says "For every read from or write to a volatile variable that would be performed by a straightforward interpreter for C, exactly one load from or store to the memory location(s) allocated to the variable must be performed."

This is wrong. It later kind of gets it right for C, explaining about sequence points, but it entirely misses that implementations are free to combine and eliminate multiple volatile accesses within the same sequence point.

Now certainly, most of what was reported were genuine bugs (and John reports a lot of correctness issues). But it does/did nobody favors to start with an incorrect definition.

Sounds like "volatile" variables don't really provide good semantics for most uses even without considering compiler bugs, so it's better to just use explicit load and store macros or functions.

"Although the symptoms of this compiler bug—spurious periodic reboots due to failure to reset the watchdog timer—may be relatively benign, the situation could be worse, for example, if the hardware register were used to lower control rods, cancel a missile launch, or open the pod bay doors."

So that's what the problem was with those pod bay doors.

i'm a little surprised at how much worse clang fared.