> The C abstract machine, despite looking a lot like a PDP-11, leaks the underlying memory and ordering semantics of the architecture being targeted. The result is that even seasoned C programmers regularly rely on architecture-specific assumptions when writing ostensibly cross-platform code: assumptions about the atomicity of reads and writes, operation ordering, coherence and visibility in self-modifying code, the safety and performance of unaligned accesses, and so forth. Each of these, apart from being a potential source of unsafety, are impossible to detect statically in the general case: they are, after all, perfectly correct (and frequently intended!) on the programmer’s host architecture.
I don't disagree with his general point that C isn't really cross-platform, but these are bad examples:
- The C memory model addresses the issues regarding atomicity, memory ordering, and the safety of unaligned accesses (although users might not like the answer for the last one).
- Coherence within a C program is guaranteed by the language, so issues with coherence only arise when interacting with an incoherent external agent. How could any language solve this?
- Are there any examples of self-modifying code that don't already make assumptions about the underlying ISA? If you're already doing that, dealing with data/instruction memory consistency doesn't seem particularly onerous. Even when only targeting AArch64, it isn't possible to write platform-independent userspace assembly code that does this because instruction cache maintenance instructions might not be enabled for EL0.
I don't disagree with his general point that C isn't really cross-platform, but these are bad examples:
- The C memory model addresses the issues regarding atomicity, memory ordering, and the safety of unaligned accesses (although users might not like the answer for the last one).
- Coherence within a C program is guaranteed by the language, so issues with coherence only arise when interacting with an incoherent external agent. How could any language solve this?
- Are there any examples of self-modifying code that don't already make assumptions about the underlying ISA? If you're already doing that, dealing with data/instruction memory consistency doesn't seem particularly onerous. Even when only targeting AArch64, it isn't possible to write platform-independent userspace assembly code that does this because instruction cache maintenance instructions might not be enabled for EL0.