Comment by throwaway_pdp09 | Hacker News Reader

Comment by throwaway_pdp09 | Hacker News Reader

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throwaway_pdp09

Yep. They have a compiler to bring it down to the metal so IDK what you're saying.

--- EDIT ---

@saagarjha, as I'm being slowposted by HN, here's my response via edit:

OK, sure! You need some agreed semantics for that, at the low level. But the hardware guys aren't likely to add actors in the silicon. And they presumably don't intend to support eg. hardware level malloc, nor hardware level general expression evaluation[0], nor hardware level function calling complete with full argument handling, nor fopen, nor much more.

BTW "The metal which largely respects C's semantics?" C semantics were modelled after real machinery, which is why C has variables which can be assigned to, and arrays which follow very closely actual memory layout, and pointers which are for the hardware's address handling. If the C designers could follow theory rather than hardware, well, look at lisp.

[0] IIRC the PDPs had polynomial evaluation in hardware.

The metal which largely respects C's semantics? For example, here are some instructions that exist to match C's atomics model: https://developer.arm.com/documentation/den0024/a/Memory-Ord...

I've done work on a proprietary embedded RTOS that has had high level versions of those barriers at least a decade before the C atomics model was standardized (and compiles them to the closest barrier supported by the target architecture).

I suspect that the OS and Architecture communities have known about one-way barriers for a very long time, and they were only recently added to the Arm architecture because people only recently started making Arm CPUs that benefit from them. And that seems like a more likely explanation than them having been plucked from the C standard.

Moreover, one-way barriers are useful regardless of what language you're using.

Note that I am specifically pointing to those exact barriers, and not "any old barriers". C's memory orderings don't really lower down to a single instruction on any other platform that I'm aware of because of subtle differences in semantics.

[0] Close, it was the VAX-11 and is the poster child for CISC madness.

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