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The provenance memory model for C

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224 points HexDecOctBin | 2 comments | 30 Jun 25 09:25 UTC | HN request time: 0s | source
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nikic ◴[30 Jun 25 20:45 UTC] No.44427669[source]▶
At least at a skim, what this specifies for exposure/synthesis for reads/writes of the object representation is concerning. One of the consequences is that dead integer loads cannot be eliminated, as they may have an exposure side effect. I guess C might be able to get away with it due to the interaction with strict aliasing rules. Still quite surprised that they are going against consensus here (and reduces the likelihood that these semantics will get adopted by implementers).
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1. alextingle ◴[01 Jul 25 09:27 UTC] No.44432092[source]▶
I don't imagine that the exposed state would need to be represented in the final compiler output, so the optimiser could mark the pointer as exposed, but still eliminate the dead integer load.

Or from a pragmatic viewpoint, perhaps if the optimiser eliminates a dead load, then don't mark the pointer as exposed? After all, the whole point is to keep track of whether a synthesised pointer might potentially refer to the exposed pointer's storage. There's zero danger of that happening if the integer load never actually occurs.

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2. Hercuros ◴[01 Jul 25 20:48 UTC] No.44437878[source]▶
I guess the internal exposure state would be “wrong” if the compiler removes the dead load (e.g in a pass that runs before provenance analysis).

However, if all of the program paths from that point onward behave the same as if the pointer was marked as exposed, that would be fine. It’s only “wrong” to track the incorrect abstract machine state when that would lead to a different behaviour in the abstract machine.

In that sense I suppose it’s no different from things like removing a variable initialisation if the variable is never used. That also has a side effect in the abstract machine, but it can still be optimised out if that abstract machine side effect is not observable.