From: Ralf Jung Date: Wed, 25 Jul 2018 07:50:04 +0000 (+0200) Subject: clarify X-Git-Url: https://git.ralfj.de/web.git/commitdiff_plain/5cdb512d4c051c4b3d6e7432f1c4cacc46dc9fa8 clarify --- diff --git a/personal/_posts/2018-07-24-pointers-and-bytes.md b/personal/_posts/2018-07-24-pointers-and-bytes.md index adf82bd..05690b3 100644 --- a/personal/_posts/2018-07-24-pointers-and-bytes.md +++ b/personal/_posts/2018-07-24-pointers-and-bytes.md @@ -137,7 +137,8 @@ However, this simple model starts to fall apart once you consider pointer-intege In miri, casting a pointer to an integer does not actually do anything, we now just have an integer variable (i.e., its *type* says it is an integer) whose *value* is a pointer (i.e., an allocation-offset pair). However, multiplying that "integer" by 2 leads to an error, because it is entirely unclear what it means to multiply such an abstract pointer by 2. -This is the most lazy thing to do, and we do it because it is not clear what else to do -- in our abstract machine, there is no single coherent "address space" that all allocations live in, that we could use to map every pointer to a distinct integer. +This is the most lazy thing to do, and we do it because it is not clear what else to do (other than not supporting these casts at all -- but this way, miri can run more programs). +In our abstract machine, there is no single coherent "address space" that all allocations live in, that we could use to map every pointer to a distinct integer. Every allocation is just identified by an (unobservable) ID. We could now start to enrich this model with extra data like a base address for each allocation, and somehow use that when casting an integer back to a pointer... but that's where it gets really complicated, and anyway discussing such a model is not the point of this post. The point it to discuss the *need* for such a model.