From: Ralf Jung Date: Tue, 16 Jul 2019 11:46:11 +0000 (+0200) Subject: wording X-Git-Url: https://git.ralfj.de/web.git/commitdiff_plain/a38f68a926b1ff86d7bf2a0f235f093121c957c3 wording --- diff --git a/personal/_posts/2019-07-14-uninit.md b/personal/_posts/2019-07-14-uninit.md index ebb1fdc..85c8dcb 100644 --- a/personal/_posts/2019-07-14-uninit.md +++ b/personal/_posts/2019-07-14-uninit.md @@ -78,7 +78,7 @@ When new memory gets allocated for a local variable (on the stack) or on the hea Every location stores a `None`. (In LLVM, this `None` corresponds to `poison`, which [has the potential to replace `undef` entirely](http://www.cs.utah.edu/~regehr/papers/undef-pldi17.pdf).) -When writing safe Rust, you do not have to worry about this, but this is the model you should have in your head when dealing with uninitialized memory in unsafe code. +When writing safe Rust, you do not have to worry about this, but this is the model that is good to have in your head when dealing with uninitialized memory in unsafe code. Alexis wrote a [great post](https://gankro.github.io/blah/initialize-me-maybe/) on which APIs to use for that in Rust; there is no need for me to repeat all that here. (In that post, Alexis says that every *bit* can be either 0, 1 or uninitialized, as opposed to every *byte* being initialized or not. Given that memory accesses happen at byte granularity, these two models are actually equivalent, at least in Rust which does not have C-style bitfields.) @@ -123,7 +123,7 @@ After all, x86 does not have "uninitialized bytes" either, and still our example Of course, to explain *why* the abstract machine is defined the way it is, we have to look at optimizations and hardware-level concerns. But without an abstract machine, it is very hard to ensure that all the optimizations a compiler performs are consistent---in fact, both [LLVM](https://bugs.llvm.org/show_bug.cgi?id=35229) and [GCC](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65752) suffer from miscompilations caused by combining optimizations that all seem fine in isolation, but together cause incorrect code generation. -The abstract machine is needed as an ultimate arbiter that shows if all of the optimizations are correct, or if some of them are in conflict with each other. +The abstract machine is needed as an ultimate arbiter that determines which optimizations can be safely combined with each other. I also think that when writing unsafe code, it is much easier to keep in your head a fixed abstract machine as opposed to a set of optimizations that might change any time, and might or might not be applied in any order. Unfortunately, in my opinion not enough of the discussion around undefined behavior in Rust/C/C++ is focused on what concretely the "abstract machine" of these languages looks like. @@ -133,7 +133,7 @@ In Rust, I am very happy that we have [Miri](https://github.com/rust-lang/miri/) I hope C/C++ will come around to do the same, and there is some [great work in that direction](https://www.cl.cam.ac.uk/~pes20/cerberus/), but only time will tell to what extend that can affect the standard itself. If you want to do me a favor, please spread the word! -I am trying hard to combat the myth of "what the hardware does" in Rust discussions whenever I see it, but I obviously don't see all the discussions---so the next time you see such an argument, no matter whether it is about uninitialized memory or [concurrency](http://hboehm.info/boehm-hotpar11.pdf) or [out-of-bounds memory accesses](https://github.com/rust-lang/rust/issues/32976#issuecomment-446775360) or anything else, please help by steering the discussion towards "what the Rust abstract machine does", and how we can design and adjust the Rust abstract machine in a way that it is most useful for programmers and optimizing compilers alike. +I am trying hard to combat the myth of "what the hardware does" in Rust discussions whenever I see it, but I obviously don't see all the discussions---so the next time you see such an argument around and undefined behavior is involved, no matter whether it is about uninitialized memory or [concurrency](http://hboehm.info/boehm-hotpar11.pdf) or [out-of-bounds memory accesses](https://github.com/rust-lang/rust/issues/32976#issuecomment-446775360) or anything else, please help by steering the discussion towards "what the Rust abstract machine does", and how we can design and adjust the Rust abstract machine in a way that it is most useful for programmers and optimizing compilers alike. As usual, if you have any comments, suggestions or questions, [let me know in the forums](https://internals.rust-lang.org/t/what-the-hardware-does-is-not-what-your-program-does-uninitialized-memory/10561).