From: Ralf Jung Date: Wed, 9 Dec 2020 12:24:47 +0000 (+0100) Subject: less 'we' X-Git-Url: https://git.ralfj.de/web.git/commitdiff_plain/0750a867ea3132ffb722bf921ff9cf0d185553ca?ds=inline;hp=c743b6ee1f0a499d86a64a29cb9377f9124909a2 less 'we' --- diff --git a/ralf/_drafts/provenance-matters.md b/ralf/_drafts/provenance-matters.md index ceb1be5..959aadd 100644 --- a/ralf/_drafts/provenance-matters.md +++ b/ralf/_drafts/provenance-matters.md @@ -15,10 +15,10 @@ There is also a larger message here about how we could prevent such issues from I will show a series of three compiler transformations that each seem "intuitively justified", but when taken together they lead to a clearly incorrect result. -We will use LLVM for these examples, but the goal is not to pick on LLVM---other compilers suffer from similar issues. +I will use LLVM for these examples, but the goal is not to pick on LLVM---other compilers suffer from similar issues. The goal is to convince you that to build a correct compiler for languages permitting unsafe pointer manipulation such as C, C++, or Rust, we need to take IR semantics (and specifically provenance) more seriously. -We use LLVM for the examples because it is particularly easy to study with its single, (comparatively) well-documented IR that a lot of infrastructure evolved around. +I use LLVM for the examples because it is particularly easy to study with its single, (comparatively) well-documented IR that a lot of infrastructure evolved around. Let's get started! ## Warm-up: Why IRs need a precise semantics @@ -112,7 +112,7 @@ if (iq == ip) { print(q[0]); } {% endhighlight %} -We are using C syntax here just as a convenient way to write programs in LLVM IR. +I am using C syntax here just as a convenient way to write programs in LLVM IR. For simplicity, we assume that `int` has the right size to hold a pointer value; just imagine we used `uintptr_t` if you want to be more general. This program has two possible behaviors: either `ip` (the address one-past-the-end of `p`) and `iq` (the address of `q`) are different, and nothing is printed. @@ -167,7 +167,7 @@ That's why the LLVM LangRef will not give us a clear answer here, and indeed obt To proceed, we will use the three optimizations that we considered above as cues: assuming that the optimization is correct for LLVM IR, what does that tell us about the semantics? -We start with the last optimization, where the `print` argument is changed from `q[0]` to `0`. +Let us start with the last optimization, where the `print` argument is changed from `q[0]` to `0`. This optimization is based on alias analysis: `q[0]` gets initialized to `0` at the beginning of the program, and the only write between that initialization and the `print` is to the pointer `p+1`. Since `q` and `p` point to different local variables, a pointer derived from `p` cannot alias `q[0]`, and hence we know that this write cannot affect the value stored at `q[0]`.