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[web.git] / personal / _posts / 2018-07-24-pointers-and-bytes.md

diff --git a/personal/_posts/2018-07-24-pointers-and-bytes.md b/personal/_posts/2018-07-24-pointers-and-bytes.md
index 8f7089ec9086348d1ac7b623d88072e7a425d69c..636fa40edd65ea89291db7e669d51c543e37821b 100644 (file)
--- a/personal/_posts/2018-07-24-pointers-and-bytes.md
+++ b/personal/_posts/2018-07-24-pointers-and-bytes.md
@@ -32,13 +32,18 @@ int test() {
 }
 {% endhighlight %}
 It would be beneficial to be able to optimize the final read of `y[0]` to just return `42`.
+C++ compilers regularly perform such optimizations as they are crucial for generating high-quality assembly.[^perf]
 The justification for this optimization is that writing to `x_ptr`, which points into `x`, cannot change `y`.
 
+[^perf]: To be fair, the are *claimed* to be crucial for generating high-quality assembly. The claim sounds plausible to me, but unfortunately, I do not know of a systematic study exploring the performance benefits of such optimizations.
+
 However, given how low-level a language C++ is, we can actually break this assumption by setting `i` to `y-x`.
 Since `&x[i]` is the same as `x+i`, this means we are actually writing `23` to `&y[0]`.
 
 Of course, that does not stop C++ compilers from doing these optimizations.
-To allow this, the standard declares our code to have [undefined behavior]({% post_url 2017-07-14-undefined-behavior %}).
+To allow this, the standard declares our code to have [undefined behavior]({% post_url 2017-07-14-undefined-behavior %}).[^0]
+
+[^0]: An argument could be made that compilers should just not do such optimizations to make the programming model simpler. This is a discussion worth having, but the point of this post is not to explore this trade-off, it is to explore the consequences of the choices made in C++.
 
 First of all, it is not allowed to perform pointer arithmetic (like `&x[i]` does) that goes [beyond either end of the array it started in](https://timsong-cpp.github.io/cppwp/n4140/expr.add#5).
 Our program violates this rule: `x[i]` is outside of `x`, so this is undefined behavior.
@@ -121,6 +126,13 @@ This is another example of using a "virtual machine" that's different from the r
 
 Here's a simple proposal (in fact, this is the model of pointers used in [CompCert](https://hal.inria.fr/hal-00703441/document) and my [RustBelt work]({% post_url 2017-07-08-rustbelt %}), and it is also how [miri](https://github.com/solson/miri/) implements [pointers](https://github.com/rust-lang/rust/blob/fefe81605d6111faa8dbb3635ab2c51d59de740a/src/librustc/mir/interpret/mod.rs#L121-L124)):
 A pointer is a pair of some kind of ID uniquely identifying the *allocation*, and an *offset* into the allocation.
+If we defined this in Rust, we might write
+{% highlight rust %}
+struct Pointer {
+    alloc_id: usize,
+    offset: isize,
+}
+{% endhighlight %}
 Adding/subtracting an integer to/from a pointer just acts on the offset, and can thus never leave the allocation.
 Subtracting a pointer from another is only allowed when both point to the same allocation (matching [C++](https://timsong-cpp.github.io/cppwp/n4140/expr.add#6)).[^2]
 
@@ -166,7 +178,7 @@ We have to say what the value of `v` is, so we have to find some way to answer t
 (And this is an entirely separate issue from the problem with multiplication that came up in the last section. We just assume some abstract type `Pointer`.)
 
 We cannot represent a byte of a pointer as an element of `0..256`.
-Essentially, if we use a naive model of memory, the extra "hidden" part of a pointer (the one that makes it more than just an integer) would be lost whne a pointer is stored to memory and loaded again.
+Essentially, if we use a naive model of memory, the extra "hidden" part of a pointer (the one that makes it more than just an integer) would be lost when a pointer is stored to memory and loaded again.
 We have to fix this, so we have to extend our notion of a "byte" to accomodate that extra state.
 So, a byte is now *either* an element of `0..256` ("raw bits"), *or* the n-th byte of some abstract pointer.
 If we were to implement our memory model in Rust, this might look as follows:
@@ -230,6 +242,8 @@ Finally, `Uninit` is also a better choice for interpreters like miri.
 Such interpreters have a hard time dealing with operations of the form "just choose any of these values" (i.e., non-deterministic operations), because if they want to fully explore all possible program executions, that means they have to try every possible value.
 Using `Uninit` instead of an arbitrary bit pattern means miri can, in a single execution, reliably tell you if your programs uses uninitialized values incorrectly.
 
+**Update:** Since writing this section, I have written an entire [post dedicated to uninitialized memory and "real hardware"]({% post_url 2019-07-14-uninit %}) with more details, examples and references. **/Update**
+
 ## Conclusion
 
 We have seen that in languages like C++ and Rust (unlike on real hardware), pointers can be different even when they point to the same address, and that a byte is more than just a number in `0..256`.