note on integer-pointer transmutation

[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 68a2cb0cf8483a4a0221b8fa5b841d6354f273e3..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`.
 }
 {% 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`.
 
 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.
 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.
 
 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.