Draft "Unreachable Switch Statement Default Cases"

Author: mmore500

blog_professional/_posts/2021-03-24-switch-default.md

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+---
+layout: post
+title: "Unreachable Switch Statement Default Cases"
+date: 2021-03-24
+---
+
+Standard C and C++ requires `switch` statements to behave harmlessly if on a value that not covered by one of their `case`'s.
+For example, in this case
+```cpp
+switch( i ) {
+  case 0:
+    j+=k;
+    break;
+  case 1;
+    j*=k;
+    break;
+  case 2:
+    j/=k;
+    break;
+}
+```
+If `i` wasn't 0, 1, or 2, the `switch` statement would just roll on by without anything happening to `j`.
+
+Behaving harmlessly is good, except that it's not free.
+In practice, this means that every time the `switch` statement executes it has to bounds-check `i`.
+In really tight loops, for example in a [byte-code interpreter](https://eli.thegreenplace.net/2012/07/12/computed-goto-for-efficient-dispatch-tables) where each case statement represents an instruction on a virtual CPU, this bounds-checking can add up!
+
+Enter [`__builtin_unreachable()`](https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html), a compiler intrinsic that lets you double-dog swear   that a section of code will never execute.
+It's not standard, but GCC and Clang at least both know about it.
+Getting the compiler to take your word that certain execution paths will never execute is useful for all sorts of mischief, but I wondered whether it might prove useful in this particular problem of disabling `switch` bounds-checking.
+
+Basically this would look like something along the lines of,
+```cpp
+switch( i ) {
+  case 0:
+    j+=k;
+    break;
+  case 1;
+    j*=k;
+    break;
+  case 2:
+    j/=k;
+    break;
+  default:
+    __builtin_unreachable();
+}
+```
+
+
+To the compiler-explorer-&-quick-bench-mobile!!
+:bat:
+
+Here's an assembly snippet from a small example I threw together [on Godbolt](https://godbolt.org/z/TrqKv77G6).
+```
+unreachable_default(int):
+        mov     edi, edi
+        mov     eax, DWORD PTR CSWTCH.2[0+rdi*4]
+        ret
+no_default(int):
+        xor     eax, eax
+        cmp     edi, 3
+        ja      .L3
+        mov     edi, edi
+        mov     eax, DWORD PTR CSWTCH.4[0+rdi*4]
+```
+
+With the `unreachable_default` function, we can skip a `cmp` (compare) and `ja` (jump if) instruction.
+I assume that in the `no_default` function those instructions were being used for the aforementioned bounds-checking.
+
+But, does it go brrr?
+[Looking at this graph](https://www.youtube.com/watch?v=sIlNIVXpIns), it does indeed seem to go brrr.
+
+![benchmarking result](/resources/switch-default-microbench.jpg){:width="100%"}
+
+Compiling with Clang, adding an unreachable default case gives ~1.3x speedup [on a toy example](https://quick-bench.com/q/1cObPCIkz3g44gHWFWeTkKDgp1Q).
+Even though I was able to see some changes in the compiler output (so `_unreachable_default()` was doing *something*) [I wasn't able to see any speedup on GCC, though](https://quick-bench.com/q/ycTTednxWKAsGKd3JsRG9YH9bx8).
+
+Kind of a nifty trick!
+I've gone ahead and pasted it in to some of my projects, like [signagp-lite](https://github.com/mmore500/signalgp-lite), that do byte-code interpretation in a tight loop.
+I did add a (debug-mode only) assert in right before the `__builtin_unreachable()`... you know, in order to properly check myself when I inevitably wreck myself.
+:man_shrugging:
+
+## Let's Chat
+
+Comments?
+Questions?
+I'd love to hear about any related hacks you're up to!
+
+I started a twitter thread (right below) so we can chat :phone: :phone: :phone:
+
+<blockquote class="twitter-tweet"><p lang="en" dir="ltr">it&#39;s been forever, but new blog post!!! <br><br>this one on mischief with __builtin_unreachable() default switch cases<br><br>perfect for the extra percent of unsafe bytecode interpreter fun 🍹🏖️💣<a href="https://t.co/rBJCM8nnJt">https://t.co/rBJCM8nnJt</a></p>&mdash; Matthew A Moreno (@MorenoMatthewA) <a href="https://twitter.com/MorenoMatthewA/status/1374891036934295555?ref_src=twsrc%5Etfw">March 25, 2021</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>