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

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 where each case statement represents an instruction on a virtual CPU, this bounds-checking can add up!

Enter __builtin_unreachable(), 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,

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!!

Here’s an assembly snippet from a small example I threw together on Godbolt.

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, it does indeed seem to go brrr.

Compiling with Clang, adding an unreachable default case gives ~1.3x speedup on a toy example. 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.

Kind of a nifty trick! I’ve gone ahead and pasted it in to some of my projects, like 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.

## 🔗 Let’s Chat

Comments? Questions? I’d love to hear about any related hacks you’re up to!