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Computed goto for efficient dispatch tables (2012)

34 points by firephox

by variadix

1 subcomments

You can probably eliminate the switch bounds check by making the default case execute __builtin_unreachable(). The switch version is safe for non-conforming opcodes where the computed goto version invokes UB.

by jdw64

2 subcomments

I understood up to the comparison operations in this article, but I'm having trouble understanding branch prediction even after reading it. It seems like they used branch prediction optimization... Sometimes when I read articles like this, I start to question whether I can really call myself a programmer

by nly

1 subcomments

by kibwen

1 subcomments

> Therefore, the standard forces the compiler to generate "safe" code for the switch. Safety, as usual, has cost, so the switch version ends up doing a bit more per loop iteration.
Safety only has a cost in this case because the switch is fundamentally just operating on an integer. With an actual enumerated type (rather than C's primitive "enums as numeric aliases"), which even a basic type system could trivially enforce, there would be no need for this check, because the domain of the value would be guaranteed at compile-time.

by froh

0 subcomment

0 subcomment