4 ms·

Is rewriting switch statements to a bunch of ifs always faster? Or is there some number of cases where the switch is faster? Seems like it should be added as a

• Karellen 3y ago
  It shouldn't be. If the fastest way to implement a particular `switch` in assembly is with the equivalent of a set of `if`s, a reasonably smart compiler "should" be able to output the assembly to do that. And I thought that gcc and clang at least have actually been smart enough to do that for a while now. But if the number of `if`s is high and the distribution sufficiently dense, where a jump table is better than a bunch of `if`s, then a `switch` should output that. OTOH, a sufficiently smart compiler could theoretically turn a bunch of `if`s into a `switch`-like jump table - but it's much harder to reason that case through correctly than it is the other way, so I'm not sure any current compilers are sufficiently smart to do that.
  • kevin_thibedeau 3y ago
    On x64, GCC is supposed to use a a jump table with more than 4 cases when the cases are dense enough (gaps of 9 or less) to minimize wasted memory, otherwise it generates sequential comparisons. Testing on Godbolt, it looks like GCC 13.1 uses 10 cases as the jump table threshold for ARM64.
• flohofwoe 3y ago
  It shouldn't if the control flow is actually identical. E.g. note how both the switch- and if-based functions generate the same code using a lookup table here: https://godbolt.org/z/qoT3M7r5G https://godbolt.org/z/qoT3M7r5G