Speed Test: Switch vs If-Else-If

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Speed Test: Switch vs If-Else-If(blackwasp.co.uk)

submitted by BlackWasp(4024) 4 years, 10 months ago

The .NET framework and the C# language provide two methods for conditional processing where multiple discrete values can be selected from. The switch statement is less flexible than the if-else-if ladder but is generally considered to be more efficient.

5 comments |category: C# |Views: 386
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posted by yesthatmcgurk(4063) 4 years, 10 months ago 0

Lemme guess... about the same, because they compile to very similar IL?

posted by yesthatmcgurk(4063) 4 years, 10 months ago 0

DOH!

posted by senfo(881) 4 years, 10 months ago 0

Interesting. For the impatient:

The results show that the switch statement is faster to execute than the if-else-if ladder. This is due to the compiler's ability to optimise the switch statement. In the case of the if-else-if ladder, the code must process each if statement in the order determined by the programmer. However, because each case within a switch statement does not rely on earlier cases, the compiler is able to re-order the testing in such a way as to provide the fastest execution.

posted by kentcb(140) 4 years, 10 months ago 0

Interesting result, but I'm baffled by the closing statement:

This is due to the compiler's ability to optimise the switch statement. In the case of the if-else-if ladder, the code must process each if statement in the order determined by the programmer. However, because each case within a switch statement does not rely on earlier cases, the compiler is able to re-order the testing in such a way as to provide the fastest execution.

Does that mean re-ordering the if/else statements would allow the same performance to be on par with the switch statement? And how could the compiler possibly know the best order for the switch cases when the input is random? At a guess, I'd say using switch is faster because it results in less assembly instructions running, not because of the order of the tests. Wish I had the time to look into it further . . .

posted by BlackWasp(4024) 4 years, 10 months ago 0

The final statement in the article alludes to the compiler’s ability to optimise the code. With a series of if statements, the executing code is forced down a decision path that may include complex comparisons rather than simple matches.

In the case of the switch statement, fall-throughs are not permitted (unlike in C++) and only matching of single, explicit values occurs. This provides the compiler with the option to optimise the ‘table’ of available match values in a similar manner to a simplified SQL indexing operation.

An example of this in operation with the initial code and IL code can be found at http://www.codeproject.com/cs/algorithms/soundex.asp

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