Nested IF or &&

[jecottrell]

Which is more efficient:

[SherpaDoug]

The best way to find out is to write yourself a little test program, compile it, and look at the .lst file.
The answer may depend on:
1) The compiler version
2) The optimization level
3) The types of the variables
4) Whether you are looking for small size or high speed
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[jecottrell]

Thanks.

I'll try that. I'm looking mostly for speed (I have yet to outgrow a part yet.)

I assume that less instructions will be faster, but it will also be based on what type of instructions get used (because different instructions take different numbers of steps?)

John

[rwyoung]

Instructions all execute in a single system clock (1/4 the Fosc rate) except for program branches.
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[Ttelmah]

I'd 'expect' that the answer will depend on the program flow. In the first case, if the condition fails, the program will branch, and not execute the second test, and be faster. However if he condition 'passes', then there will have been the extra instruction times associated with this branch, before it executes the second test (remember a 'skip not zero' type branch, uses 2 instruction times if it either skips, or does not).
So potentially, if you can have the most common program flow being the condition passing the least tests, this will give the fastest normal execution, but possibly at the 'cost' of actually taking longer for the states where a lot of tests are involved. The compiler does not perform the optimisation often associated with 'sequential' logic tests, of performing them seperately (on some compilers, if you perform a pait of 'anded' tests as shown, it performs the first test, and if it solves to 'false', knows there is no point in performing the second - this gives exactly the same structure as the sequential 'if' statements).
The answer will also depend on the nature of 'x', and 'y'. If for instance these statements involve an array access, there will be a massive overhead in this, and the potential advantage of skipping the second test rises.

Best Wishes