code optimization

Question

please explain each optimization too!

Comments

1. operation y = y*2 ( this is costly operation compare to ) y = y <<1 , so we can replace that operation. 

this is called strength reduction.

2. y = 10 remove it from high-frequency( like loop) region to low frequency is called code motion.

3.we can write both the operation y = y<<1 and x++ in the single loop ( loop unrolling)

4. x = x*0, can be written like this x = 0 ( constant folding)

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okay,thanks!

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on what basis we should judge that $x$ is a costlier operator and $y$ is a cheaper operator?