One aspect of GCC optimisation that still amazes me is it's ability to convert specific repetitive operations into highly-efficient mathematical sequence equations.
One example is that GCC can convert an function in where you sum all the integers up to the number specified as an input variable using a for loop, into the equation
x * (x + 1) / 2.
This is covered in more detail at http://blog.xebia.com/gcc-compiler-optimizations-dissection-of-a-benchmark/.
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