Other Strength Reduction Operations
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- This material is disputed. It is better described as peephole optimizations and instruction assignment.
Operator strength reduction uses mathematical identities to replace slow math operations with faster operations. The benefits depend on the target CPU and sometimes on the surrounding code (which can affect the availability of other functional units within the CPU).
- replacing integer division or multiplication by a power of 2 with an arithmetic shift or logical shift
- replacing integer multiplication by a constant with a combination of shifts, adds or subtracts
- replacing integer division by a constant with a multiplication, taking advantage of the limited range of machine integers.
| original calculation | replacement calculation |
|---|---|
| y = x / 8 | y = x >> 3 |
| y = x * 64 | y = x << 6 |
| y = x * 2 | y = x << 1 |
| y = x * 15 | y = (x << 4) - x |
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