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In software engineering, strength reduction is a compiler optimization where expensive operations are replaced with equivalent but less expensive operations. The classic example of strength reduction converts "strong" multiplications inside a loop into "weaker" additions – something that frequently occurs in array addressing. Examples of strength reduction include: * replacing a multiplication within a loop with an addition * replacing an exponentiation within a loop with a multiplication == Code analysis == Most of a program's execution time is typically spent in a small section of code (called a hot spot), and that code is often inside a loop that is executed over and over. A compiler uses methods to identify loops and recognize the characteristics of register values within those loops. For strength reduction, the compiler is interested in *loop invariants. These are the values that do not change within the body of a loop. *induction variables. These are the values that are being iterated each time through the loop. Loop invariants are essentially constants within a loop, but their value may change outside of the loop. Induction variables are changing by known amounts. The terms are relative to a particular loop. When loops are nested, an induction variable in the outer loop can be a loop invariant in the inner loop. Strength reduction looks for expressions involving a loop invariant and an induction variable. Some of those expressions can be simplified. For example, the multiplication of loop invariant c and induction variable ic = 7; for (i = 0; i < N; i++) can be replaced with successive weaker additions c = 7; k = 0; for (i = 0; i < N; i++) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Strength reduction」の詳細全文を読む スポンサード リンク
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