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In compiler construction, strength reduction is a compiler optimization where expensive operations are replaced with equivalent but less expensive operations. [1] The classic example of strength reduction converts strong multiplications inside a loop into weaker additions – something that frequently occurs in array addressing.
Constant folding is the process of recognizing and evaluating constant expressions at compile time rather than computing them at runtime. Terms in constant expressions are typically simple literals, such as the integer literal 2, but they may also be variables whose values are known at compile time.
In computer science, instruction selection is the stage of a compiler backend that transforms its middle-level intermediate representation (IR) into a low-level IR. In a typical compiler, instruction selection precedes both instruction scheduling and register allocation; hence its output IR has an infinite set of pseudo-registers (often known as temporaries) and may still be – and typically ...
Peephole optimization is an optimization technique performed on a small set of compiler-generated instructions, known as a peephole or window, [1] [2] that involves replacing the instructions with a logically equivalent set that has better performance.
The condition 0 == 0 is always true, so the compiler can replace the line marked (2) with the consequent, tmp += 0 (which does nothing). The compiler can rewrite the condition y+1 == 0 to y == -1. The compiler can reduce the expression (y + 1) - 1 to y. The expressions y and y+1 cannot both equal zero. This lets the compiler eliminate one test ...
The following is the same as above, but with loop unrolling implemented at a factor of 4. Note again that the size of one element of the arrays (a double) is 8 bytes; thus the 0, 8, 16, 24 displacements and the 32 displacement on each loop.
The ETH Oberon-2 compiler was one of the first public projects to incorporate "GSA", a variant of SSA. The Open64 compiler used SSA form in its global scalar optimizer, though the code is brought into SSA form before and taken out of SSA form afterwards. Open64 uses extensions to SSA form to represent memory in SSA form as well as scalar values.
int i = 0; while (i < n) {x = y + z; a [i] = 6 * i + x * x; ++ i;} Although the calculation x = y + z and x * x is loop-invariant, precautions must be taken before moving the code outside the loop. It is possible that the loop condition is false (for example, if n holds a negative value), and in such case, the loop body should not be executed ...