Transformation loop unrolling

As an example of transformation. Figure 5 shows a partial loop unrolling applied to EXAMPLE. The simplified dependency graph (Figure 4) was duplicated twice, and only the real dependencies are shown. Operation 1 of the second iteration does not depend on the result D of operation 8, which is only used by operation 5. Similarly, only operation 6 uses the result C of operation 7. Thus, operations 7 and 8 can be executed in parallel with the test at the beginning of the next loop iteration (operation 1). The partially-unrolled dependency graph captures this information. 

Supports behavioral transformations, primarily to improve the efficiency of the control structure constant folding, common subexpression elimination, dead procedure elimination, inline expansion and formation of procedures, code motion into and out of the branches of decoding operations, and loop unrolling. 

Applies optimizing compiler transformations, including constant folding, common subexpression elimination, dead code elimination, loop retiming, loop pipelining, loop unrolling, and loop jamming. 

Loop unrolling—transforming a loop into a linear sequence by making copies of the loop body—illustrates how automata-theoretic algorithms implement structural transformations. Consider a machine which executes a definite loq) (one whose iteration bounds are known at compile time) such a machine can be built from a sequencer and a counter as shown in Figure 7. The sequencer resets the counter at the start of the loop, increments it at the end of each iteration, and exits the loop when the counter reaches its limit. The loop body performs a simple assignment The state transition table for the sequencer component is shown in Figure 8 we have used more states than necessary in the sequencer to more clearly illustrate how the loop unrolls. 

For-loop unrolling. Fixed-iteation loops are unrolled to ineease the scope of subsequent optimizations. This is accomplished in BIF by replacing the For-loop node with a Block node, where each child of the block node is a duplicate of the loop body for a particular value of loop index. The example below illustrates the lo( unrolling transformation. 

Automatically applies block-level transformations to produce the dacon with the fastest implementation. Transformations supported merge adjacent bloi s, combine a block followed by two or more conditionally executed blocks into a single block, unroll loops, and move the test at the top of a loop to the bottom of the loop. 

Uses transformations to eliminate useless registers, replace addition by 1 with an increment operation, unroll loops, and perform dead code elimination, etc. 

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