Code Generation from DAG
| S 1= 4 * i
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S 1 = 4 * i
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| S2 = addr(A)-4
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S 2 = addr(A)-4
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| S3 = S 2 [S 1 ]
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S 3 = S2 [S 1 ]
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| S 4 = 4 * i
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| S5 = addr(B)-4
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S 5= addr(B)-4
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| S 6 = S 5 [S4 ] |
S6 = S5 [S 4 ] |
| S7 = S 3 * S6
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S 7 = S3 * S 6
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| S8 = prod+S7
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| prod = S8
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prod = prod + S 7
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| S9 = I+1
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| I = S9
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I = I + 1
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| If I <= 20 goto (1)
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If I <= 20 goto (1)
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We see how to generate code for a basic block from its DAG representation. The advantage of doing so is that from a DAG we can more easily see how to rearrange the order of the final computation sequence than we can starting from a linear sequence of three-address statements or quadruples. If the DAG is a tree, we can generate code that we can prove is optimal under such criteria as program length or the fewest number of temporaries used. The algorithm for optimal code generation from a tree is also useful when the intermediate code is a parse tree.
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