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When solving backward data-flow problems, many resources (Wikipedia and many presentations found online) recommend traversing the control-flow graph (CFG) in post-order for fastest convergence, which makes sense.

Other resources (such as the "Engineering a Compiler" book) suggest to use reverse post-order (RPO) on the reversed CFG instead.

... and PO(graph) != RPO(rev(graph)) in some cases involving cycles.

My question is: which resource is right? Post-order makes sense, so why not just use that and save the CFG inversion? Why would I use RPO on reversed CFG instead?

D.W.
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Eli Bendersky
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1 Answers1

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It is mostly due to the speed of convergence. To clarify, RPO visits as many predecessors possible before visiting a node so in case of forward data flow problems (like Dominator computation) this would help to converge faster. You can find more details in Engineering a compiler book (which you already know!)

username_4567
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