[PATCH] Estimate DCE effect in heuristic for estimating complete-unroll optimization effects.

[Chandler Carruth]

Before we go here, I think you should handle dead CFG paths which is a *much* simpler problem.

Specifically, you should simplify the condition for branches or the input for switch, and only add a single successor when it folds to a constant.

Once that's handled, I think this should work the other way. The problem with doing it as you have is that walking all the users of an instruction is very expensive (linked list traversal, to cache hostile). Instead, when we first see an instruction in the body of the loop without side-effects we should add it to a dead set rather than accumulating its cost, and we should remove all operands of all instructions we see in the body of the loop from the dead set (and if it was removed, account for its cost there). On the last iteration, we can also remove all live-out instructions from the dead set using the LCSSA PHI node (or just not use the dead set for the last iteration).

Does that make sense?


http://reviews.llvm.org/D8817

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