Re: [Bug tree-optimization/21356] [4.1 Regression] Dominance errorafter aggressive dead code elimination (cd_dce)

[Jeffrey A Law <law at redhat dot com>]

DCE in aggressive mode sometimes is able to remove control structures
and thus edge from the CFG.  Sometimes removal of edges from the CFG
changes the dominator tree, but we make no attempt to actually keep
the dominators up-to-date.

In this testcase failure to keep the dominators up-to-date leads to
a checking failure.  This is trivially addressed by arranging for the
dominators to be recomputed if we remove edges from the CFG.  An
enterprising individual might be able to incrementally update the
dominators, but for now this is safe and given that removal of 
control structures by DCE is rare, the compile-time cost is minimal.

Bootstrapped and regression tested on i686-pc-linux-gnu.

Jeff

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