[PATCH] Select Elimination in InstCombine

Chandler Carruth chandlerc at gmail.com
Wed Sep 10 17:08:13 PDT 2014


On Wed, Sep 10, 2014 at 5:00 PM, Gerolf Hoflehner <ghoflehner at apple.com>
wrote:

> Perhaps I should phrase it differently. DT is an analysis required by
> other passes like LICM which runs after the combiner.  The combiner does
> not destroy the Dominator Tree. So what is the root of your concern?


Requiring DT will likely cause DT to be run more times during the
compilation (I'm pretty sure there is at least one place where nothing else
needs it and there is a pass that invalidates it). As a consequence, this
could *in theory* have an impact on compile time.

However, I don't actually expect it to have such an impact. I just think
its reasonable to measure the compile time impact because sometimes my
expectations are quite wrong. =]

(The only reason I mention a separate thread is to try to help make the
code review progress faster -- the code is fine regardless of what the
compile time results show, and we could potentially look at ways to get DT
some of the time when it will be used later anyways but not always to avoid
any compile time regressions.)
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