[PATCH] Make GVN more iterative

[Daniel Berlin]

We have cases where GVN is *really slow* (IE 29 seconds and 80% of
compile time). Iterating it again is likely to make that worse.  Happy
to give them to you (or test it with this patch)

You say you saw a 5% speedup in compile time, which seems really odd.

What exactly did you run it on?

Additionally, it kind of sounds like you are saying all this does is
remove 1 additional load for this testcase. Do you have more general
performance numbers?
Iterating all of GVN to eliminate a single load seems like a pretty
heavy hammer.



On Mon, Aug 11, 2014 at 11:29 AM, James Molloy <James.Molloy at arm.com> wrote:
> Hi all,
>
>
>
> GVN currently iterates until it can do no more, then performs PRE. There’s a
> FIXME, saying we should try GVN again if PRE made changes.
>
>
>
> This patch changes GVN to do this, motivated by the attached testcase
> (reduced from SPEC) where GVN currently leaves a redundant load.
>
>
>
> The FIXME mentions memory dependence checking, but it looks to me like the
> memory dependence updating got implemented after the FIXME was written, so
> it’s out of date.
>
>
>
> I’ve tested this for compile time and there are no non-noise regressions (in
> fact, the geomean was a 5% speedup, interestingly).
>
>
>
> What do you think?
>
>
>
> Cheers,
>
>
>
> James
>
>
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