[Bug 90887] PhiMovesPass in register allocator broken - bugzilla-daemon-CC+yJ3UmIYqDUpFQwHEjaQ

From: bugzilla-daemon-CC+yJ3UmIYqDUpFQwHEjaQ@public.gmane.org
To: nouveau-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org
Subject: [Bug 90887] PhiMovesPass in register allocator broken
Date: Wed, 17 Jun 2015 04:44:08 +0000	[thread overview]
Message-ID: <bug-90887-8800-tiTqu7jnWX@http.bugs.freedesktop.org/> (raw)
In-Reply-To: <bug-90887-8800-V0hAGp6uBxMKqLRl/0Ahz6D7qz1kEfGD2LY78lusg7I@public.gmane.org/>

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https://bugs.freedesktop.org/show_bug.cgi?id=90887

--- Comment #5 from Ilia Mirkin <imirkin-FrUbXkNCsVf2fBVCVOL8/A@public.gmane.org> ---
(In reply to jr from comment #4)
> I've been thinking a little about a better way to fix this inbound edge
> ordering dependency. Unfortunately it seems that unless there is a way to
> efficiently recompute the information there always will be an invariant that
> has to be respected by graph transformations (and which attach and detach
> simply cannot guarantee on their own).

The way that LLVM deals with this is that the source actually contains a
reference to the inbound bb as well. That way you can do whatever you want with
the edges, each src of a phi node knows which bb it's coming from.

It'd be a bit of a pain to introduce here... my strategy so far has been to
hold my nose and just fix things up as they come, which I guess is what you've
done as well. I need to look at this logic very carefully though.

The fact that we were previously adding a FORWARD edge and aren't anymore is a
bit worrying -- the meaning of those things is explained somewhere btw -- TREE
== part of the MST; FORWARD/etc are various other edges.

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