[LLVMdev] Updating PHI for Instruction Domination?

Andrew Trick
Mon, 06 Feb 2012 22:23:23 -0000

You're creating a new path that doesn't include L. For all values
defined in L and used outside of L, you need to determine the new reaching def.
That's specific to your transformation and can't be automated. Once you
do that, creating the phi in F is natural.
-Andy

On Feb 6, 2012, at 11:51 AM, Ryan Taylor <ryta1203 at gmail.com> wrote:

> I guess not since Value is a superclass of Instruction.
>
> On Mon, Feb 6, 2012 at 10:36 AM, Ryan Taylor <ryta1203 at gmail.com>
wrote:
>
>
> ---------- Forwarded message ----------
> From: Ryan Taylor <ryta1203 at gmail.com>
> Date: Mon, Feb 6, 2012 at 10:36 AM
> Subject: Re: [LLVMdev] Updating PHI for Instruction Domination?
> To: Eric Christopher <echristo at apple.com>
>
>
> Since I'm not sure which instructions I might want to replicate, would
it be possible to cast the Value from "PHINode::getIncomingValue" to
"Instruction" and then copy that and cast it back?
>
>
> On Sun, Feb 5, 2012 at 8:49 PM, Ryan Taylor <ryta1203 at gmail.com>
wrote:
>
>
> On Sun, Feb 5, 2012 at 5:15 PM, Eric Christopher <echristo at
apple.com> wrote:
>
> On Feb 2, 2012, at 5:13 PM, Ryan Taylor wrote:
>
> > So my best bet is to try and work in reg2mem mode and then go back to
mem2reg?
> >
>
> I wouldn't. That sounds painful.
>
> > I'm curious, it seems though when you split a block that the phis
get updated, right?
>
> Sure. There's code in splitBasicBlock to do this. It should just take a
little bit of work to get what you want done.
>
> -eric
>
> Yes, the first DOES sound painful (I want to avoid it) and the second is
not an option. I'm adding a second path in a loop.
>
> The old path: BB1->BB2/BB3, BB2->BB1/BB3, BB3->n/a
>
> The new path: BB1->H/BB2, BB2->F/K, H->F, K->F,  F->BB1/G,
G->BB3
>
> There are PHI nodes in BB1 that come in from BB2 for values defined in BB2.
With the new possible path: BB1->H->F, BB2. Without adding new PHIs or
Values, there exists a dominance instruction issue. So I want to create PHIs in
F for each value in BB2 that is used in BB1. The update the PHI Values in BB1
(that used to come from BB2) to the new values in F (which are getting values
from BB2/H).
>
> The problem is that there is no way to copy the value from BB2, since there
exists no Value constructor!?
>
> This seems like a pretty large limitation anytime anyone wants to add a
branch inside a loop? Is there an easier way to get what I want that I'm
missing?
>
>
>
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