[LLVMdev] Updating PHI for Instruction Domination?

[Andrew Trick]

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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