[LLVMdev] Passes propose passes

Devang Patel dpatel at apple.com
Mon Dec 5 09:27:31 PST 2011

On Dec 2, 2011, at 11:56 AM, Carl-Philip Hänsch wrote:

> While trying to find a solution for this bug http://llvm.org/bugs/show_bug.cgi?id=11235 ,
> I came to the conclusion that the following things can happen while optimizing:
>  - after gvn, I get new constants which are inserted into br and add/sum/..., so there should be at least one more jump-threading and/or instsimplify
>  - after instsimplify, I get new constants which are inserted into br, so there should be one more jump-threading pass
>  - after jump-threading, new bigger blocks occur with redundant loads which need an other gvn pass
> At least for -O3 we will need those optimizations.
> Is there a kind of "I need at least one more $XYZ pass" call that can be invoked from a Pass?
> So my suggestion to implement in LLVM is:
>  - Detect where exactly constants are inserted
>  - Detect which pass is responsible to continue folding it
>  - Insert that pass if allowed and if it is not in the queue by a ProposePass function
> ProposePass should decide wether it's worth to further constant fold the block by running the proposed pass. Once accepted, the pass should be inserted into the pass queue at a position where it is most effective.
> What do you think about that idea? How much would be the effort of implementing it? Does it fit the layering design?

This looks like a slippery slope towards finding optimal optimization pass sequence. 

Why not fix pass sequence manually by updating PassManagerBuilder directly ? 

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