[llvm-commits] [llvm] r85016 - /llvm/trunk/lib/Transforms/Scalar/LoopRotation.cpp

[Chris Lattner]

On Oct 26, 2009, at 8:58 AM, Dan Gohman wrote:
>>> Author: djg
>>> Date: Sat Oct 24 18:19:52 2009
>>> New Revision: 85016
>>> 
>>> URL: http://llvm.org/viewvc/llvm-project?rev=85016&view=rev
>>> Log:
>>> Rewrite LoopRotation's SSA updating code using SSAUpdater.
>> 
>> Awesome, I didn't remember loop rotate had its own ssa update code.
>> 
>> If, in your travels, you run into any cases where SSAUpdate is a significant compile time issue, please give me a testcase.  I have many less-than-crazy ideas for speeding it up.
> 
> One problem I'm seeing is redundant PHIs. Sphereflake for example
> now gets code like this:
> 
>  %p.013.i = phi %struct.node_t* [ %p.0.be.i, %bb5.backedge.i ], [ %78, %bb9.i ] ; <%struct.node_t*> [#uses=1]
>  %p.012.i = phi %struct.node_t* [ %p.0.be.i, %bb5.backedge.i ], [ %78, %bb9.i ] ; <%struct.node_t*> [#uses=1]
>  %p.011.i = phi %struct.node_t* [ %p.0.be.i, %bb5.backedge.i ], [ %78, %bb9.i ] ; <%struct.node_t*> [#uses=1]
>  %p.010.i = phi %struct.node_t* [ %p.0.be.i, %bb5.backedge.i ], [ %78, %bb9.i ] ; <%struct.node_t*> [#uses=1]
>  %p.09.i = phi %struct.node_t* [ %p.0.be.i, %bb5.backedge.i ], [ %78, %bb9.i ] ; <%struct.node_t*> [#uses=7]
> 
> Indvars can't eliminate these because they aren't a function of the
> canonical induction variable.   These PHIs stick around and end up
> being allocated registers, which is suboptimal.

I finally found a good example that demonstrated this, PR5837, which is now fixed.  I recall that you added phi merging code to simplifycfg or instcombine somewhere, do you think it makes sense to remove this (potentially really expensive) optimization now?
 
-Chris