[LLVMdev] Requiring a pass to run before/after a pass? (Adding PHIs and updating uses)

[Devang Patel]

On Oct 16, 2008, at 8:29 AM, Edward Lee wrote:

> Is there a simple way to require a pass, e.g., Reg2Mem/Mem2Reg, to  
> run before/after my transformation pass? Or do I do something like:

One simplest way is to handle this is to add these passes around your  
pass in the pass manager.

pm.add(Reg2Mem)
pm.add(MyPass)
pm.add(Mem2Reg)

>
>
> struct myOpt {
>   myOpt() {
>     mBefore = createDemoteRegisterToMemoryPass();
>     mAfter = createPromoteMemoryToRegisterPass();
>   }
>
>   getAnalysisUsage(AU) {
>     AU.addRequired(my stuff);
>     mBefore.getAnalysisUsage(AU);
>     mAfter.getAnalysisUsage(AU);
>   }
>
>   runOnFunction(aF) {
>     changed = mBefore(F);
>     do my stuff;
>     changed |= mAfter(F);
>   }
> }
>
> I'm trying to transform a CFG where A flows into B and C, and I'm  
> making a copy of A, A_copy, which will also flow into B and C. Right  
> now I'm making a merge point mergeB where both A and A_copy flow  
> into it, and it then flows into B. Same for C.
>
>   A
>  / \
> B   C
>
>
>  A     A_copy
>  |  /  \  |
> mergeB mergeC
>   |      |
>   B      C
>
> For any values %v deffed in A and A_copy, mergeB has the phis  
> %v.phi.B, and I replace the original uses of %v in B with %v.phi.B.  
> However, if both B and C use %v, I need to make sure B gets updated  
> to use %v.phi.B and C gets %v.phi.C (and not %v.phi.B).
>
> I figured I could maybe make the code cleaner by pushing values into  
> alloca slots then doing my transformation and converting back to  
> registers with Reg2Mem then Mem2Reg.
>
> Ed
>
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