[LLVMdev] How to eliminate PHI nodes on pointer types?

[Duncan Sands]

Hi Andrew,

> Somewhere during optimization PHI nodes on pointer types (including
> alloca instructions) are being introduced, and they persist through the
> scalar replacement of aggregates pass and others.  I can't seem to find
> a combination of passes or transformations to get rid of them.  Has
> anyone had this problem before, and know a transformation to eliminate
> it?  My optimization passes rely on everything being in pure SSA form.

why is this a problem?  Presumably you are happy with phi nodes of integer
or float type, why are phi nodes of pointer type problematic?

Ciao, Duncan.

PS: I'm not sure what you mean by "pure SSA form".  Phi nodes are needed for
values of pointer type exactly because these values are in SSA form...

>
> One more note, this particular optimization problem only seems to occur
> in llvm 2.8, not 3.0 - though I can't say for sure whether it never
> happens in 3.0.
>
> Here's an example from the compiled code:
>
>
> define private void @"vop_bindST at FFIFFI"(double* %news, double* %ss,
> i32* %isSConnected, double* %newt, double* %tt, i32* %isTConnected)
> nounwind alwaysinline {
> entry:
>     %"reg2mem alloca point" = bitcast i32 0 to i32
>     %isSConnected1 = load i32* %isSConnected
>     %0 = icmp eq i32 %isSConnected1, 0
>     br i1 %0, label %false, label %true
>
> true:                                             ; preds = %entry
>     br label %end
>
> false:                                            ; preds = %entry
>     br label %end
>
> end:                                              ; preds = %false, %true
>     %phi.in.reg2mem.0 = phi double* [ @s, %false ], [ %ss, %true ]
>     %phi = load double* %phi.in.reg2mem.0
>     store double %phi, double* %news
>     %isTConnected4 = load i32* %isTConnected
>     %1 = icmp eq i32 %isTConnected4, 0
>     br i1 %1, label %false6, label %true5
>
> true5:                                            ; preds = %end
>     br label %end7
>
> false6:                                           ; preds = %end
>     br label %end7
>
> end7:                                             ; preds = %false6, %true5
>     %phi9.in.reg2mem.0 = phi double* [ @t, %false6 ], [ %tt, %true5 ]
>     %phi9 = load double* %phi9.in.reg2mem.0
>     store double %phi9, double* %newt
>     ret void
> }
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