[PATCH] D92285: [VPlan] Manage scalarized values using VPValues.

[Florian Hahn via Phabricator via llvm-commits]

fhahn added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:9207
+    State.set(this, VPhi, VPhi, Part);
+    State.set(getOperand(0), VPhi, Part);
   } else {
----------------
fhahn wrote:
> gilr wrote:
> > This is porting the resetVectorValue to State operations, which I think is fine by itself (I tend to agree with passing nullptr as the underlying value to VPPredInstPHI), but something else bothers me: It seems VPPredInstPHI doesn't take over the VPReplicate as the VPValue mapped to the predicated IR value during VPlan construction, which means that recipes constructed for users of predicated instructions use the ReplicateRecipe although it doesn't dominate them. We get away with this by overriding the IR's vector value, but for correct VP def-use we should have VPPredInstPHI take over as the registered VPValue in VPlan during construction. Does all that make sense?
> I think the problem here is how we generate code for a scalarization region, like the one below. Uses after the region use the `PHI-PREDICATED-INSTRUCTION ` VPValue.
> 
> But when generating code for the region itself, we execute the same blocks once for each vector element. So in the second iteration `  "REPLICATE ir<%div> = udiv ir<16>, ir<%counter1.0> (S->V)\l" ` currently needs to pack its result in the result vector from the previous iteration. Currently that is done by overwriting the state of the def `"REPLICATE ir<%div> = udiv ir<16>, ir<%counter1.0> (S->V)\l"` with the predicated PHI from the previous iteration, so the vector-pack logic picks it up.
> 
> It might be possible to instead change the code to just have scalar phis and do the packing as part of the phi creation. I think for transformations, the VPValue def-use chain should be correct, and it is 'just' a code-gen improvement. Do you think we need to fix this before this one goes in?
> 
> 
> 
> ```
>   subgraph cluster_N2 {
>     fontname=Courier
>     label="\<xVFxUF\> pred.udiv"
>     N1 [label =
>       "pred.udiv.entry:\n" +
>         " +
>         "BRANCH-ON-MASK vp<%4>\l"\l" +
>          "CondBit: vp<%4> (for.body3)\l"
>     ]
>     N1 -> N3 [ label="T"]
>     N1 -> N4 [ label="F"]
>     N3 [label =
>       "pred.udiv.if:\n" +
>         "REPLICATE ir<%div> = udiv ir<16>, ir<%counter1.0> (S->V)\l"
>     ]
>     N3 -> N4 [ label=""]
>     N4 [label =
>       "pred.udiv.continue:\n" +
>         "PHI-PREDICATED-INSTRUCTION vp<%6> = ir<%div>\l"
>     ]
>   }
> ```
@gilr I added a note explaining why we need to also update the operand. Do you think that is sufficient to move forward with this patch?


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https://reviews.llvm.org/D92285