[llvm] Don't collect from phi nodes which are not fully constructed yet (PR #140806)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Manuel Drehwald (ZuseZ4)

<details>
<summary>Changes</summary>

This fixes two issues reported for Enzyme+Rust in https://github.com/EnzymeAD/rust/issues/196,
based on help by @<!-- -->wsmoses 

I have minimized one of the issues into a minimal IR reproducer, which crashes Enzyme when it calls llvm's scev.
https://github.com/EnzymeAD/Enzyme/issues/2310#issuecomment-2869004751

However, I'm unsure how to make a unit test for it, since it seems like opt seems to automatically verify input while parsing.

So if I either remove a branch towards a bb with a phi node (but not the corresponding entry from the phi node), or vice versa, I get

> PHINode should have one entry for each predecessor of its parent basic block!

cc @<!-- -->nikic since we discussed it last week

---
Full diff: https://github.com/llvm/llvm-project/pull/140806.diff


1 Files Affected:

- (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+3-1) 


``````````diff
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 5e01c29615fab..9a35eb1312df0 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -15823,7 +15823,9 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
       Depth < MaxLoopGuardCollectionDepth) {
     SmallDenseMap<const BasicBlock *, LoopGuards> IncomingGuards;
     for (auto &Phi : Pair.second->phis())
-      collectFromPHI(SE, Guards, Phi, VisitedBlocks, IncomingGuards, Depth);
+      // We don't collect from PHIs that are under construction.
+      if (Pair.second->hasNPredecessors(Phi.getNumIncomingValues()))
+        collectFromPHI(SE, Guards, Phi, VisitedBlocks, IncomingGuards, Depth);
   }
 
   // Now apply the information from the collected conditions to

``````````

</details>


https://github.com/llvm/llvm-project/pull/140806