[llvm] [Utils] Fix incorrect LCSSA PHI nodes when splitting critical edges with MergeIdenticalEdges (PR #131744)

[Nikita Popov via llvm-commits]

================
@@ -438,6 +438,63 @@ define void @crit_edge(i1 %cond0, i1 %cond1) {
   EXPECT_TRUE(PDT.verify());
 }
 
+TEST(BasicBlockUtils, SplitLoopCriticalEdge) {
+  LLVMContext C;
+  std::unique_ptr<Module> M = parseIR(C, R"IR(
+declare i1 @predicate(ptr %p)
+
+define ptr @Parse(ptr %gp) {
+entry:
+  br label %for.inc
+
+for.inc:
+  %phi = phi ptr [ %gp, %entry ], [ %cp, %while.cond ], [ %cp, %while.cond ]
+  %cond = call i1 @predicate(ptr %phi)
+  %inc= getelementptr inbounds i8, ptr %phi, i64 1
+  br i1 %cond, label %while.cond, label %exit
+
+while.cond:
+  %cp = phi ptr [ %inc, %for.inc ], [ %incdec, %while.body ]
+  %val = load i8, ptr %cp, align 1
+  switch i8 %val, label %while.body [
+    i8 10, label %for.inc
+    i8 0, label %for.inc
+  ]
+
+while.body:
+  %incdec = getelementptr inbounds i8, ptr %cp, i64 1
+  br label %while.cond
+
+exit:
+  ret ptr %phi
+}
+)IR");
+  Function *F = M->getFunction("Parse");
+  DominatorTree DT(*F);
+  LoopInfo LI(DT);
+
+  CriticalEdgeSplittingOptions CESO =
+      CriticalEdgeSplittingOptions(nullptr, &LI, nullptr)
+          .setMergeIdenticalEdges()
+          .setPreserveLCSSA();
+  EXPECT_EQ(2u, SplitAllCriticalEdges(*F, CESO));
+
+  BasicBlock *WhileBB = getBasicBlockByName(*F, "while.cond");
+  BasicBlock *SplitBB = WhileBB->getTerminator()->getSuccessor(1);
+  // The only 1 successor of SplitBB is %for.inc
+  EXPECT_EQ(1u, SplitBB->getTerminator()->getNumSuccessors());
+  // MergeIdenticalEdges: SplitBB has two identical predecessors, %while.cond.
+  EXPECT_EQ(WhileBB, SplitBB->getUniquePredecessor());
+  EXPECT_TRUE(SplitBB->hasNPredecessors(2));
+
+  auto *PN = dyn_cast<PHINode>(&SplitBB->front());
+  // PreserveLCSSA: should insert a PHI node in front of SplitBB
+  EXPECT_EQ(nullptr, PN);
----------------
nikic wrote:

```suggestion
  EXPECT_NE(nullptr, PN);
```

https://github.com/llvm/llvm-project/pull/131744