[LoopSimplifyCFG] Add check for missing loop preheader

Adds a check for a missing loop preheader during analysis.
This fixes a nullptr dereference that happened whenever
LoopSimplify was unable to generate a preheader because
the loop was entered by an indirectbr instruction (as
stated in the LoopSimplify.cpp doc comment).

Author: Justus2308

llvm/lib/Transforms/Scalar/LoopSimplifyCFG.cpp

@@ -128,6 +128,8 @@ class ConstantTerminatorFoldingImpl {
   // from any other block. So this variable set to true means that loop's latch
   // has become unreachable from loop header.
   bool DeleteCurrentLoop = false;
+  // Whether or not we enter the loop through an indirectbr.
+  bool HasIndirectEntry = false;
 
   // The blocks of the original loop that will still be reachable from entry
   // after the constant folding.
@@ -216,6 +218,17 @@ class ConstantTerminatorFoldingImpl {
       return;
     }
 
+    // We need a loop preheader to split in handleDeadExits(). If LoopSimplify
+    // wasn't able to form one because the loop can be entered through an
+    // indirectbr we cannot continue.
+    if (!L.getLoopPreheader()) {
+      assert(any_of(predecessors(L.getHeader()), [&](BasicBlock *Pred) {
+        return isa<IndirectBrInst>(Pred->getTerminator());
+      }) && "Loop should have preheader if it is not entered indirectly");
+      HasIndirectEntry = true;
+      return;
+    }
+
     // Collect live and dead loop blocks and exits.
     LiveLoopBlocks.insert(L.getHeader());
     for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
@@ -546,6 +559,12 @@ class ConstantTerminatorFoldingImpl {
       return false;
     }
 
+    if (HasIndirectEntry) {
+      LLVM_DEBUG(dbgs() << "Loops which can be entered indirectly are not"
+                           " supported!\n");
+      return false;
+    }
+
     // Nothing to constant-fold.
     if (FoldCandidates.empty()) {
       LLVM_DEBUG(

llvm/test/Transforms/LoopSimplifyCFG/enter-through-indirectbr.ll

@@ -0,0 +1,16 @@
+; RUN: opt -S -enable-loop-simplifycfg-term-folding=true -passes='require<domtree>,loop(loop-simplifycfg)' -verify-loop-info -verify-dom-info -verify-loop-lcssa < %s | FileCheck %s
+ 
+define void @test() {
+; CHECK-LABEL: @test(
+
+  indirectbr ptr null, [label %A, label %C]
+
+A:
+  br i1 true, label %B, label %C
+
+B:
+  br i1 true, label %A, label %C
+
+C:
+  unreachable
+}