[IR][JumpThreading] Fix infinite recursion on compare self-reference

In unreachable code, constant PHI nodes may appear and be replaced by
their single value. As a result, instructions may become
self-referencing. This commit adds checks to avoid going into infinite
recursion when handling self-referencing compare instructions in
`evaluateOnPredecessorEdge()`.

Author: ro-i

llvm/lib/Transforms/Scalar/JumpThreading.cpp

@@ -1519,11 +1519,15 @@ Constant *JumpThreadingPass::evaluateOnPredecessorEdge(BasicBlock *BB,
   }
 
   // If we have a CmpInst, try to fold it for each incoming edge into PredBB.
+  // Note that during the execution of the pass, phi nodes may become constant
+  // and may be removed, which can lead to self-referencing instructions in
+  // code that becomes unreachable. Consequently, we need to handle those
+  // instructions in unreachable code and check before going into recursion.
   if (CmpInst *CondCmp = dyn_cast<CmpInst>(V)) {
     if (CondCmp->getParent() == BB) {
-      Constant *Op0 =
+      Constant *Op0 = CondCmp->getOperand(0) == CondCmp ? nullptr :
           evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(0), DL);
-      Constant *Op1 =
+      Constant *Op1 = CondCmp->getOperand(1) == CondCmp ? nullptr :
           evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(1), DL);
       if (Op0 && Op1) {
         return ConstantFoldCompareInstOperands(CondCmp->getPredicate(), Op0,

llvm/test/Transforms/JumpThreading/unreachable-loops.ll

@@ -180,4 +180,63 @@ cleanup2343.loopexit4:                            ; preds = %cleanup1491
   unreachable
 }
 
+; This segfaults due to recursion in %C4. Reason: %L6 is identified to be a
+; "partially redundant load" and is replaced by a PHI node. The PHI node is then
+; simplified to be constant and is removed. This leads to %L6 being replaced by
+; %C4, which makes %C4 invalid since it uses %L6.
+; The test case has been generated by the AMD Fuzzing project and simplified
+; manually and by llvm-reduce.
+
+define i32 @constant_phi_leads_to_self_reference(ptr %ptr) {
+; CHECK-LABEL: @constant_phi_leads_to_self_reference(
+; CHECK-NEXT:    [[A9:%.*]] = alloca i1, align 1
+; CHECK-NEXT:    br label [[F6:%.*]]
+; CHECK:       T3:
+; CHECK-NEXT:    br label [[BB5:%.*]]
+; CHECK:       BB5:
+; CHECK-NEXT:    [[L10:%.*]] = load i1, ptr [[A9]], align 1
+; CHECK-NEXT:    br i1 [[L10]], label [[BB6:%.*]], label [[F6]]
+; CHECK:       BB6:
+; CHECK-NEXT:    [[LGV3:%.*]] = load i1, ptr [[PTR:%.*]], align 1
+; CHECK-NEXT:    [[C4:%.*]] = icmp sle i1 [[C4]], true
+; CHECK-NEXT:    store i1 [[C4]], ptr [[PTR]], align 1
+; CHECK-NEXT:    br i1 [[C4]], label [[F6]], label [[T3:%.*]]
+; CHECK:       F6:
+; CHECK-NEXT:    ret i32 0
+; CHECK:       F7:
+; CHECK-NEXT:    br label [[BB5]]
+;
+  %A9 = alloca i1, align 1
+  br i1 false, label %BB4, label %F6
+
+BB4:                                              ; preds = %0
+  br i1 false, label %F6, label %F1
+
+F1:                                               ; preds = %BB4
+  br i1 false, label %T4, label %T3
+
+T3:                                               ; preds = %T4, %BB6, %F1
+  %L6 = load i1, ptr %ptr, align 1
+  br label %BB5
+
+BB5:                                              ; preds = %F7, %T3
+  %L10 = load i1, ptr %A9, align 1
+  br i1 %L10, label %BB6, label %F6
+
+BB6:                                              ; preds = %BB5
+  %LGV3 = load i1, ptr %ptr, align 1
+  %C4 = icmp sle i1 %L6, true
+  store i1 %C4, ptr %ptr, align 1
+  br i1 %L6, label %F6, label %T3
+
+T4:                                               ; preds = %F1
+  br label %T3
+
+F6:                                               ; preds = %BB6, %BB5, %BB4, %0
+  ret i32 0
+
+F7:                                               ; No predecessors!
+  br label %BB5
+}
+
 !0 = !{!"branch_weights", i32 2146410443, i32 1073205}