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

llvm/lib/IR/BasicBlock.cpp

@@ -556,8 +556,14 @@ void BasicBlock::removePredecessor(BasicBlock *Pred,
     if (NumPreds == 1)
       continue;
 
-    // Try to replace the PHI node with a constant value.
-    if (Value *PhiConstant = Phi.hasConstantValue()) {
+    // Try to replace the PHI node with a constant value, but make sure that
+    // this value isn't using the PHI node. (Except it's a PHI node itself. PHI
+    // nodes are allowed to reference themselves.)
+    if (Value *PhiConstant = Phi.hasConstantValue();
+        PhiConstant &&
+        (!isa<Instruction>(PhiConstant) || isa<PHINode>(PhiConstant) ||
+         llvm::all_of(Phi.users(),
+                      [PhiConstant](User *U) { return U != PhiConstant; }))) {
       Phi.replaceAllUsesWith(PhiConstant);
       Phi.eraseFromParent();
     }

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

@@ -180,4 +180,64 @@ 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() {
+; CHECK-LABEL: @constant_phi_leads_to_self_reference(
+; CHECK-NEXT:    [[A9:%.*]] = alloca i1, align 1
+; CHECK-NEXT:    br label [[F6:%.*]]
+; CHECK:       T3:
+; CHECK-NEXT:    [[L6:%.*]] = phi i1 [ [[C4:%.*]], [[BB6:%.*]] ]
+; 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 null, align 1
+; CHECK-NEXT:    [[C4]] = icmp sle i1 [[L6]], true
+; CHECK-NEXT:    store i1 [[C4]], ptr null, align 1
+; CHECK-NEXT:    br i1 [[L6]], 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 null, 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 null, align 1
+  %C4 = icmp sle i1 %L6, true
+  store i1 %C4, ptr null, 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}