Index: lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp
+++ lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp
@@ -1423,9 +1423,10 @@
 
       // If one or more successors has been unscheduled, then the current
       // node is no longer available.
-      if (!TrySU->isAvailable)
+      if (!TrySU->isAvailable || !TrySU->NodeQueueId)
         CurSU = AvailableQueue->pop();
       else {
+        // Available and in AvailableQueue
         AvailableQueue->remove(TrySU);
         CurSU = TrySU;
       }
Index: test/CodeGen/Generic/scheduler-backtracking.ll
===================================================================
--- /dev/null
+++ test/CodeGen/Generic/scheduler-backtracking.ll
@@ -0,0 +1,51 @@
+; RUN: llc < %s -pre-RA-sched=list-ilp    | FileCheck %s
+; RUN: llc < %s -pre-RA-sched=list-hybrid | FileCheck %s
+; RUN: llc < %s -pre-RA-sched=source      | FileCheck %s
+; RUN: llc < %s -pre-RA-sched=list-burr   | FileCheck %s
+; RUN: llc < %s -pre-RA-sched=linearize   | FileCheck %s
+
+; PR22304 https://llvm.org/bugs/show_bug.cgi?id=22304
+; Tests checking backtracking in source scheduler. llc used to crash on them.
+
+; CHECK-LABEL: test1
+define i256 @test1(i256 %a) {
+  %b = add i256 %a, 1 
+  %m = shl i256 %b, 1
+  %p = add i256 %m, 1
+  %v = lshr i256 %b, %p
+  %t = trunc i256 %v to i1
+  %c = shl i256 1, %p
+  %f = select i1 %t, i256 undef, i256 %c
+  ret i256 %f
+}
+
+; CHECK-LABEL: test2
+define i256 @test2(i256 %a) {
+  %b = sub i256 0, %a
+  %c = and i256 %b, %a
+  %d = call i256 @llvm.ctlz.i256(i256 %c, i1 false)
+  ret i256 %d
+}
+
+; CHECK-LABEL: test3
+define i256 @test3(i256 %n) {
+  %m = sub i256 -1, %n
+  %x = sub i256 0, %n
+  %y = and i256 %x, %m
+  %z = call i256 @llvm.ctlz.i256(i256 %y, i1 false)
+  ret i256 %z
+}
+
+declare i256 @llvm.ctlz.i256(i256, i1) nounwind readnone
+
+; CHECK-LABEL: test4
+define i64 @test4(i64 %a, i64 %b) {
+  %r = zext i64 %b to i256
+  %u = add i256 %r, 1
+  %w = and i256 %u, 1461501637330902918203684832716283019655932542975
+  %x = zext i64 %a to i256
+  %c = icmp uge i256 %w, %x
+  %y = select i1 %c, i64 0, i64 1
+  %z = add i64 %y, 1
+  ret i64 %z
+}