- MachineOperand **Head = &getRegUseDefListHead(MO->getReg());
-
- // For SSA values, we prefer to keep the definition at the start of the list.
- // we do this by skipping over the definition if it is at the head of the
- // list.
- if (*Head && (*Head)->isDef())
- Head = &(*Head)->Contents.Reg.Next;
-
- MO->Contents.Reg.Next = *Head;
- if (MO->Contents.Reg.Next) {
- assert(MO->getReg() == MO->Contents.Reg.Next->getReg() &&
- "Different regs on the same list!");
- MO->Contents.Reg.Next->Contents.Reg.Prev = &MO->Contents.Reg.Next;
+ MachineOperand *&HeadRef = getRegUseDefListHead(MO->getReg());
+ MachineOperand *const Head = HeadRef;
+
+ // Head points to the first list element.
+ // Next is NULL on the last list element.
+ // Prev pointers are circular, so Head->Prev == Last.
+
+ // Head is NULL for an empty list.
+ if (!Head) {
+ MO->Contents.Reg.Prev = MO;
+ MO->Contents.Reg.Next = 0;
+ HeadRef = MO;
+ return;
+ }
+ assert(MO->getReg() == Head->getReg() && "Different regs on the same list!");
+
+ // Insert MO between Last and Head in the circular Prev chain.
+ MachineOperand *Last = Head->Contents.Reg.Prev;
+ assert(Last && "Inconsistent use list");
+ assert(MO->getReg() == Last->getReg() && "Different regs on the same list!");
+ Head->Contents.Reg.Prev = MO;
+ MO->Contents.Reg.Prev = Last;
+
+ // Def operands always precede uses. This allows def_iterator to stop early.
+ // Insert def operands at the front, and use operands at the back.
+ if (MO->isDef()) {
+ // Insert def at the front.
+ MO->Contents.Reg.Next = Head;
+ HeadRef = MO;
+ } else {
+ // Insert use at the end.
+ MO->Contents.Reg.Next = 0;
+ Last->Contents.Reg.Next = MO;