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Thanks Mehdi. Below is an example of "dead def", it looks like unused definition as you mentioned, could you confirm? I also saw dead defs in some generated code before register allocation, like "%reg1030 = ADD8rr %reg1028, %reg1029, %EFLAGS<imp-def,dead>", does it mean similar for EFLAGS here?

// AggressiveAntiDepBreaker.cpp

void AggressiveAntiDepBreaker::PrescanInstruction(MachineInstr \*MI,

unsigned Count,

std::set& PassthruRegs) {

std::vector &DefIndices = State->GetDefIndices();

std::multimap&

RegRefs = State->GetRegRefs();

// Handle dead defs by simulating a last-use of the register just

// after the def. A dead def can occur because the def is truly

// dead, or because only a subregister is live at the def. If we

// don't do this the dead def will be incorrectly merged into the

// previous def.

for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {

MachineOperand &MO = MI->getOperand(i);

if (!MO.isReg() || !MO.isDef()) continue;

unsigned Reg = MO.getReg();

if (Reg == 0) continue;

HandleLastUse(Reg, Count + 1, "", "\\tDead Def: ", "\\n");

}