[LLVMdev] What is dead def? (original) (raw)

Thomson lilotom at gmail.com
Fri Jan 2 19:15:33 PST 2015

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<unsigned, AggressiveAntiDepState::RegisterReference>& 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, "", "\t*Dead Def*: ", "\n");

}

On Fri, Jan 2, 2015 at 4:38 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:

> On Jan 1, 2015, at 7:24 AM, Thomson <lilotom at gmail.com> wrote: > > I saw there is reference to dead def on registers in LLVM source code. I am not aware of this concept from the traditional course material. What are the properties of dead def? Could it be a def that is never used (hence dead)? If you mention where you saw it, it may be easier to help :) Best, Mehdi

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