[InlineSpiller] Don't spill fully undef values
Althought it is not wrong to spill undef values, it is useless and harms
both code size and runtime. Before spilling a value, check that its
content actually matters.
http://www.llvm.org/PR33311
llvm-svn: 304752
diff --git a/llvm/lib/CodeGen/InlineSpiller.cpp b/llvm/lib/CodeGen/InlineSpiller.cpp
index b7ab404..68f2a2e 100644
--- a/llvm/lib/CodeGen/InlineSpiller.cpp
+++ b/llvm/lib/CodeGen/InlineSpiller.cpp
@@ -857,14 +857,36 @@
++NumReloads;
}
+/// Check if \p Def fully defines a VReg with an undefined value.
+/// If that's the case, that means the value of VReg is actually
+/// not relevant.
+static bool isFullUndefDef(const MachineInstr &Def) {
+ if (!Def.isImplicitDef())
+ return false;
+ assert(Def.getNumOperands() == 1 &&
+ "Implicit def with more than one definition");
+ // We can say that the VReg defined by Def is undef, only if it is
+ // fully defined by Def. Otherwise, some of the lanes may not be
+ // undef and the value of the VReg matters.
+ return !Def.getOperand(0).getSubReg();
+}
+
/// insertSpill - Insert a spill of NewVReg after MI.
void InlineSpiller::insertSpill(unsigned NewVReg, bool isKill,
MachineBasicBlock::iterator MI) {
MachineBasicBlock &MBB = *MI->getParent();
MachineInstrSpan MIS(MI);
- TII.storeRegToStackSlot(MBB, std::next(MI), NewVReg, isKill, StackSlot,
- MRI.getRegClass(NewVReg), &TRI);
+ if (isFullUndefDef(*MI))
+ // Don't spill undef value.
+ // Anything works for undef, in particular keeping the memory
+ // uninitialized is a viable option and it saves code size and
+ // run time.
+ BuildMI(MBB, std::next(MI), MI->getDebugLoc(), TII.get(TargetOpcode::KILL))
+ .addReg(NewVReg, getKillRegState(isKill));
+ else
+ TII.storeRegToStackSlot(MBB, std::next(MI), NewVReg, isKill, StackSlot,
+ MRI.getRegClass(NewVReg), &TRI);
LIS.InsertMachineInstrRangeInMaps(std::next(MI), MIS.end());