+// FIXME: This is very simple and misses some cases which should be handled when
+// motivating examples are found.
+//
+// The copy rewriting logic should look at uses as well as defs and be able to
+// eliminate copies across blocks.
+//
+// Later copies that are subregister extracts will also not be eliminated since
+// only the first copy is considered.
+//
+// e.g.
+// %vreg1 = COPY %vreg0
+// %vreg2 = COPY %vreg0:sub1
+//
+// Should replace %vreg2 uses with %vreg1:sub1
+bool PeepholeOptimizer::foldRedundantCopy(
+ MachineInstr *MI,
+ SmallSet<unsigned, 4> &CopySrcRegs,
+ DenseMap<unsigned, MachineInstr *> &CopyMIs) {
+ assert(MI->isCopy());
+
+ unsigned SrcReg = MI->getOperand(1).getReg();
+ if (!TargetRegisterInfo::isVirtualRegister(SrcReg))
+ return false;
+
+ unsigned DstReg = MI->getOperand(0).getReg();
+ if (!TargetRegisterInfo::isVirtualRegister(DstReg))
+ return false;
+
+ if (CopySrcRegs.insert(SrcReg).second) {
+ // First copy of this reg seen.
+ CopyMIs.insert(std::make_pair(SrcReg, MI));
+ return false;
+ }
+
+ MachineInstr *PrevCopy = CopyMIs.find(SrcReg)->second;
+
+ unsigned SrcSubReg = MI->getOperand(1).getSubReg();
+ unsigned PrevSrcSubReg = PrevCopy->getOperand(1).getSubReg();
+
+ // Can't replace different subregister extracts.
+ if (SrcSubReg != PrevSrcSubReg)
+ return false;
+
+ unsigned PrevDstReg = PrevCopy->getOperand(0).getReg();
+
+ // Only replace if the copy register class is the same.
+ //
+ // TODO: If we have multiple copies to different register classes, we may want
+ // to track multiple copies of the same source register.
+ if (MRI->getRegClass(DstReg) != MRI->getRegClass(PrevDstReg))
+ return false;
+
+ MRI->replaceRegWith(DstReg, PrevDstReg);
+
+ // Lifetime of the previous copy has been extended.
+ MRI->clearKillFlags(PrevDstReg);
+ return true;
+}
+