VirtRegMap: Replace some identity copies with KILL instructions.
An identity COPY like this:
%AL = COPY %AL, %EAX<imp-def>
has no semantic effect, but encodes liveness information: Further users
of %EAX only depend on this instruction even though it does not define
the full register.
Replace the COPY with a KILL instruction in those cases to maintain this
liveness information. (This reverts a small part of r238588 but this
time adds a comment explaining why a KILL instruction is useful).
llvm-svn: 274952
diff --git a/llvm/lib/CodeGen/VirtRegMap.cpp b/llvm/lib/CodeGen/VirtRegMap.cpp
index 7713adb..8a3a032 100644
--- a/llvm/lib/CodeGen/VirtRegMap.cpp
+++ b/llvm/lib/CodeGen/VirtRegMap.cpp
@@ -166,6 +166,7 @@
void addMBBLiveIns();
bool readsUndefSubreg(const MachineOperand &MO) const;
void addLiveInsForSubRanges(const LiveInterval &LI, unsigned PhysReg) const;
+ void handleIdentityCopy(MachineInstr &MI) const;
public:
static char ID;
@@ -346,6 +347,30 @@
return true;
}
+void VirtRegRewriter::handleIdentityCopy(MachineInstr &MI) const {
+ if (!MI.isIdentityCopy())
+ return;
+ DEBUG(dbgs() << "Identity copy: " << MI);
+ ++NumIdCopies;
+
+ // Copies like:
+ // %R0 = COPY %R0<undef>
+ // %AL = COPY %AL, %EAX<imp-def>
+ // give us additional liveness information: The target (super-)register
+ // must not be valid before this point. Replace the COPY with a KILL
+ // instruction to maintain this information.
+ if (MI.getOperand(0).isUndef() || MI.getNumOperands() > 2) {
+ MI.setDesc(TII->get(TargetOpcode::KILL));
+ DEBUG(dbgs() << " replace by: " << MI);
+ return;
+ }
+
+ if (Indexes)
+ Indexes->removeMachineInstrFromMaps(MI);
+ MI.eraseFromParent();
+ DEBUG(dbgs() << " deleted.\n");
+}
+
void VirtRegRewriter::rewrite() {
bool NoSubRegLiveness = !MRI->subRegLivenessEnabled();
SmallVector<unsigned, 8> SuperDeads;
@@ -435,15 +460,8 @@
DEBUG(dbgs() << "> " << *MI);
- // Finally, remove any identity copies.
- if (MI->isIdentityCopy()) {
- ++NumIdCopies;
- DEBUG(dbgs() << "Deleting identity copy.\n");
- if (Indexes)
- Indexes->removeMachineInstrFromMaps(*MI);
- // It's safe to erase MI because MII has already been incremented.
- MI->eraseFromParent();
- }
+ // We can remove identity copies right now.
+ handleIdentityCopy(*MI);
}
}
}