In Machine CSE pass, the source register of a COPY machine instruction can
be propagated to all its users, and this propagation could increase the
probability of finding common subexpressions. If the COPY has only one user,
the COPY itself can be removed.
llvm-svn: 215344
diff --git a/llvm/lib/CodeGen/MachineCSE.cpp b/llvm/lib/CodeGen/MachineCSE.cpp
index b032c7d..ae26967 100644
--- a/llvm/lib/CodeGen/MachineCSE.cpp
+++ b/llvm/lib/CodeGen/MachineCSE.cpp
@@ -79,7 +79,8 @@
SmallVector<MachineInstr*, 64> Exps;
unsigned CurrVN;
- bool PerformTrivialCoalescing(MachineInstr *MI, MachineBasicBlock *MBB);
+ bool PerformTrivialCopyPropagation(MachineInstr *MI,
+ MachineBasicBlock *MBB);
bool isPhysDefTriviallyDead(unsigned Reg,
MachineBasicBlock::const_iterator I,
MachineBasicBlock::const_iterator E) const;
@@ -113,8 +114,12 @@
INITIALIZE_PASS_END(MachineCSE, "machine-cse",
"Machine Common Subexpression Elimination", false, false)
-bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
- MachineBasicBlock *MBB) {
+/// The source register of a COPY machine instruction can be propagated to all
+/// its users, and this propagation could increase the probability of finding
+/// common subexpressions. If the COPY has only one user, the COPY itself can
+/// be removed.
+bool MachineCSE::PerformTrivialCopyPropagation(MachineInstr *MI,
+ MachineBasicBlock *MBB) {
bool Changed = false;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
@@ -123,10 +128,7 @@
unsigned Reg = MO.getReg();
if (!TargetRegisterInfo::isVirtualRegister(Reg))
continue;
- if (!MRI->hasOneNonDBGUse(Reg))
- // Only coalesce single use copies. This ensure the copy will be
- // deleted.
- continue;
+ bool OnlyOneUse = MRI->hasOneNonDBGUse(Reg);
MachineInstr *DefMI = MRI->getVRegDef(Reg);
if (!DefMI->isCopy())
continue;
@@ -154,10 +156,14 @@
continue;
DEBUG(dbgs() << "Coalescing: " << *DefMI);
DEBUG(dbgs() << "*** to: " << *MI);
+ // Propagate SrcReg of copies to MI.
MO.setReg(SrcReg);
MRI->clearKillFlags(SrcReg);
- DefMI->eraseFromParent();
- ++NumCoalesces;
+ // Coalesce single use copies.
+ if (OnlyOneUse) {
+ DefMI->eraseFromParent();
+ ++NumCoalesces;
+ }
Changed = true;
}
@@ -454,13 +460,15 @@
bool FoundCSE = VNT.count(MI);
if (!FoundCSE) {
- // Look for trivial copy coalescing opportunities.
- if (PerformTrivialCoalescing(MI, MBB)) {
+ // Using trivial copy propagation to find more CSE opportunities.
+ if (PerformTrivialCopyPropagation(MI, MBB)) {
Changed = true;
// After coalescing MI itself may become a copy.
if (MI->isCopyLike())
continue;
+
+ // Try again to see if CSE is possible.
FoundCSE = VNT.count(MI);
}
}