[ARM] Remove a dead ADD during the creation of TBBs

During the optimisation of jump tables in the constant island pass,
an extra ADD could be left over, now dead but not removed.

Differential Revision: https://reviews.llvm.org/D31389

llvm-svn: 299634

1 files changed, 42 insertions, 1 deletions

diff --git a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
index 0de628b2c19..23722f1b7f3 100644
--- a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -2017,6 +2017,44 @@ static bool jumpTableFollowsTB(MachineInstr *JTMI, MachineInstr *CPEMI) {
          &*MBB->begin() == CPEMI;
 }
 
+static void RemoveDeadAddBetweenLEAAndJT(MachineInstr *LEAMI,
+                                         MachineInstr *JumpMI,
+                                         unsigned &DeadSize) {
+  // Remove a dead add between the LEA and JT, which used to compute EntryReg,
+  // but the JT now uses PC. Finds the last ADD (if any) that def's EntryReg
+  // and is not clobbered / used.
+  MachineInstr *RemovableAdd = nullptr;
+  unsigned EntryReg = JumpMI->getOperand(0).getReg();
+
+  // Find the last ADD to set EntryReg
+  MachineBasicBlock::iterator I(LEAMI);
+  for (++I; &*I != JumpMI; ++I) {
+    if (I->getOpcode() == ARM::t2ADDrs && I->getOperand(0).getReg() == EntryReg)
+      RemovableAdd = &*I;
+  }
+
+  if (!RemovableAdd)
+    return;
+
+  // Ensure EntryReg is not clobbered or used.
+  MachineBasicBlock::iterator J(RemovableAdd);
+  for (++J; &*J != JumpMI; ++J) {
+    for (unsigned K = 0, E = J->getNumOperands(); K != E; ++K) {
+      const MachineOperand &MO = J->getOperand(K);
+      if (!MO.isReg() || !MO.getReg())
+        continue;
+      if (MO.isDef() && MO.getReg() == EntryReg)
+        return;
+      if (MO.isUse() && MO.getReg() == EntryReg)
+        return;
+    }
+  }
+
+  DEBUG(dbgs() << "Removing Dead Add: " << *RemovableAdd);
+  RemovableAdd->eraseFromParent();
+  DeadSize += 4;
+}
+
 static bool registerDefinedBetween(unsigned Reg,
                                    MachineBasicBlock::iterator From,
                                    MachineBasicBlock::iterator To,
@@ -2172,7 +2210,10 @@ bool ARMConstantIslands::optimizeThumb2JumpTables() {
       NewJTMI->getOperand(0).setReg(ARM::PC);
       NewJTMI->getOperand(0).setIsKill(false);
 
-      if (CanDeleteLEA)  {
+      if (CanDeleteLEA) {
+        if (isThumb2)
+          RemoveDeadAddBetweenLEAAndJT(User.MI, MI, DeadSize);
+
         User.MI->eraseFromParent();
         DeadSize += isThumb2 ? 4 : 2;