[PowerPC] optimize conditional branch on CRSET/CRUNSET

This patch adds a check to optimize conditional branch (BC and BCn) based on a constant set by CRSET or CRUNSET.
Other optimizers, such as block placement, may generate such code and hence
I do this at the very end of the optimization in pre-emit peephole pass.

A conditional branch based on a constant is eliminated or converted into unconditional branch. 
Also CRSET/CRUNSET is eliminated if the condition code register is not used
by instruction other than the branch to be optimized.

Differential Revision: https://reviews.llvm.org/D52345

llvm-svn: 343100

1 files changed, 71 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp b/llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp
index 1892d1e3dc2..55da0a295aa 100644
--- a/llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp
+++ b/llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp
@@ -18,6 +18,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/LivePhysRegs.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
@@ -60,6 +61,7 @@ namespace {
         return false;
       bool Changed = false;
       const PPCInstrInfo *TII = MF.getSubtarget<PPCSubtarget>().getInstrInfo();
+      const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
       SmallVector<MachineInstr *, 4> InstrsToErase;
       for (MachineBasicBlock &MBB : MF) {
         for (MachineInstr &MI : MBB) {
@@ -74,6 +76,75 @@ namespace {
             }
           }
         }
+
+        // Eliminate conditional branch based on a constant CR bit by
+        // CRSET or CRUNSET. We eliminate the conditional branch or
+        // convert it into an unconditional branch. Also, if the CR bit
+        // is not used by other instructions, we eliminate CRSET as well.
+        auto I = MBB.getFirstInstrTerminator();
+        if (I == MBB.instr_end())
+          continue;
+        MachineInstr *Br = &*I;
+        if (Br->getOpcode() != PPC::BC && Br->getOpcode() != PPC::BCn)
+          continue;
+        MachineInstr *CRSetMI = nullptr;
+        unsigned CRBit = Br->getOperand(0).getReg();
+        unsigned CRReg = getCRFromCRBit(CRBit);
+        bool SeenUse = false;
+        MachineBasicBlock::reverse_iterator It = Br, Er = MBB.rend();
+        for (It++; It != Er; It++) {
+          if (It->modifiesRegister(CRBit, TRI)) {
+            if ((It->getOpcode() == PPC::CRUNSET ||
+                 It->getOpcode() == PPC::CRSET) &&
+                It->getOperand(0).getReg() == CRBit)
+              CRSetMI = &*It;
+            break;
+          }
+          if (It->readsRegister(CRBit, TRI))
+            SeenUse = true;
+        }
+        if (!CRSetMI) continue;
+
+        unsigned CRSetOp = CRSetMI->getOpcode();
+        if ((Br->getOpcode() == PPC::BCn && CRSetOp == PPC::CRSET) ||
+            (Br->getOpcode() == PPC::BC  && CRSetOp == PPC::CRUNSET)) {
+          // Remove this branch since it cannot be taken.
+          InstrsToErase.push_back(Br);
+          MBB.removeSuccessor(Br->getOperand(1).getMBB());
+        }
+        else {
+          // This conditional branch is always taken. So, remove all branches
+          // and insert an unconditional branch to the destination of this.
+          MachineBasicBlock::iterator It = Br, Er = MBB.end();
+          for (; It != Er && !SeenUse; It++) {
+            if (It->isDebugInstr()) continue;
+            assert(It->isTerminator() && "Non-terminator after a terminator");
+            InstrsToErase.push_back(&*It);
+          }
+          if (!MBB.isLayoutSuccessor(Br->getOperand(1).getMBB())) {
+            ArrayRef<MachineOperand> NoCond;
+            TII->insertBranch(MBB, Br->getOperand(1).getMBB(), nullptr,
+                              NoCond, Br->getDebugLoc());
+          }
+          for (auto &Succ : MBB.successors())
+            if (Succ != Br->getOperand(1).getMBB()) {
+              MBB.removeSuccessor(Succ);
+              break;
+            }
+        }
+
+        // If the CRBit is not used by another instruction, we can eliminate
+        // CRSET/CRUNSET instruction.
+        if (!SeenUse) {
+          // We need to check use of the CRBit in successors.
+          for (auto &SuccMBB : MBB.successors())
+            if (SuccMBB->isLiveIn(CRBit) || SuccMBB->isLiveIn(CRReg)) {
+              SeenUse = true;
+              break;
+            }
+          if (!SeenUse)
+            InstrsToErase.push_back(CRSetMI);
+        }
       }
       for (MachineInstr *MI : InstrsToErase) {
         LLVM_DEBUG(dbgs() << "PPC pre-emit peephole: erasing instruction: ");