[PATCH] D65267: [PowerPC][PreEmitPeephole] Remove redundant copies of immediate numbers after TailDup

[Kai Luo via Phabricator via llvm-commits]

lkail created this revision.
lkail added reviewers: hfinkel, steven.zhang, jsji, nemanjai, shchenz.
Herald added subscribers: llvm-commits, MaskRay, kbarton, hiraditya.
Herald added a project: LLVM.

After tailduplication, we have copies generated in termination BBs. If the source of a copy is an immediate number, we can fold such patterns.


Repository:
  rL LLVM

https://reviews.llvm.org/D65267

Files:
  llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp
  llvm/test/CodeGen/PowerPC/redundant-copy-after-tail-dup.ll


Index: llvm/test/CodeGen/PowerPC/redundant-copy-after-tail-dup.ll
===================================================================
--- llvm/test/CodeGen/PowerPC/redundant-copy-after-tail-dup.ll
+++ llvm/test/CodeGen/PowerPC/redundant-copy-after-tail-dup.ll
@@ -26,8 +26,7 @@
 ; CHECK-P9-NEXT:    cmplwi r3, 2
 ; CHECK-P9-NEXT:    bge- cr0, .LBB0_6
 ; CHECK-P9-NEXT:  # %bb.3: # %land.lhs.true.1
-; CHECK-P9-NEXT:    li r5, 0
-; CHECK-P9-NEXT:    mr r3, r5
+; CHECK-P9-NEXT:    li r3, 0
 ; CHECK-P9-NEXT:    blr
 ; CHECK-P9-NEXT:  .LBB0_4: # %lor.lhs.false
 ; CHECK-P9-NEXT:    cmplwi cr0, r4, 0
Index: llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp
===================================================================
--- llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp
+++ llvm/lib/Target/PowerPC/PPCPreEmitPeephole.cpp
@@ -35,6 +35,7 @@
           "Number of instructions deleted in pre-emit peephole");
 STATISTIC(NumberOfSelfCopies,
           "Number of self copy instructions eliminated");
+STATISTIC(NumCopyOfImmediateFolded, "Number of copies of immediates folded");
 
 static cl::opt<bool>
 RunPreEmitPeephole("ppc-late-peephole", cl::Hidden, cl::init(true),
@@ -57,6 +58,62 @@
           MachineFunctionProperties::Property::NoVRegs);
     }
 
+    /// Fold following pattern which is generated after tailduplication:
+    /// ```
+    /// li r6, 0
+    /// mr r3, r6
+    /// blr
+    /// ```
+    /// into
+    /// ```
+    /// li r3, 0
+    /// blr
+    /// ```
+    bool foldCopyOfImmediate(MachineInstr &MI,
+                             SmallVector<MachineInstr *, 4> &InstrsToErase) {
+      MachineBasicBlock *BB = MI.getParent();
+      MachineRegisterInfo *MRI = &BB->getParent()->getRegInfo();
+      // FIXME: Since we only find such patterns in termination BB after
+      // TailDuplication, no need to bother ourselves to seek a general
+      // implementation currently. We can extend it gradually if such patterns
+      // are found in non-termination BB.
+      if (MRI->isSSA() || !BB->succ_empty())
+        return false;
+      unsigned Opc = MI.getOpcode();
+      if (!((Opc == PPC::OR || Opc == PPC::OR8) &&
+            MI.getOperand(1).getReg() == MI.getOperand(2).getReg()))
+        return false;
+      unsigned Reg = MI.getOperand(1).getReg();
+      bool SeenIntermediateUse = false;
+      const PPCInstrInfo *TII =
+          BB->getParent()->getSubtarget<PPCSubtarget>().getInstrInfo();
+      MachineInstr *DefMI = TII->getDefMIPostRA(Reg, MI, SeenIntermediateUse);
+      if (DefMI == nullptr)
+        return false;
+      unsigned DefOpc = DefMI->getOpcode();
+      if (DefOpc != PPC::LI && DefOpc != PPC::LI8 && DefOpc != PPC::LIS &&
+          DefOpc != PPC::LIS8)
+        return false;
+      assert(DefMI->getOperand(1).isImm() && "Should be an immediate");
+      MachineInstr *NewInstr =
+          BuildMI(*MI.getParent(), &MI, MI.getDebugLoc(),
+                  TII->get(DefMI->getOpcode()), MI.getOperand(0).getReg())
+              .add(DefMI->getOperand(1));
+      (void)NewInstr;
+
+      NumCopyOfImmediateFolded++;
+      LLVM_DEBUG(dbgs() << "Fold instructions: ");
+      LLVM_DEBUG(DefMI->dump());
+      LLVM_DEBUG(MI.dump());
+      LLVM_DEBUG(dbgs() << "To: ");
+      LLVM_DEBUG(NewInstr->dump());
+
+      InstrsToErase.push_back(&MI);
+      if (!SeenIntermediateUse && MI.getOperand(1).isKill())
+        InstrsToErase.push_back(DefMI);
+      return true;
+    }
+
     // This function removes any redundant load immediates. It has two level
     // loops - The outer loop finds the load immediates BBI that could be used
     // to replace following redundancy. The inner loop scans instructions that
@@ -201,6 +258,7 @@
               InstrsToErase.push_back(DefMIToErase);
             }
           }
+          Changed |= foldCopyOfImmediate(MI, InstrsToErase);
         }
 
         // Eliminate conditional branch based on a constant CR bit by


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