Add a new pass on machine instructions to optimize away PHI cycles that

reduce down to a single value.  InstCombine already does this transformation
but DAG legalization may introduce new opportunities.  This has turned out to
be important for ARM where 64-bit values are split up during type legalization:
InstCombine is not able to remove the PHI cycles on the 64-bit values but
the separate 32-bit values can be optimized.  I measured the compile time 
impact of this (running llc on 176.gcc) and it was not significant.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95951 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 141 insertions, 0 deletions

diff --git a/lib/CodeGen/OptimizePHIs.cpp b/lib/CodeGen/OptimizePHIs.cpp
new file mode 100644
index 0000000000..5b3fa6a68a
--- /dev/null
+++ b/lib/CodeGen/OptimizePHIs.cpp
@@ -0,0 +1,141 @@
+//===-- OptimizePHIs.cpp - Optimize machine instruction PHIs --------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass optimizes machine instruction PHIs to take advantage of
+// opportunities created during DAG legalization.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "phi-opt"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Function.h"
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/Statistic.h"
+using namespace llvm;
+
+STATISTIC(NumPHICycles, "Number of PHI cycles replaced");
+
+namespace {
+  class OptimizePHIs : public MachineFunctionPass {
+    MachineRegisterInfo *MRI;
+    const TargetInstrInfo *TII;
+
+  public:
+    static char ID; // Pass identification
+    OptimizePHIs() : MachineFunctionPass(&ID) {}
+
+    virtual bool runOnMachineFunction(MachineFunction &MF);
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.setPreservesCFG();
+      MachineFunctionPass::getAnalysisUsage(AU);
+    }
+
+  private:
+    bool IsSingleValuePHICycle(const MachineInstr *MI, unsigned &SingleValReg,
+                               SmallSet<unsigned, 16> &RegsInCycle);
+    bool ReplacePHICycles(MachineBasicBlock &MBB);
+  };
+}
+
+char OptimizePHIs::ID = 0;
+static RegisterPass<OptimizePHIs>
+X("opt-phis", "Optimize machine instruction PHIs");
+
+FunctionPass *llvm::createOptimizePHIsPass() { return new OptimizePHIs(); }
+
+bool OptimizePHIs::runOnMachineFunction(MachineFunction &Fn) {
+  MRI = &Fn.getRegInfo();
+  TII = Fn.getTarget().getInstrInfo();
+
+  // Find PHI cycles that can be replaced by a single value.  InstCombine
+  // does this, but DAG legalization may introduce new opportunities, e.g.,
+  // when i64 values are split up for 32-bit targets.
+  bool Changed = false;
+  for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
+    Changed |= ReplacePHICycles(*I);
+
+  return Changed;
+}
+
+/// IsSingleValuePHICycle - Check if MI is a PHI where all the source operands
+/// are copies of SingleValReg, possibly via copies through other PHIs.  If
+/// SingleValReg is zero on entry, it is set to the register with the single
+/// non-copy value.  RegsInCycle is a set used to keep track of the PHIs that
+/// have been scanned.
+bool OptimizePHIs::IsSingleValuePHICycle(const MachineInstr *MI,
+                                         unsigned &SingleValReg,
+                                         SmallSet<unsigned, 16> &RegsInCycle) {
+  assert(MI->isPHI() && "IsSingleValuePHICycle expects a PHI instruction");
+  unsigned DstReg = MI->getOperand(0).getReg();
+
+  // See if we already saw this register.
+  if (!RegsInCycle.insert(DstReg))
+    return true;
+
+  // Don't scan crazily complex things.
+  if (RegsInCycle.size() == 16)
+    return false;
+
+  // Scan the PHI operands.
+  for (unsigned i = 1; i != MI->getNumOperands(); i += 2) {
+    unsigned SrcReg = MI->getOperand(i).getReg();
+    if (SrcReg == DstReg)
+      continue;
+    const MachineInstr *SrcMI = MRI->getVRegDef(SrcReg);
+
+    // Skip over register-to-register moves.
+    unsigned MvSrcReg, MvDstReg, SrcSubIdx, DstSubIdx;
+    if (SrcMI &&
+        TII->isMoveInstr(*SrcMI, MvSrcReg, MvDstReg, SrcSubIdx, DstSubIdx) &&
+        SrcSubIdx == 0 && DstSubIdx == 0 &&
+        TargetRegisterInfo::isVirtualRegister(MvSrcReg))
+      SrcMI = MRI->getVRegDef(MvSrcReg);
+    if (!SrcMI)
+      return false;
+
+    if (SrcMI->isPHI()) {
+      if (!IsSingleValuePHICycle(SrcMI, SingleValReg, RegsInCycle))
+        return false;
+    } else {
+      // Fail if there is more than one non-phi/non-move register.
+      if (SingleValReg != 0)
+        return false;
+      SingleValReg = SrcReg;
+    }
+  }
+  return true;
+}
+
+/// ReplacePHICycles - Find PHI cycles that can be replaced by a single
+/// value and remove them.
+bool OptimizePHIs::ReplacePHICycles(MachineBasicBlock &MBB) {
+  bool Changed = false;
+  for (MachineBasicBlock::iterator
+         MII = MBB.begin(), E = MBB.end(); MII != E; ) {
+    MachineInstr *MI = &*MII++;
+    if (!MI->isPHI())
+      break;
+
+    unsigned SingleValReg = 0;
+    SmallSet<unsigned, 16> RegsInCycle;
+    if (IsSingleValuePHICycle(MI, SingleValReg, RegsInCycle) &&
+        SingleValReg != 0) {
+      MRI->replaceRegWith(MI->getOperand(0).getReg(), SingleValReg);
+      MI->eraseFromParent();
+      ++NumPHICycles;
+      Changed = true;
+    }
+  }
+  return Changed;
+}