[SystemZ] Split out comparison elimination into a separate pass

Perhaps predictably, doing comparison elimination on the fly during
SystemZLongBranch turned out to be a bad idea.  The next patches make
use of LOAD AND TEST and BRANCH ON COUNT, both of which require
changes to earlier instructions.

No functionality change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187718 91177308-0d34-0410-b5e6-96231b3b80d8

5 files changed, 346 insertions, 270 deletions

diff --git a/lib/Target/SystemZ/CMakeLists.txt b/lib/Target/SystemZ/CMakeLists.txt
index 04bbec5..cd01f1a 100644
--- a/lib/Target/SystemZ/CMakeLists.txt
+++ b/lib/Target/SystemZ/CMakeLists.txt
@@ -15,6 +15,7 @@ add_llvm_target(SystemZCodeGen
   SystemZAsmPrinter.cpp
   SystemZCallingConv.cpp
   SystemZConstantPoolValue.cpp
+  SystemZElimCompare.cpp
   SystemZFrameLowering.cpp
   SystemZISelDAGToDAG.cpp
   SystemZISelLowering.cpp
diff --git a/lib/Target/SystemZ/SystemZ.h b/lib/Target/SystemZ/SystemZ.h
index 4c1e81a..eccc2aa 100644
--- a/lib/Target/SystemZ/SystemZ.h
+++ b/lib/Target/SystemZ/SystemZ.h
@@ -85,6 +85,7 @@ namespace llvm {
 
   FunctionPass *createSystemZISelDag(SystemZTargetMachine &TM,
                                      CodeGenOpt::Level OptLevel);
+  FunctionPass *createSystemZElimComparePass(SystemZTargetMachine &TM);
   FunctionPass *createSystemZLongBranchPass(SystemZTargetMachine &TM);
 } // end namespace llvm;
 #endif
diff --git a/lib/Target/SystemZ/SystemZElimCompare.cpp b/lib/Target/SystemZ/SystemZElimCompare.cpp
new file mode 100644
index 0000000..9b0bdd8
--- /dev/null
+++ b/lib/Target/SystemZ/SystemZElimCompare.cpp
@@ -0,0 +1,311 @@
+//===-- SystemZElimCompare.cpp - Eliminate comparison instructions --------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass:
+// (1) tries to remove compares if CC already contains the required information
+// (2) fuses compares and branches into COMPARE AND BRANCH instructions
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "systemz-elim-compare"
+
+#include "SystemZTargetMachine.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/IR/Function.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+
+using namespace llvm;
+
+STATISTIC(EliminatedComparisons, "Number of eliminated comparisons");
+STATISTIC(FusedComparisons, "Number of fused compare-and-branch instructions");
+
+namespace {
+  class SystemZElimCompare : public MachineFunctionPass {
+  public:
+    static char ID;
+    SystemZElimCompare(const SystemZTargetMachine &tm)
+      : MachineFunctionPass(ID), TII(0), TRI(0) {}
+
+    virtual const char *getPassName() const {
+      return "SystemZ Comparison Elimination";
+    }
+
+    bool processBlock(MachineBasicBlock *MBB);
+    bool runOnMachineFunction(MachineFunction &F);
+
+  private:
+    bool adjustCCMasksForInstr(MachineInstr *MI, MachineInstr *Compare,
+                               SmallVectorImpl<MachineInstr *> &CCUsers);
+    bool optimizeCompareZero(MachineInstr *Compare,
+                             SmallVectorImpl<MachineInstr *> &CCUsers);
+    bool fuseCompareAndBranch(MachineInstr *Compare,
+                              SmallVectorImpl<MachineInstr *> &CCUsers);
+
+    const SystemZInstrInfo *TII;
+    const TargetRegisterInfo *TRI;
+  };
+
+  char SystemZElimCompare::ID = 0;
+} // end of anonymous namespace
+
+FunctionPass *llvm::createSystemZElimComparePass(SystemZTargetMachine &TM) {
+  return new SystemZElimCompare(TM);
+}
+
+// Return true if CC is live out of MBB.
+static bool isCCLiveOut(MachineBasicBlock *MBB) {
+  for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
+         SE = MBB->succ_end(); SI != SE; ++SI)
+    if ((*SI)->isLiveIn(SystemZ::CC))
+      return true;
+  return false;
+}
+
+// Return true if any CC result of MI would reflect the value of subreg
+// SubReg of Reg.
+static bool resultTests(MachineInstr *MI, unsigned Reg, unsigned SubReg) {
+  if (MI->getNumOperands() > 0 &&
+      MI->getOperand(0).isReg() &&
+      MI->getOperand(0).isDef() &&
+      MI->getOperand(0).getReg() == Reg &&
+      MI->getOperand(0).getSubReg() == SubReg)
+    return true;
+
+  return false;
+}
+
+// The CC users in CCUsers are testing the result of a comparison of some
+// value X against zero and we know that any CC value produced by MI
+// would also reflect the value of X.  Try to adjust CCUsers so that
+// they test the result of MI directly, returning true on success.
+// Leave everything unchanged on failure.
+bool SystemZElimCompare::
+adjustCCMasksForInstr(MachineInstr *MI, MachineInstr *Compare,
+                      SmallVectorImpl<MachineInstr *> &CCUsers) {
+  int Opcode = MI->getOpcode();
+  const MCInstrDesc &Desc = TII->get(Opcode);
+  unsigned MIFlags = Desc.TSFlags;
+
+  // See which compare-style condition codes are available.
+  unsigned ReusableCCMask = 0;
+  if (MIFlags & SystemZII::CCHasZero)
+    ReusableCCMask |= SystemZ::CCMASK_CMP_EQ;
+
+  // For unsigned comparisons with zero, only equality makes sense.
+  unsigned CompareFlags = Compare->getDesc().TSFlags;
+  if (!(CompareFlags & SystemZII::IsLogical) &&
+      (MIFlags & SystemZII::CCHasOrder))
+    ReusableCCMask |= SystemZ::CCMASK_CMP_LT | SystemZ::CCMASK_CMP_GT;
+
+  if (ReusableCCMask == 0)
+    return false;
+
+  unsigned CCValues = SystemZII::getCCValues(MIFlags);
+  assert((ReusableCCMask & ~CCValues) == 0 && "Invalid CCValues");
+
+  // Now check whether these flags are enough for all users.
+  SmallVector<MachineOperand *, 4> AlterMasks;
+  for (unsigned int I = 0, E = CCUsers.size(); I != E; ++I) {
+    MachineInstr *MI = CCUsers[I];
+
+    // Fail if this isn't a use of CC that we understand.
+    unsigned Flags = MI->getDesc().TSFlags;
+    unsigned FirstOpNum;
+    if (Flags & SystemZII::CCMaskFirst)
+      FirstOpNum = 0;
+    else if (Flags & SystemZII::CCMaskLast)
+      FirstOpNum = MI->getNumExplicitOperands() - 2;
+    else
+      return false;
+
+    // Check whether the instruction predicate treats all CC values
+    // outside of ReusableCCMask in the same way.  In that case it
+    // doesn't matter what those CC values mean.
+    unsigned CCValid = MI->getOperand(FirstOpNum).getImm();
+    unsigned CCMask = MI->getOperand(FirstOpNum + 1).getImm();
+    unsigned OutValid = ~ReusableCCMask & CCValid;
+    unsigned OutMask = ~ReusableCCMask & CCMask;
+    if (OutMask != 0 && OutMask != OutValid)
+      return false;
+
+    AlterMasks.push_back(&MI->getOperand(FirstOpNum));
+    AlterMasks.push_back(&MI->getOperand(FirstOpNum + 1));
+  }
+
+  // All users are OK.  Adjust the masks for MI.
+  for (unsigned I = 0, E = AlterMasks.size(); I != E; I += 2) {
+    AlterMasks[I]->setImm(CCValues);
+    unsigned CCMask = AlterMasks[I + 1]->getImm();
+    if (CCMask & ~ReusableCCMask)
+      AlterMasks[I + 1]->setImm((CCMask & ReusableCCMask) |
+                                (CCValues & ~ReusableCCMask));
+  }
+
+  // CC is now live after MI.
+  int CCDef = MI->findRegisterDefOperandIdx(SystemZ::CC, false, true, TRI);
+  assert(CCDef >= 0 && "Couldn't find CC set");
+  MI->getOperand(CCDef).setIsDead(false);
+
+  // Clear any intervening kills of CC.
+  MachineBasicBlock::iterator MBBI = MI, MBBE = Compare;
+  for (++MBBI; MBBI != MBBE; ++MBBI)
+    MBBI->clearRegisterKills(SystemZ::CC, TRI);
+
+  return true;
+}
+
+// Try to optimize cases where comparison instruction Compare is testing
+// a value against zero.  Return true on success and if Compare should be
+// deleted as dead.  CCUsers is the list of instructions that use the CC
+// value produced by Compare.
+bool SystemZElimCompare::
+optimizeCompareZero(MachineInstr *Compare,
+                    SmallVectorImpl<MachineInstr *> &CCUsers) {
+  // Check whether this is a comparison against zero.
+  if (Compare->getNumExplicitOperands() != 2 ||
+      !Compare->getOperand(1).isImm() ||
+      Compare->getOperand(1).getImm() != 0)
+    return false;
+
+  // Search back for CC results that are based on the first operand.
+  unsigned SrcReg = Compare->getOperand(0).getReg();
+  unsigned SrcSubReg = Compare->getOperand(0).getSubReg();
+  MachineBasicBlock *MBB = Compare->getParent();
+  MachineBasicBlock::iterator MBBI = Compare, MBBE = MBB->begin();
+  while (MBBI != MBBE) {
+    --MBBI;
+    MachineInstr *MI = MBBI;
+    if (resultTests(MI, SrcReg, SrcSubReg) &&
+        adjustCCMasksForInstr(MI, Compare, CCUsers)) {
+      EliminatedComparisons += 1;
+      return true;
+    }
+    if (MI->modifiesRegister(SrcReg, TRI) ||
+        MI->modifiesRegister(SystemZ::CC, TRI))
+      return false;
+  }
+  return false;
+}
+
+// Try to fuse comparison instruction Compare into a later branch.
+// Return true on success and if Compare is therefore redundant.
+bool SystemZElimCompare::
+fuseCompareAndBranch(MachineInstr *Compare,
+                     SmallVectorImpl<MachineInstr *> &CCUsers) {
+  // See whether we have a comparison that can be fused.
+  unsigned FusedOpcode = TII->getCompareAndBranch(Compare->getOpcode(),
+                                                  Compare);
+  if (!FusedOpcode)
+    return false;
+
+  // See whether we have a single branch with which to fuse.
+  if (CCUsers.size() != 1)
+    return false;
+  MachineInstr *Branch = CCUsers[0];
+  if (Branch->getOpcode() != SystemZ::BRC)
+    return false;
+
+  // Make sure that the operands are available at the branch.
+  unsigned SrcReg = Compare->getOperand(0).getReg();
+  unsigned SrcReg2 = (Compare->getOperand(1).isReg() ?
+                      Compare->getOperand(1).getReg() : 0);
+  MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
+  for (++MBBI; MBBI != MBBE; ++MBBI)
+    if (MBBI->modifiesRegister(SrcReg, TRI) ||
+        (SrcReg2 && MBBI->modifiesRegister(SrcReg2, TRI)))
+      return false;
+
+  // Read the branch mask and target.
+  MachineOperand CCMask(MBBI->getOperand(1));
+  MachineOperand Target(MBBI->getOperand(2));
+  assert((CCMask.getImm() & ~SystemZ::CCMASK_ICMP) == 0 &&
+         "Invalid condition-code mask for integer comparison");
+
+  // Clear out all current operands.
+  int CCUse = MBBI->findRegisterUseOperandIdx(SystemZ::CC, false, TRI);
+  assert(CCUse >= 0 && "BRC must use CC");
+  Branch->RemoveOperand(CCUse);
+  Branch->RemoveOperand(2);
+  Branch->RemoveOperand(1);
+  Branch->RemoveOperand(0);
+
+  // Rebuild Branch as a fused compare and branch.
+  Branch->setDesc(TII->get(FusedOpcode));
+  MachineInstrBuilder(*Branch->getParent()->getParent(), Branch)
+    .addOperand(Compare->getOperand(0))
+    .addOperand(Compare->getOperand(1))
+    .addOperand(CCMask)
+    .addOperand(Target)
+    .addReg(SystemZ::CC, RegState::ImplicitDefine);
+
+  // Clear any intervening kills of SrcReg and SrcReg2.
+  MBBI = Compare;
+  for (++MBBI; MBBI != MBBE; ++MBBI) {
+    MBBI->clearRegisterKills(SrcReg, TRI);
+    if (SrcReg2)
+      MBBI->clearRegisterKills(SrcReg2, TRI);
+  }
+  FusedComparisons += 1;
+  return true;
+}
+
+// Process all comparison instructions in MBB.  Return true if something
+// changed.
+bool SystemZElimCompare::processBlock(MachineBasicBlock *MBB) {
+  bool Changed = false;
+
+  // Walk backwards through the block looking for comparisons, recording
+  // all CC users as we go.  The subroutines can delete Compare and
+  // instructions before it.
+  bool CompleteCCUsers = !isCCLiveOut(MBB);
+  SmallVector<MachineInstr *, 4> CCUsers;
+  MachineBasicBlock::iterator MBBI = MBB->end();
+  while (MBBI != MBB->begin()) {
+    MachineInstr *MI = --MBBI;
+    if (CompleteCCUsers &&
+        MI->isCompare() &&
+        (optimizeCompareZero(MI, CCUsers) ||
+         fuseCompareAndBranch(MI, CCUsers))) {
+      ++MBBI;
+      MI->removeFromParent();
+      Changed = true;
+      CCUsers.clear();
+      CompleteCCUsers = true;
+      continue;
+    }
+
+    if (MI->definesRegister(SystemZ::CC, TRI)) {
+      CCUsers.clear();
+      CompleteCCUsers = true;
+    } else if (MI->modifiesRegister(SystemZ::CC, TRI))
+      CompleteCCUsers = false;
+
+    if (CompleteCCUsers && MI->readsRegister(SystemZ::CC, TRI))
+      CCUsers.push_back(MI);
+  }
+  return Changed;
+}
+
+bool SystemZElimCompare::runOnMachineFunction(MachineFunction &F) {
+  TII = static_cast<const SystemZInstrInfo *>(F.getTarget().getInstrInfo());
+  TRI = &TII->getRegisterInfo();
+
+  bool Changed = false;
+  for (MachineFunction::iterator MFI = F.begin(), MFE = F.end();
+       MFI != MFE; ++MFI)
+    Changed |= processBlock(MFI);
+
+  return Changed;
+}
diff --git a/lib/Target/SystemZ/SystemZLongBranch.cpp b/lib/Target/SystemZ/SystemZLongBranch.cpp
index f0ea3e2..c5c4cab 100644
--- a/lib/Target/SystemZ/SystemZLongBranch.cpp
+++ b/lib/Target/SystemZ/SystemZLongBranch.cpp
@@ -7,44 +7,16 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This pass does three things:
-// (1) try to remove compares if CC already contains the required information
-// (2) fuse compares and branches into COMPARE AND BRANCH instructions
-// (3) make sure that all branches are in range.
-//
-// We do (1) here rather than earlier because some transformations can
-// change the set of available CC values and we generally want those
-// transformations to have priority over (1).  This is especially true in
-// the commonest case where the CC value is used by a single in-range branch
-// instruction, since (2) will then be able to fuse the compare and the
-// branch instead.
-//
-// For example, two-address NILF can sometimes be converted into
-// three-address RISBLG.  NILF produces a CC value that indicates whether
-// the low word is zero, but RISBLG does not modify CC at all.  On the
-// other hand, 64-bit ANDs like NILL can sometimes be converted to RISBG.
-// The CC value produced by NILL isn't useful for our purposes, but the
-// value produced by RISBG can be used for any comparison with zero
-// (not just equality).  So there are some transformations that lose
-// CC values (while still being worthwhile) and others that happen to make
-// the CC result more useful than it was originally.
-//
-// We do (2) here rather than earlier because the fused form prevents
-// predication.  It also has to happen after (1).
-//
-// Doing (2) so late makes it more likely that a register will be reused
-// between the compare and the branch, but it isn't clear whether preventing
-// that would be a win or not.
-//
-// There are several ways in which (3) could be done.  One aggressive
-// approach is to assume that all branches are in range and successively
-// replace those that turn out not to be in range with a longer form
-// (branch relaxation).  A simple implementation is to continually walk
-// through the function relaxing branches until no more changes are
-// needed and a fixed point is reached.  However, in the pathological
-// worst case, this implementation is quadratic in the number of blocks;
-// relaxing branch N can make branch N-1 go out of range, which in turn
-// can make branch N-2 go out of range, and so on.
+// This pass makes sure that all branches are in range.  There are several ways
+// in which this could be done.  One aggressive approach is to assume that all
+// branches are in range and successively replace those that turn out not
+// to be in range with a longer form (branch relaxation).  A simple
+// implementation is to continually walk through the function relaxing
+// branches until no more changes are needed and a fixed point is reached.
+// However, in the pathological worst case, this implementation is
+// quadratic in the number of blocks; relaxing branch N can make branch N-1
+// go out of range, which in turn can make branch N-2 go out of range,
+// and so on.
 //
 // An alternative approach is to assume that all branches must be
 // converted to their long forms, then reinstate the short forms of
@@ -99,8 +71,6 @@ using namespace llvm;
 STATISTIC(LongBranches, "Number of long branches.");
 
 namespace {
-  typedef MachineBasicBlock::iterator Iter;
-
   // Represents positional information about a basic block.
   struct MBBInfo {
     // The address that we currently assume the block has.
@@ -174,8 +144,6 @@ namespace {
     void skipTerminator(BlockPosition &Position, TerminatorInfo &Terminator,
                         bool AssumeRelaxed);
     TerminatorInfo describeTerminator(MachineInstr *MI);
-    bool optimizeCompareZero(MachineInstr *PrevCCSetter, MachineInstr *Compare);
-    bool fuseCompareAndBranch(MachineInstr *Compare);
     uint64_t initMBBInfo();
     bool mustRelaxBranch(const TerminatorInfo &Terminator, uint64_t Address);
     bool mustRelaxABranch();
@@ -273,226 +241,10 @@ TerminatorInfo SystemZLongBranch::describeTerminator(MachineInstr *MI) {
   return Terminator;
 }
 
-// Return true if CC is live out of MBB.
-static bool isCCLiveOut(MachineBasicBlock *MBB) {
-  for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
-         SE = MBB->succ_end(); SI != SE; ++SI)
-    if ((*SI)->isLiveIn(SystemZ::CC))
-      return true;
-  return false;
-}
-
-// Return true if CC is live after MBBI.
-static bool isCCLiveAfter(MachineBasicBlock::iterator MBBI,
-                          const TargetRegisterInfo *TRI) {
-  if (MBBI->killsRegister(SystemZ::CC, TRI))
-    return false;
-
-  MachineBasicBlock *MBB = MBBI->getParent();
-  MachineBasicBlock::iterator MBBE = MBB->end();
-  for (++MBBI; MBBI != MBBE; ++MBBI) {
-    if (MBBI->readsRegister(SystemZ::CC, TRI))
-      return true;
-    if (MBBI->definesRegister(SystemZ::CC, TRI))
-      return false;
-  }
-
-  return isCCLiveOut(MBB);
-}
-
-// Return true if all uses of the CC value produced by MBBI could make do
-// with the CC values in ReusableCCMask.  When returning true, point AlterMasks
-// to the "CC valid" and "CC mask" operands for each condition.
-static bool canRestrictCCMask(MachineBasicBlock::iterator MBBI,
-                              unsigned ReusableCCMask,
-                              SmallVectorImpl<MachineOperand *> &AlterMasks,
-                              const TargetRegisterInfo *TRI) {
-  MachineBasicBlock *MBB = MBBI->getParent();
-  MachineBasicBlock::iterator MBBE = MBB->end();
-  for (++MBBI; MBBI != MBBE; ++MBBI) {
-    if (MBBI->readsRegister(SystemZ::CC, TRI)) {
-      // Fail if this isn't a use of CC that we understand.
-      unsigned MBBIFlags = MBBI->getDesc().TSFlags;
-      unsigned FirstOpNum;
-      if (MBBIFlags & SystemZII::CCMaskFirst)
-        FirstOpNum = 0;
-      else if (MBBIFlags & SystemZII::CCMaskLast)
-        FirstOpNum = MBBI->getNumExplicitOperands() - 2;
-      else
-        return false;
-
-      // Check whether the instruction predicate treats all CC values
-      // outside of ReusableCCMask in the same way.  In that case it
-      // doesn't matter what those CC values mean.
-      unsigned CCValid = MBBI->getOperand(FirstOpNum).getImm();
-      unsigned CCMask = MBBI->getOperand(FirstOpNum + 1).getImm();
-      unsigned OutValid = ~ReusableCCMask & CCValid;
-      unsigned OutMask = ~ReusableCCMask & CCMask;
-      if (OutMask != 0 && OutMask != OutValid)
-        return false;
-
-      AlterMasks.push_back(&MBBI->getOperand(FirstOpNum));
-      AlterMasks.push_back(&MBBI->getOperand(FirstOpNum + 1));
-
-      // Succeed if this was the final use of the CC value.
-      if (MBBI->killsRegister(SystemZ::CC, TRI))
-        return true;
-    }
-    // Succeed if the instruction redefines CC.
-    if (MBBI->definesRegister(SystemZ::CC, TRI))
-      return true;
-  }
-  // Fail if there are other uses of CC that we didn't see.
-  return !isCCLiveOut(MBB);
-}
-
-// Try to make Compare redundant with PrevCCSetter, the previous setter of CC,
-// by looking for cases where Compare compares the result of PrevCCSetter
-// against zero.  Return true on success and if Compare can therefore
-// be deleted.
-bool SystemZLongBranch::optimizeCompareZero(MachineInstr *PrevCCSetter,
-                                            MachineInstr *Compare) {
-  if (MF->getTarget().getOptLevel() == CodeGenOpt::None)
-    return false;
-
-  // Check whether this is a comparison against zero.
-  if (Compare->getNumExplicitOperands() != 2 ||
-      !Compare->getOperand(1).isImm() ||
-      Compare->getOperand(1).getImm() != 0)
-    return false;
-
-  // See which compare-style condition codes are available after PrevCCSetter.
-  unsigned PrevFlags = PrevCCSetter->getDesc().TSFlags;
-  unsigned ReusableCCMask = 0;
-  if (PrevFlags & SystemZII::CCHasZero)
-    ReusableCCMask |= SystemZ::CCMASK_CMP_EQ;
-
-  // For unsigned comparisons with zero, only equality makes sense.
-  unsigned CompareFlags = Compare->getDesc().TSFlags;
-  if (!(CompareFlags & SystemZII::IsLogical) &&
-      (PrevFlags & SystemZII::CCHasOrder))
-    ReusableCCMask |= SystemZ::CCMASK_CMP_LT | SystemZ::CCMASK_CMP_GT;
-
-  if (ReusableCCMask == 0)
-    return false;
-
-  // Make sure that PrevCCSetter sets the value being compared.
-  unsigned SrcReg = Compare->getOperand(0).getReg();
-  unsigned SrcSubReg = Compare->getOperand(0).getSubReg();
-  if (!PrevCCSetter->getOperand(0).isReg() ||
-      !PrevCCSetter->getOperand(0).isDef() ||
-      PrevCCSetter->getOperand(0).getReg() != SrcReg ||
-      PrevCCSetter->getOperand(0).getSubReg() != SrcSubReg)
-    return false;
-
-  // Make sure that SrcReg survives until Compare.
-  MachineBasicBlock::iterator MBBI = PrevCCSetter, MBBE = Compare;
-  const TargetRegisterInfo *TRI = &TII->getRegisterInfo();
-  for (++MBBI; MBBI != MBBE; ++MBBI)
-    if (MBBI->modifiesRegister(SrcReg, TRI))
-      return false;
-
-  // See whether all uses of Compare's CC value could make do with
-  // the values produced by PrevCCSetter.
-  SmallVector<MachineOperand *, 4> AlterMasks;
-  if (!canRestrictCCMask(Compare, ReusableCCMask, AlterMasks, TRI))
-    return false;
-
-  // Alter the CC masks that canRestrictCCMask says need to be altered.
-  unsigned CCValues = SystemZII::getCCValues(PrevFlags);
-  assert((ReusableCCMask & ~CCValues) == 0 && "Invalid CCValues");
-  for (unsigned I = 0, E = AlterMasks.size(); I != E; I += 2) {
-    AlterMasks[I]->setImm(CCValues);
-    unsigned CCMask = AlterMasks[I + 1]->getImm();
-    if (CCMask & ~ReusableCCMask)
-      AlterMasks[I + 1]->setImm((CCMask & ReusableCCMask) |
-                                (CCValues & ~ReusableCCMask));
-  }
-
-  // CC is now live after PrevCCSetter.
-  int CCDef = PrevCCSetter->findRegisterDefOperandIdx(SystemZ::CC, false,
-                                                      true, TRI);
-  assert(CCDef >= 0 && "Couldn't find CC set");
-  PrevCCSetter->getOperand(CCDef).setIsDead(false);
-
-  // Clear any intervening kills of CC.
-  MBBI = PrevCCSetter;
-  for (++MBBI; MBBI != MBBE; ++MBBI)
-    MBBI->clearRegisterKills(SystemZ::CC, TRI);
-
-  return true;
-}
-
-// Try to fuse compare instruction Compare into a later branch.  Return
-// true on success and if Compare is therefore redundant.
-bool SystemZLongBranch::fuseCompareAndBranch(MachineInstr *Compare) {
-  if (MF->getTarget().getOptLevel() == CodeGenOpt::None)
-    return false;
-
-  unsigned FusedOpcode = TII->getCompareAndBranch(Compare->getOpcode(),
-                                                  Compare);
-  if (!FusedOpcode)
-    return false;
-
-  unsigned SrcReg = Compare->getOperand(0).getReg();
-  unsigned SrcReg2 = (Compare->getOperand(1).isReg() ?
-                      Compare->getOperand(1).getReg() : 0);
-  const TargetRegisterInfo *TRI = &TII->getRegisterInfo();
-  MachineBasicBlock *MBB = Compare->getParent();
-  MachineBasicBlock::iterator MBBI = Compare, MBBE = MBB->end();
-  for (++MBBI; MBBI != MBBE; ++MBBI) {
-    if (MBBI->getOpcode() == SystemZ::BRC && !isCCLiveAfter(MBBI, TRI)) {
-      // Read the branch mask and target.
-      MachineOperand CCMask(MBBI->getOperand(1));
-      MachineOperand Target(MBBI->getOperand(2));
-      assert((CCMask.getImm() & ~SystemZ::CCMASK_ICMP) == 0 &&
-             "Invalid condition-code mask for integer comparison");
-
-      // Clear out all current operands.
-      int CCUse = MBBI->findRegisterUseOperandIdx(SystemZ::CC, false, TRI);
-      assert(CCUse >= 0 && "BRC must use CC");
-      MBBI->RemoveOperand(CCUse);
-      MBBI->RemoveOperand(2);
-      MBBI->RemoveOperand(1);
-      MBBI->RemoveOperand(0);
-
-      // Rebuild MBBI as a fused compare and branch.
-      MBBI->setDesc(TII->get(FusedOpcode));
-      MachineInstrBuilder(*MBB->getParent(), MBBI)
-        .addOperand(Compare->getOperand(0))
-        .addOperand(Compare->getOperand(1))
-        .addOperand(CCMask)
-        .addOperand(Target);
-
-      // Clear any intervening kills of SrcReg and SrcReg2.
-      MBBI = Compare;
-      for (++MBBI; MBBI != MBBE; ++MBBI) {
-        MBBI->clearRegisterKills(SrcReg, TRI);
-        if (SrcReg2)
-          MBBI->clearRegisterKills(SrcReg2, TRI);
-      }
-      return true;
-    }
-
-    // Stop if we find another reference to CC before a branch.
-    if (MBBI->readsRegister(SystemZ::CC, TRI) ||
-        MBBI->modifiesRegister(SystemZ::CC, TRI))
-      return false;
-
-    // Stop if we find another assignment to the registers before the branch.
-    if (MBBI->modifiesRegister(SrcReg, TRI) ||
-        (SrcReg2 && MBBI->modifiesRegister(SrcReg2, TRI)))
-      return false;
-  }
-  return false;
-}
-
 // Fill MBBs and Terminators, setting the addresses on the assumption
 // that no branches need relaxation.  Return the size of the function under
 // this assumption.
 uint64_t SystemZLongBranch::initMBBInfo() {
-  const TargetRegisterInfo *TRI = &TII->getRegisterInfo();
-
   MF->RenumberBlocks();
   unsigned NumBlocks = MF->size();
 
@@ -513,20 +265,9 @@ uint64_t SystemZLongBranch::initMBBInfo() {
     // Calculate the size of the fixed part of the block.
     MachineBasicBlock::iterator MI = MBB->begin();
     MachineBasicBlock::iterator End = MBB->end();
-    MachineInstr *PrevCCSetter = 0;
     while (MI != End && !MI->isTerminator()) {
-      MachineInstr *Current = MI;
+      Block.Size += TII->getInstSizeInBytes(MI);
       ++MI;
-      if (Current->isCompare()) {
-        if ((PrevCCSetter && optimizeCompareZero(PrevCCSetter, Current)) ||
-            fuseCompareAndBranch(Current)) {
-          Current->removeFromParent();
-          continue;
-        }
-      }
-      if (Current->modifiesRegister(SystemZ::CC, TRI))
-        PrevCCSetter = Current;
-      Block.Size += TII->getInstSizeInBytes(Current);
     }
     skipNonTerminators(Position, Block);
 
diff --git a/lib/Target/SystemZ/SystemZTargetMachine.cpp b/lib/Target/SystemZ/SystemZTargetMachine.cpp
index 437ea61..2bacc2b 100644
--- a/lib/Target/SystemZ/SystemZTargetMachine.cpp
+++ b/lib/Target/SystemZ/SystemZTargetMachine.cpp
@@ -65,6 +65,28 @@ bool SystemZPassConfig::addPreSched2() {
 }
 
 bool SystemZPassConfig::addPreEmitPass() {
+  // We eliminate comparisons here rather than earlier because some
+  // transformations can change the set of available CC values and we
+  // generally want those transformations to have priority.  This is
+  // especially true in the commonest case where the result of the comparison
+  // is used by a single in-range branch instruction, since we will then
+  // be able to fuse the compare and the branch instead.
+  //
+  // For example, two-address NILF can sometimes be converted into
+  // three-address RISBLG.  NILF produces a CC value that indicates whether
+  // the low word is zero, but RISBLG does not modify CC at all.  On the
+  // other hand, 64-bit ANDs like NILL can sometimes be converted to RISBG.
+  // The CC value produced by NILL isn't useful for our purposes, but the
+  // value produced by RISBG can be used for any comparison with zero
+  // (not just equality).  So there are some transformations that lose
+  // CC values (while still being worthwhile) and others that happen to make
+  // the CC result more useful than it was originally.
+  //
+  // Doing it so late makes it more likely that a register will be reused
+  // between the comparison and the branch, but it isn't clear whether
+  // preventing that would be a win or not.
+  if (getOptLevel() != CodeGenOpt::None)
+    addPass(createSystemZElimComparePass(getSystemZTargetMachine()));
   addPass(createSystemZLongBranchPass(getSystemZTargetMachine()));
   return true;
 }