1//===-- SystemZElimCompare.cpp - Eliminate comparison instructions --------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass:
11// (1) tries to remove compares if CC already contains the required information
12// (2) fuses compares and branches into COMPARE AND BRANCH instructions
13//
14//===----------------------------------------------------------------------===//
15
16#include "SystemZTargetMachine.h"
17#include "llvm/ADT/Statistic.h"
18#include "llvm/CodeGen/MachineFunctionPass.h"
19#include "llvm/CodeGen/MachineInstrBuilder.h"
20#include "llvm/IR/Function.h"
21#include "llvm/Support/CommandLine.h"
22#include "llvm/Support/MathExtras.h"
23#include "llvm/Target/TargetInstrInfo.h"
24#include "llvm/Target/TargetMachine.h"
25#include "llvm/Target/TargetRegisterInfo.h"
26
27using namespace llvm;
28
29#define DEBUG_TYPE "systemz-elim-compare"
30
31STATISTIC(BranchOnCounts, "Number of branch-on-count instructions");
32STATISTIC(EliminatedComparisons, "Number of eliminated comparisons");
33STATISTIC(FusedComparisons, "Number of fused compare-and-branch instructions");
34
35namespace {
36// Represents the references to a particular register in one or more
37// instructions.
38struct Reference {
39  Reference()
40    : Def(false), Use(false), IndirectDef(false), IndirectUse(false) {}
41
42  Reference &operator|=(const Reference &Other) {
43    Def |= Other.Def;
44    IndirectDef |= Other.IndirectDef;
45    Use |= Other.Use;
46    IndirectUse |= Other.IndirectUse;
47    return *this;
48  }
49
50  explicit operator bool() const { return Def || Use; }
51
52  // True if the register is defined or used in some form, either directly or
53  // via a sub- or super-register.
54  bool Def;
55  bool Use;
56
57  // True if the register is defined or used indirectly, by a sub- or
58  // super-register.
59  bool IndirectDef;
60  bool IndirectUse;
61};
62
63class SystemZElimCompare : public MachineFunctionPass {
64public:
65  static char ID;
66  SystemZElimCompare(const SystemZTargetMachine &tm)
67    : MachineFunctionPass(ID), TII(nullptr), TRI(nullptr) {}
68
69  const char *getPassName() const override {
70    return "SystemZ Comparison Elimination";
71  }
72
73  bool processBlock(MachineBasicBlock &MBB);
74  bool runOnMachineFunction(MachineFunction &F) override;
75
76private:
77  Reference getRegReferences(MachineInstr *MI, unsigned Reg);
78  bool convertToBRCT(MachineInstr *MI, MachineInstr *Compare,
79                     SmallVectorImpl<MachineInstr *> &CCUsers);
80  bool convertToLoadAndTest(MachineInstr *MI);
81  bool adjustCCMasksForInstr(MachineInstr *MI, MachineInstr *Compare,
82                             SmallVectorImpl<MachineInstr *> &CCUsers);
83  bool optimizeCompareZero(MachineInstr *Compare,
84                           SmallVectorImpl<MachineInstr *> &CCUsers);
85  bool fuseCompareAndBranch(MachineInstr *Compare,
86                            SmallVectorImpl<MachineInstr *> &CCUsers);
87
88  const SystemZInstrInfo *TII;
89  const TargetRegisterInfo *TRI;
90};
91
92char SystemZElimCompare::ID = 0;
93} // end anonymous namespace
94
95FunctionPass *llvm::createSystemZElimComparePass(SystemZTargetMachine &TM) {
96  return new SystemZElimCompare(TM);
97}
98
99// Return true if CC is live out of MBB.
100static bool isCCLiveOut(MachineBasicBlock &MBB) {
101  for (auto SI = MBB.succ_begin(), SE = MBB.succ_end(); SI != SE; ++SI)
102    if ((*SI)->isLiveIn(SystemZ::CC))
103      return true;
104  return false;
105}
106
107// Return true if any CC result of MI would reflect the value of subreg
108// SubReg of Reg.
109static bool resultTests(MachineInstr *MI, unsigned Reg, unsigned SubReg) {
110  if (MI->getNumOperands() > 0 &&
111      MI->getOperand(0).isReg() &&
112      MI->getOperand(0).isDef() &&
113      MI->getOperand(0).getReg() == Reg &&
114      MI->getOperand(0).getSubReg() == SubReg)
115    return true;
116
117  switch (MI->getOpcode()) {
118  case SystemZ::LR:
119  case SystemZ::LGR:
120  case SystemZ::LGFR:
121  case SystemZ::LTR:
122  case SystemZ::LTGR:
123  case SystemZ::LTGFR:
124  case SystemZ::LER:
125  case SystemZ::LDR:
126  case SystemZ::LXR:
127  case SystemZ::LTEBR:
128  case SystemZ::LTDBR:
129  case SystemZ::LTXBR:
130    if (MI->getOperand(1).getReg() == Reg &&
131        MI->getOperand(1).getSubReg() == SubReg)
132      return true;
133  }
134
135  return false;
136}
137
138// Describe the references to Reg in MI, including sub- and super-registers.
139Reference SystemZElimCompare::getRegReferences(MachineInstr *MI, unsigned Reg) {
140  Reference Ref;
141  for (unsigned I = 0, E = MI->getNumOperands(); I != E; ++I) {
142    const MachineOperand &MO = MI->getOperand(I);
143    if (MO.isReg()) {
144      if (unsigned MOReg = MO.getReg()) {
145        if (MOReg == Reg || TRI->regsOverlap(MOReg, Reg)) {
146          if (MO.isUse()) {
147            Ref.Use = true;
148            Ref.IndirectUse |= (MOReg != Reg);
149          }
150          if (MO.isDef()) {
151            Ref.Def = true;
152            Ref.IndirectDef |= (MOReg != Reg);
153          }
154        }
155      }
156    }
157  }
158  return Ref;
159}
160
161// Compare compares the result of MI against zero.  If MI is an addition
162// of -1 and if CCUsers is a single branch on nonzero, eliminate the addition
163// and convert the branch to a BRCT(G).  Return true on success.
164bool
165SystemZElimCompare::convertToBRCT(MachineInstr *MI, MachineInstr *Compare,
166                                  SmallVectorImpl<MachineInstr *> &CCUsers) {
167  // Check whether we have an addition of -1.
168  unsigned Opcode = MI->getOpcode();
169  unsigned BRCT;
170  if (Opcode == SystemZ::AHI)
171    BRCT = SystemZ::BRCT;
172  else if (Opcode == SystemZ::AGHI)
173    BRCT = SystemZ::BRCTG;
174  else
175    return false;
176  if (MI->getOperand(2).getImm() != -1)
177    return false;
178
179  // Check whether we have a single JLH.
180  if (CCUsers.size() != 1)
181    return false;
182  MachineInstr *Branch = CCUsers[0];
183  if (Branch->getOpcode() != SystemZ::BRC ||
184      Branch->getOperand(0).getImm() != SystemZ::CCMASK_ICMP ||
185      Branch->getOperand(1).getImm() != SystemZ::CCMASK_CMP_NE)
186    return false;
187
188  // We already know that there are no references to the register between
189  // MI and Compare.  Make sure that there are also no references between
190  // Compare and Branch.
191  unsigned SrcReg = Compare->getOperand(0).getReg();
192  MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
193  for (++MBBI; MBBI != MBBE; ++MBBI)
194    if (getRegReferences(MBBI, SrcReg))
195      return false;
196
197  // The transformation is OK.  Rebuild Branch as a BRCT(G).
198  MachineOperand Target(Branch->getOperand(2));
199  Branch->RemoveOperand(2);
200  Branch->RemoveOperand(1);
201  Branch->RemoveOperand(0);
202  Branch->setDesc(TII->get(BRCT));
203  MachineInstrBuilder(*Branch->getParent()->getParent(), Branch)
204    .addOperand(MI->getOperand(0))
205    .addOperand(MI->getOperand(1))
206    .addOperand(Target)
207    .addReg(SystemZ::CC, RegState::ImplicitDefine);
208  MI->removeFromParent();
209  return true;
210}
211
212// If MI is a load instruction, try to convert it into a LOAD AND TEST.
213// Return true on success.
214bool SystemZElimCompare::convertToLoadAndTest(MachineInstr *MI) {
215  unsigned Opcode = TII->getLoadAndTest(MI->getOpcode());
216  if (!Opcode)
217    return false;
218
219  MI->setDesc(TII->get(Opcode));
220  MachineInstrBuilder(*MI->getParent()->getParent(), MI)
221    .addReg(SystemZ::CC, RegState::ImplicitDefine);
222  return true;
223}
224
225// The CC users in CCUsers are testing the result of a comparison of some
226// value X against zero and we know that any CC value produced by MI
227// would also reflect the value of X.  Try to adjust CCUsers so that
228// they test the result of MI directly, returning true on success.
229// Leave everything unchanged on failure.
230bool SystemZElimCompare::
231adjustCCMasksForInstr(MachineInstr *MI, MachineInstr *Compare,
232                      SmallVectorImpl<MachineInstr *> &CCUsers) {
233  int Opcode = MI->getOpcode();
234  const MCInstrDesc &Desc = TII->get(Opcode);
235  unsigned MIFlags = Desc.TSFlags;
236
237  // See which compare-style condition codes are available.
238  unsigned ReusableCCMask = SystemZII::getCompareZeroCCMask(MIFlags);
239
240  // For unsigned comparisons with zero, only equality makes sense.
241  unsigned CompareFlags = Compare->getDesc().TSFlags;
242  if (CompareFlags & SystemZII::IsLogical)
243    ReusableCCMask &= SystemZ::CCMASK_CMP_EQ;
244
245  if (ReusableCCMask == 0)
246    return false;
247
248  unsigned CCValues = SystemZII::getCCValues(MIFlags);
249  assert((ReusableCCMask & ~CCValues) == 0 && "Invalid CCValues");
250
251  // Now check whether these flags are enough for all users.
252  SmallVector<MachineOperand *, 4> AlterMasks;
253  for (unsigned int I = 0, E = CCUsers.size(); I != E; ++I) {
254    MachineInstr *MI = CCUsers[I];
255
256    // Fail if this isn't a use of CC that we understand.
257    unsigned Flags = MI->getDesc().TSFlags;
258    unsigned FirstOpNum;
259    if (Flags & SystemZII::CCMaskFirst)
260      FirstOpNum = 0;
261    else if (Flags & SystemZII::CCMaskLast)
262      FirstOpNum = MI->getNumExplicitOperands() - 2;
263    else
264      return false;
265
266    // Check whether the instruction predicate treats all CC values
267    // outside of ReusableCCMask in the same way.  In that case it
268    // doesn't matter what those CC values mean.
269    unsigned CCValid = MI->getOperand(FirstOpNum).getImm();
270    unsigned CCMask = MI->getOperand(FirstOpNum + 1).getImm();
271    unsigned OutValid = ~ReusableCCMask & CCValid;
272    unsigned OutMask = ~ReusableCCMask & CCMask;
273    if (OutMask != 0 && OutMask != OutValid)
274      return false;
275
276    AlterMasks.push_back(&MI->getOperand(FirstOpNum));
277    AlterMasks.push_back(&MI->getOperand(FirstOpNum + 1));
278  }
279
280  // All users are OK.  Adjust the masks for MI.
281  for (unsigned I = 0, E = AlterMasks.size(); I != E; I += 2) {
282    AlterMasks[I]->setImm(CCValues);
283    unsigned CCMask = AlterMasks[I + 1]->getImm();
284    if (CCMask & ~ReusableCCMask)
285      AlterMasks[I + 1]->setImm((CCMask & ReusableCCMask) |
286                                (CCValues & ~ReusableCCMask));
287  }
288
289  // CC is now live after MI.
290  int CCDef = MI->findRegisterDefOperandIdx(SystemZ::CC, false, true, TRI);
291  assert(CCDef >= 0 && "Couldn't find CC set");
292  MI->getOperand(CCDef).setIsDead(false);
293
294  // Clear any intervening kills of CC.
295  MachineBasicBlock::iterator MBBI = MI, MBBE = Compare;
296  for (++MBBI; MBBI != MBBE; ++MBBI)
297    MBBI->clearRegisterKills(SystemZ::CC, TRI);
298
299  return true;
300}
301
302// Return true if Compare is a comparison against zero.
303static bool isCompareZero(MachineInstr *Compare) {
304  switch (Compare->getOpcode()) {
305  case SystemZ::LTEBRCompare:
306  case SystemZ::LTDBRCompare:
307  case SystemZ::LTXBRCompare:
308    return true;
309
310  default:
311    return (Compare->getNumExplicitOperands() == 2 &&
312            Compare->getOperand(1).isImm() &&
313            Compare->getOperand(1).getImm() == 0);
314  }
315}
316
317// Try to optimize cases where comparison instruction Compare is testing
318// a value against zero.  Return true on success and if Compare should be
319// deleted as dead.  CCUsers is the list of instructions that use the CC
320// value produced by Compare.
321bool SystemZElimCompare::
322optimizeCompareZero(MachineInstr *Compare,
323                    SmallVectorImpl<MachineInstr *> &CCUsers) {
324  if (!isCompareZero(Compare))
325    return false;
326
327  // Search back for CC results that are based on the first operand.
328  unsigned SrcReg = Compare->getOperand(0).getReg();
329  unsigned SrcSubReg = Compare->getOperand(0).getSubReg();
330  MachineBasicBlock &MBB = *Compare->getParent();
331  MachineBasicBlock::iterator MBBI = Compare, MBBE = MBB.begin();
332  Reference CCRefs;
333  Reference SrcRefs;
334  while (MBBI != MBBE) {
335    --MBBI;
336    MachineInstr *MI = MBBI;
337    if (resultTests(MI, SrcReg, SrcSubReg)) {
338      // Try to remove both MI and Compare by converting a branch to BRCT(G).
339      // We don't care in this case whether CC is modified between MI and
340      // Compare.
341      if (!CCRefs.Use && !SrcRefs && convertToBRCT(MI, Compare, CCUsers)) {
342        BranchOnCounts += 1;
343        return true;
344      }
345      // Try to eliminate Compare by reusing a CC result from MI.
346      if ((!CCRefs && convertToLoadAndTest(MI)) ||
347          (!CCRefs.Def && adjustCCMasksForInstr(MI, Compare, CCUsers))) {
348        EliminatedComparisons += 1;
349        return true;
350      }
351    }
352    SrcRefs |= getRegReferences(MI, SrcReg);
353    if (SrcRefs.Def)
354      return false;
355    CCRefs |= getRegReferences(MI, SystemZ::CC);
356    if (CCRefs.Use && CCRefs.Def)
357      return false;
358  }
359  return false;
360}
361
362// Try to fuse comparison instruction Compare into a later branch.
363// Return true on success and if Compare is therefore redundant.
364bool SystemZElimCompare::
365fuseCompareAndBranch(MachineInstr *Compare,
366                     SmallVectorImpl<MachineInstr *> &CCUsers) {
367  // See whether we have a comparison that can be fused.
368  unsigned FusedOpcode = TII->getCompareAndBranch(Compare->getOpcode(),
369                                                  Compare);
370  if (!FusedOpcode)
371    return false;
372
373  // See whether we have a single branch with which to fuse.
374  if (CCUsers.size() != 1)
375    return false;
376  MachineInstr *Branch = CCUsers[0];
377  if (Branch->getOpcode() != SystemZ::BRC)
378    return false;
379
380  // Make sure that the operands are available at the branch.
381  unsigned SrcReg = Compare->getOperand(0).getReg();
382  unsigned SrcReg2 = (Compare->getOperand(1).isReg() ?
383                      Compare->getOperand(1).getReg() : 0);
384  MachineBasicBlock::iterator MBBI = Compare, MBBE = Branch;
385  for (++MBBI; MBBI != MBBE; ++MBBI)
386    if (MBBI->modifiesRegister(SrcReg, TRI) ||
387        (SrcReg2 && MBBI->modifiesRegister(SrcReg2, TRI)))
388      return false;
389
390  // Read the branch mask and target.
391  MachineOperand CCMask(MBBI->getOperand(1));
392  MachineOperand Target(MBBI->getOperand(2));
393  assert((CCMask.getImm() & ~SystemZ::CCMASK_ICMP) == 0 &&
394         "Invalid condition-code mask for integer comparison");
395
396  // Clear out all current operands.
397  int CCUse = MBBI->findRegisterUseOperandIdx(SystemZ::CC, false, TRI);
398  assert(CCUse >= 0 && "BRC must use CC");
399  Branch->RemoveOperand(CCUse);
400  Branch->RemoveOperand(2);
401  Branch->RemoveOperand(1);
402  Branch->RemoveOperand(0);
403
404  // Rebuild Branch as a fused compare and branch.
405  Branch->setDesc(TII->get(FusedOpcode));
406  MachineInstrBuilder(*Branch->getParent()->getParent(), Branch)
407    .addOperand(Compare->getOperand(0))
408    .addOperand(Compare->getOperand(1))
409    .addOperand(CCMask)
410    .addOperand(Target)
411    .addReg(SystemZ::CC, RegState::ImplicitDefine);
412
413  // Clear any intervening kills of SrcReg and SrcReg2.
414  MBBI = Compare;
415  for (++MBBI; MBBI != MBBE; ++MBBI) {
416    MBBI->clearRegisterKills(SrcReg, TRI);
417    if (SrcReg2)
418      MBBI->clearRegisterKills(SrcReg2, TRI);
419  }
420  FusedComparisons += 1;
421  return true;
422}
423
424// Process all comparison instructions in MBB.  Return true if something
425// changed.
426bool SystemZElimCompare::processBlock(MachineBasicBlock &MBB) {
427  bool Changed = false;
428
429  // Walk backwards through the block looking for comparisons, recording
430  // all CC users as we go.  The subroutines can delete Compare and
431  // instructions before it.
432  bool CompleteCCUsers = !isCCLiveOut(MBB);
433  SmallVector<MachineInstr *, 4> CCUsers;
434  MachineBasicBlock::iterator MBBI = MBB.end();
435  while (MBBI != MBB.begin()) {
436    MachineInstr *MI = --MBBI;
437    if (CompleteCCUsers &&
438        MI->isCompare() &&
439        (optimizeCompareZero(MI, CCUsers) ||
440         fuseCompareAndBranch(MI, CCUsers))) {
441      ++MBBI;
442      MI->removeFromParent();
443      Changed = true;
444      CCUsers.clear();
445      CompleteCCUsers = true;
446      continue;
447    }
448
449    Reference CCRefs(getRegReferences(MI, SystemZ::CC));
450    if (CCRefs.Def) {
451      CCUsers.clear();
452      CompleteCCUsers = !CCRefs.IndirectDef;
453    }
454    if (CompleteCCUsers && CCRefs.Use)
455      CCUsers.push_back(MI);
456  }
457  return Changed;
458}
459
460bool SystemZElimCompare::runOnMachineFunction(MachineFunction &F) {
461  TII = static_cast<const SystemZInstrInfo *>(F.getSubtarget().getInstrInfo());
462  TRI = &TII->getRegisterInfo();
463
464  bool Changed = false;
465  for (auto &MBB : F)
466    Changed |= processBlock(MBB);
467
468  return Changed;
469}
470