[SystemZ] Use LOAD AND TEST to eliminate comparisons against zero

llvm-svn: 187720

3 files changed, 48 insertions, 1 deletions

diff --git a/llvm/lib/Target/SystemZ/SystemZElimCompare.cpp b/llvm/lib/Target/SystemZ/SystemZElimCompare.cpp
index 9b0bdd8505e..bcdc5b728f0 100644
--- a/llvm/lib/Target/SystemZ/SystemZElimCompare.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZElimCompare.cpp
@@ -46,6 +46,7 @@ namespace {
     bool runOnMachineFunction(MachineFunction &F);
 
   private:
+    bool convertToLoadAndTest(MachineInstr *MI);
     bool adjustCCMasksForInstr(MachineInstr *MI, MachineInstr *Compare,
                                SmallVectorImpl<MachineInstr *> &CCUsers);
     bool optimizeCompareZero(MachineInstr *Compare,
@@ -83,9 +84,34 @@ static bool resultTests(MachineInstr *MI, unsigned Reg, unsigned SubReg) {
       MI->getOperand(0).getSubReg() == SubReg)
     return true;
 
+  switch (MI->getOpcode()) {
+  case SystemZ::LR:
+  case SystemZ::LGR:
+  case SystemZ::LGFR:
+  case SystemZ::LTR:
+  case SystemZ::LTGR:
+  case SystemZ::LTGFR:
+    if (MI->getOperand(1).getReg() == Reg &&
+        MI->getOperand(1).getSubReg() == SubReg)
+      return true;
+  }
+
   return false;
 }
 
+// If MI is a load instruction, try to convert it into a LOAD AND TEST.
+// Return true on success.
+bool SystemZElimCompare::convertToLoadAndTest(MachineInstr *MI) {
+  unsigned Opcode = TII->getLoadAndTest(MI->getOpcode());
+  if (!Opcode)
+    return false;
+
+  MI->setDesc(TII->get(Opcode));
+  MachineInstrBuilder(*MI->getParent()->getParent(), MI)
+    .addReg(SystemZ::CC, RegState::ImplicitDefine);
+  return true;
+}
+
 // 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
@@ -184,17 +210,21 @@ optimizeCompareZero(MachineInstr *Compare,
   unsigned SrcSubReg = Compare->getOperand(0).getSubReg();
   MachineBasicBlock *MBB = Compare->getParent();
   MachineBasicBlock::iterator MBBI = Compare, MBBE = MBB->begin();
+  bool SeenUseOfCC = false;
   while (MBBI != MBBE) {
     --MBBI;
     MachineInstr *MI = MBBI;
     if (resultTests(MI, SrcReg, SrcSubReg) &&
-        adjustCCMasksForInstr(MI, Compare, CCUsers)) {
+        ((!SeenUseOfCC && convertToLoadAndTest(MI)) ||
+         adjustCCMasksForInstr(MI, Compare, CCUsers))) {
       EliminatedComparisons += 1;
       return true;
     }
     if (MI->modifiesRegister(SrcReg, TRI) ||
         MI->modifiesRegister(SystemZ::CC, TRI))
       return false;
+    if (MI->readsRegister(SystemZ::CC, TRI))
+      SeenUseOfCC = true;
   }
   return false;
 }
diff --git a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
index 9913db7b0e4..5dd8d98d27d 100644
--- a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
@@ -753,6 +753,19 @@ unsigned SystemZInstrInfo::getOpcodeForOffset(unsigned Opcode,
   return 0;
 }
 
+unsigned SystemZInstrInfo::getLoadAndTest(unsigned Opcode) const {
+  switch (Opcode) {
+  case SystemZ::L:    return SystemZ::LT;
+  case SystemZ::LY:   return SystemZ::LT;
+  case SystemZ::LG:   return SystemZ::LTG;
+  case SystemZ::LGF:  return SystemZ::LTGF;
+  case SystemZ::LR:   return SystemZ::LTR;
+  case SystemZ::LGFR: return SystemZ::LTGFR;
+  case SystemZ::LGR:  return SystemZ::LTGR;
+  default:            return 0;
+  }
+}
+
 // Return true if Mask matches the regexp 0*1+0*, given that zero masks
 // have already been filtered out.  Store the first set bit in LSB and
 // the number of set bits in Length if so.
diff --git a/llvm/lib/Target/SystemZ/SystemZInstrInfo.h b/llvm/lib/Target/SystemZ/SystemZInstrInfo.h
index 763a3956fc1..1392745672f 100644
--- a/llvm/lib/Target/SystemZ/SystemZInstrInfo.h
+++ b/llvm/lib/Target/SystemZ/SystemZInstrInfo.h
@@ -193,6 +193,10 @@ public:
   // exists.
   unsigned getOpcodeForOffset(unsigned Opcode, int64_t Offset) const;
 
+  // If Opcode is a load instruction that has a LOAD AND TEST form,
+  // return the opcode for the testing form, otherwise return 0.
+  unsigned getLoadAndTest(unsigned Opcode) const;
+
   // Return true if ROTATE AND ... SELECTED BITS can be used to select bits
   // Mask of the R2 operand, given that only the low BitSize bits of Mask are
   // significant.  Set Start and End to the I3 and I4 operands if so.