[SystemZ] Rework compare and branch support

Before the patch we took advantage of the fact that the compare and
branch are glued together in the selection DAG and fused them together
(where possible) while emitting them.  This seemed to work well in practice.
However, fusing the compare so early makes it harder to remove redundant
compares in cases where CC already has a suitable value.  This patch
therefore uses the peephole analyzeCompare/optimizeCompareInstr pair of
functions instead.

No behavioral change intended, but it paves the way for a later patch.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187116 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 2 insertions, 54 deletions

diff --git a/lib/Target/SystemZ/SystemZISelLowering.cpp b/lib/Target/SystemZ/SystemZISelLowering.cpp
index e70f775169..87710022c9 100644
--- a/lib/Target/SystemZ/SystemZISelLowering.cpp
+++ b/lib/Target/SystemZ/SystemZISelLowering.cpp
@@ -1695,34 +1695,6 @@ static MachineBasicBlock *splitBlockAfter(MachineInstr *MI,
   return NewMBB;
 }
 
-bool SystemZTargetLowering::
-convertPrevCompareToBranch(MachineBasicBlock *MBB,
-                           MachineBasicBlock::iterator MBBI,
-                           unsigned CCMask, MachineBasicBlock *Target) const {
-  MachineBasicBlock::iterator Compare = MBBI;
-  MachineBasicBlock::iterator Begin = MBB->begin();
-  do
-    {
-      if (Compare == Begin)
-        return false;
-      --Compare;
-    }
-  while (Compare->isDebugValue());
-
-  const SystemZInstrInfo *TII = TM.getInstrInfo();
-  unsigned FusedOpcode = TII->getCompareAndBranch(Compare->getOpcode(),
-                                                  Compare);
-  if (!FusedOpcode)
-    return false;
-
-  DebugLoc DL = Compare->getDebugLoc();
-  BuildMI(*MBB, MBBI, DL, TII->get(FusedOpcode))
-    .addOperand(Compare->getOperand(0)).addOperand(Compare->getOperand(1))
-    .addImm(CCMask).addMBB(Target);
-  Compare->removeFromParent();
-  return true;
-}
-
 // Implement EmitInstrWithCustomInserter for pseudo Select* instruction MI.
 MachineBasicBlock *
 SystemZTargetLowering::emitSelect(MachineInstr *MI,
@@ -1742,15 +1714,8 @@ SystemZTargetLowering::emitSelect(MachineInstr *MI,
   //  StartMBB:
   //   BRC CCMask, JoinMBB
   //   # fallthrough to FalseMBB
-  //
-  // The original DAG glues comparisons to their uses, both to ensure
-  // that no CC-clobbering instructions are inserted between them, and
-  // to ensure that comparison results are not reused.  This means that
-  // this Select is the sole user of any preceding comparison instruction
-  // and that we can try to use a fused compare and branch instead.
   MBB = StartMBB;
-  if (!convertPrevCompareToBranch(MBB, MI, CCMask, JoinMBB))
-    BuildMI(MBB, DL, TII->get(SystemZ::BRC)).addImm(CCMask).addMBB(JoinMBB);
+  BuildMI(MBB, DL, TII->get(SystemZ::BRC)).addImm(CCMask).addMBB(JoinMBB);
   MBB->addSuccessor(JoinMBB);
   MBB->addSuccessor(FalseMBB);
 
@@ -1814,15 +1779,8 @@ SystemZTargetLowering::emitCondStore(MachineInstr *MI,
   //  StartMBB:
   //   BRC CCMask, JoinMBB
   //   # fallthrough to FalseMBB
-  //
-  // The original DAG glues comparisons to their uses, both to ensure
-  // that no CC-clobbering instructions are inserted between them, and
-  // to ensure that comparison results are not reused.  This means that
-  // this CondStore is the sole user of any preceding comparison instruction
-  // and that we can try to use a fused compare and branch instead.
   MBB = StartMBB;
-  if (!convertPrevCompareToBranch(MBB, MI, CCMask, JoinMBB))
-    BuildMI(MBB, DL, TII->get(SystemZ::BRC)).addImm(CCMask).addMBB(JoinMBB);
+  BuildMI(MBB, DL, TII->get(SystemZ::BRC)).addImm(CCMask).addMBB(JoinMBB);
   MBB->addSuccessor(JoinMBB);
   MBB->addSuccessor(FalseMBB);
 
@@ -2475,16 +2433,6 @@ EmitInstrWithCustomInserter(MachineInstr *MI, MachineBasicBlock *MBB) const {
 
   case SystemZ::ATOMIC_CMP_SWAPW:
     return emitAtomicCmpSwapW(MI, MBB);
-  case SystemZ::BRC:
-    // The original DAG glues comparisons to their uses, both to ensure
-    // that no CC-clobbering instructions are inserted between them, and
-    // to ensure that comparison results are not reused.  This means that
-    // a BRC is the sole user of a preceding comparison and that we can
-    // try to use a fused compare and branch instead.
-    if (convertPrevCompareToBranch(MBB, MI, MI->getOperand(0).getImm(),
-                                   MI->getOperand(1).getMBB()))
-      MI->eraseFromParent();
-    return MBB;
   case SystemZ::MVCWrapper:
     return emitMVCWrapper(MI, MBB);
   default: