[Sparc][NFC] Clean up SelectCC lowering

The ARM, BPF, MSP430, Sparc and Mips backends all use a similar code sequence 
for lowering SelectCC. As pointed out by @reames in D29937, this code isn't 
particularly clear and in most of these backends doesn't actually match the 
comments. This patch makes the code sequence clearer for the Sparc backend 
through better variable naming and more accurate comments (e.g. we are 
inserting triangle control flow, _not_ diamond). There is no functional 
change.

Differential Revision: https://reviews.llvm.org/D37194


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312713 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/Sparc/SparcISelLowering.cpp

@@ -3132,57 +3132,53 @@ SparcTargetLowering::expandSelectCC(MachineInstr &MI, MachineBasicBlock *BB,
   DebugLoc dl = MI.getDebugLoc();
   unsigned CC = (SPCC::CondCodes)MI.getOperand(3).getImm();
 
-  // To "insert" a SELECT_CC instruction, we actually have to insert the diamond
-  // control-flow pattern.  The incoming instruction knows the destination vreg
-  // to set, the condition code register to branch on, the true/false values to
-  // select between, and a branch opcode to use.
+  // To "insert" a SELECT_CC instruction, we actually have to insert the
+  // triangle control-flow pattern. The incoming instruction knows the
+  // destination vreg to set, the condition code register to branch on, the
+  // true/false values to select between, and the condition code for the branch.
+  //
+  // We produce the following control flow:
+  //     ThisMBB
+  //     |  \
+  //     |  IfFalseMBB
+  //     | /
+  //    SinkMBB
   const BasicBlock *LLVM_BB = BB->getBasicBlock();
   MachineFunction::iterator It = ++BB->getIterator();
 
-  //  thisMBB:
-  //  ...
-  //   TrueVal = ...
-  //   [f]bCC copy1MBB
-  //   fallthrough --> copy0MBB
-  MachineBasicBlock *thisMBB = BB;
+  MachineBasicBlock *ThisMBB = BB;
   MachineFunction *F = BB->getParent();
-  MachineBasicBlock *copy0MBB = F->CreateMachineBasicBlock(LLVM_BB);
-  MachineBasicBlock *sinkMBB = F->CreateMachineBasicBlock(LLVM_BB);
-  F->insert(It, copy0MBB);
-  F->insert(It, sinkMBB);
-
-  // Transfer the remainder of BB and its successor edges to sinkMBB.
-  sinkMBB->splice(sinkMBB->begin(), BB,
-                  std::next(MachineBasicBlock::iterator(MI)),
-                  BB->end());
-  sinkMBB->transferSuccessorsAndUpdatePHIs(BB);
-
-  // Add the true and fallthrough blocks as its successors.
-  BB->addSuccessor(copy0MBB);
-  BB->addSuccessor(sinkMBB);
-
-  BuildMI(BB, dl, TII.get(BROpcode)).addMBB(sinkMBB).addImm(CC);
-
-  //  copy0MBB:
-  //   %FalseValue = ...
-  //   # fallthrough to sinkMBB
-  BB = copy0MBB;
-
-  // Update machine-CFG edges
-  BB->addSuccessor(sinkMBB);
-
-  //  sinkMBB:
-  //   %Result = phi [ %FalseValue, copy0MBB ], [ %TrueValue, thisMBB ]
-  //  ...
-  BB = sinkMBB;
-  BuildMI(*BB, BB->begin(), dl, TII.get(SP::PHI), MI.getOperand(0).getReg())
-      .addReg(MI.getOperand(2).getReg())
-      .addMBB(copy0MBB)
+  MachineBasicBlock *IfFalseMBB = F->CreateMachineBasicBlock(LLVM_BB);
+  MachineBasicBlock *SinkMBB = F->CreateMachineBasicBlock(LLVM_BB);
+  F->insert(It, IfFalseMBB);
+  F->insert(It, SinkMBB);
+
+  // Transfer the remainder of ThisMBB and its successor edges to SinkMBB.
+  SinkMBB->splice(SinkMBB->begin(), ThisMBB,
+                  std::next(MachineBasicBlock::iterator(MI)), ThisMBB->end());
+  SinkMBB->transferSuccessorsAndUpdatePHIs(ThisMBB);
+
+  // Set the new successors for ThisMBB.
+  ThisMBB->addSuccessor(IfFalseMBB);
+  ThisMBB->addSuccessor(SinkMBB);
+
+  BuildMI(ThisMBB, dl, TII.get(BROpcode))
+    .addMBB(SinkMBB)
+    .addImm(CC);
+
+  // IfFalseMBB just falls through to SinkMBB.
+  IfFalseMBB->addSuccessor(SinkMBB);
+
+  // %Result = phi [ %TrueValue, ThisMBB ], [ %FalseValue, IfFalseMBB ]
+  BuildMI(*SinkMBB, SinkMBB->begin(), dl, TII.get(SP::PHI),
+          MI.getOperand(0).getReg())
       .addReg(MI.getOperand(1).getReg())
-      .addMBB(thisMBB);
+      .addMBB(ThisMBB)
+      .addReg(MI.getOperand(2).getReg())
+      .addMBB(IfFalseMBB);
 
   MI.eraseFromParent(); // The pseudo instruction is gone now.
-  return BB;
+  return SinkMBB;
 }
 
 MachineBasicBlock *