- Fix SelectionDAG to generate correct CFGs.

- Add a basic machine-level dead block eliminator.

These two have to go together, since many other parts of the code generator are unable to handle the unreachable blocks otherwise created.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54333 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/LiveVariables.h

@@ -247,10 +247,8 @@ public:
     assert(Removed && "Register is not defined by this instruction!");
     return true;
   }
-
-  virtual void getAnalysisUsage(AnalysisUsage &AU) const {
-    AU.setPreservesAll();
-  }
+  
+  void getAnalysisUsage(AnalysisUsage &AU) const;
 
   virtual void releaseMemory() {
     VirtRegInfo.clear();

include/llvm/CodeGen/Passes.h

@@ -71,6 +71,10 @@ namespace llvm {
   /// register allocators.
   extern const PassInfo *const TwoAddressInstructionPassID;
 
+  /// UnreachableMachineBlockElimination pass - This pass removes unreachable
+  /// machine basic blocks.
+  extern const PassInfo *const UnreachableMachineBlockElimID;
+
   /// Creates a register allocator as the user specified on the command line.
   ///
   FunctionPass *createRegisterAllocator();

include/llvm/Target/TargetOptions.h

@@ -101,6 +101,11 @@ namespace llvm {
   /// DisableJumpTables - This flag indicates jump tables should not be 
   /// generated.
   extern bool DisableJumpTables;
+  
+  /// DisableCorrectBranchFolding - This flag indicates whether the instruction
+  /// selector should take care to update the CFG properly when
+  /// folding branches.
+  extern bool DisableCorrectBranchFolding;
 } // End llvm namespace
 
 #endif

lib/CodeGen/LiveVariables.cpp

@@ -29,6 +29,7 @@
 #include "llvm/CodeGen/LiveVariables.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/Passes.h"
 #include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetMachine.h"
@@ -43,6 +44,12 @@ using namespace llvm;
 char LiveVariables::ID = 0;
 static RegisterPass<LiveVariables> X("livevars", "Live Variable Analysis");
 
+
+void LiveVariables::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequiredID(UnreachableMachineBlockElimID);
+  AU.setPreservesAll();
+}
+
 void LiveVariables::VarInfo::dump() const {
   cerr << "  Alive in blocks: ";
   for (unsigned i = 0, e = AliveBlocks.size(); i != e; ++i)

lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

@@ -1632,11 +1632,24 @@ void SelectionDAGLowering::visitSwitchCase(SelectionDAGISel::CaseBlock &CB) {
   }
   SDValue BrCond = DAG.getNode(ISD::BRCOND, MVT::Other, getControlRoot(), Cond,
                                  DAG.getBasicBlock(CB.TrueBB));
-  if (CB.FalseBB == NextBlock)
+  
+  // If the branch was constant folded, fix up the CFG.
+  if (BrCond.getOpcode() == ISD::BR) {
+    if (!DisableCorrectBranchFolding)
+      CurMBB->removeSuccessor(CB.FalseBB);
     DAG.setRoot(BrCond);
-  else
-    DAG.setRoot(DAG.getNode(ISD::BR, MVT::Other, BrCond, 
-                            DAG.getBasicBlock(CB.FalseBB)));
+  } else {
+    // Otherwise, go ahead and insert the false branch.
+    if (BrCond == getControlRoot()) 
+      if (!DisableCorrectBranchFolding)
+        CurMBB->removeSuccessor(CB.TrueBB);
+    
+    if (CB.FalseBB == NextBlock)
+      DAG.setRoot(BrCond);
+    else
+      DAG.setRoot(DAG.getNode(ISD::BR, MVT::Other, BrCond, 
+                              DAG.getBasicBlock(CB.FalseBB)));
+  }
 }
 
 /// visitJumpTable - Emit JumpTable node in the current MBB

lib/CodeGen/UnreachableBlockElim.cpp

@@ -26,8 +26,10 @@
 #include "llvm/Function.h"
 #include "llvm/Pass.h"
 #include "llvm/Type.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/ADT/DepthFirstIterator.h"
 using namespace llvm;
 
@@ -71,11 +73,93 @@ bool UnreachableBlockElim::runOnFunction(Function &F) {
       BB->dropAllReferences();
     }
 
-  if (DeadBlocks.empty()) return false;
+  // Actually remove the blocks now.
+  for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
+    DeadBlocks[i]->eraseFromParent();
+
+  return DeadBlocks.size();
+}
+
+
+namespace {
+  class VISIBILITY_HIDDEN UnreachableMachineBlockElim : 
+        public MachineFunctionPass {
+    virtual bool runOnMachineFunction(MachineFunction &F);
+    bool iterateOnFunction(MachineFunction& F);
+    
+  public:
+    static char ID; // Pass identification, replacement for typeid
+    UnreachableMachineBlockElim() : MachineFunctionPass((intptr_t)&ID) {}
+  };
+}
+char UnreachableMachineBlockElim::ID = 0;
+
+static RegisterPass<UnreachableMachineBlockElim>
+Y("unreachable-mbb-elimination",
+  "Remove unreachable machine basic blocks");
+
+const PassInfo *const llvm::UnreachableMachineBlockElimID = &Y;
+
+bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
+  bool changed = true;
+  bool result = false;
+  
+  while (changed) {
+    changed = iterateOnFunction(F);
+    result |= changed;
+  }
+  
+  if (result)
+    F.RenumberBlocks();
+  
+  return result;
+}
+
+bool UnreachableMachineBlockElim::iterateOnFunction(MachineFunction &F) {
+  std::set<MachineBasicBlock*> Reachable;
+
+  // Mark all reachable blocks.
+  for (df_ext_iterator<MachineFunction*> I = df_ext_begin(&F, Reachable),
+         E = df_ext_end(&F, Reachable); I != E; ++I)
+    /* Mark all reachable blocks */;
+
+  // Loop over all dead blocks, remembering them and deleting all instructions
+  // in them.
+  std::vector<MachineBasicBlock*> DeadBlocks;
+  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I)
+    if (!Reachable.count(I)) {
+      MachineBasicBlock *BB = I;
+      DeadBlocks.push_back(BB);
+      
+      while (BB->succ_begin() != BB->succ_end()) {
+        MachineBasicBlock* succ = *BB->succ_begin();
+        
+        MachineBasicBlock::iterator start = succ->begin();
+        while (start != succ->end() &&
+               start->getOpcode() == TargetInstrInfo::PHI) {
+          for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
+            if (start->getOperand(i).isMBB() &&
+                start->getOperand(i).getMBB() == BB) {
+              start->RemoveOperand(i);
+              start->RemoveOperand(i-1);
+            }
+          
+          if (start->getNumOperands() == 1) {
+            MachineInstr* phi = start;
+            start++;
+            phi->eraseFromParent();
+          } else
+            start++;
+        }
+        
+        BB->removeSuccessor(BB->succ_begin());
+      }
+    }
 
   // Actually remove the blocks now.
   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
-    F.getBasicBlockList().erase(DeadBlocks[i]);
+    DeadBlocks[i]->eraseFromParent();
 
-  return true;
+  return DeadBlocks.size();
 }
+

lib/Target/TargetMachine.cpp

@@ -40,6 +40,7 @@ namespace llvm {
   bool RealignStack;
   bool VerboseAsm;
   bool DisableJumpTables;
+  bool DisableCorrectBranchFolding;
 }
 
 static cl::opt<bool, true> PrintCode("print-machineinstrs",
@@ -163,6 +164,11 @@ DisableSwitchTables(cl::Hidden, "disable-jump-tables",
            cl::location(DisableJumpTables),
            cl::init(false));
 
+static cl::opt<bool, true>
+DisableCorrectISelFolding(cl::Hidden, "disable-correct-folding",
+cl::desc("Do not perform correct branch folding in the instruction selector."),
+cl::location(DisableCorrectBranchFolding), cl::init(false));
+
 //---------------------------------------------------------------------------
 // TargetMachine Class
 //