Re-apply r127953 with fixes: eliminate empty return block if it has no predecessors; update dominator tree if cfg is modified.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127981 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetLowering.h

@@ -1287,6 +1287,14 @@ public:
     return false;
   }
 
+  /// mayBeEmittedAsTailCall - Return true if the target may be able emit the
+  /// call instruction as a tail call. This is used by optimization passes to
+  /// determine if it's profitable to duplicate return instructions to enable
+  /// tailcall optimization.
+  virtual bool mayBeEmittedAsTailCall(CallInst *CI) const {
+    return false;
+  }
+
   /// getTypeForExtArgOrReturn - Return the type that should be used to zero or
   /// sign extend a zeroext/signext integer argument or return value.
   /// FIXME: Most C calling convention requires the return type to be promoted,

lib/Target/ARM/ARMISelLowering.cpp

@@ -1805,6 +1805,16 @@ bool ARMTargetLowering::isUsedByReturnOnly(SDNode *N) const {
   return HasRet;
 }
 
+bool ARMTargetLowering::mayBeEmittedAsTailCall(CallInst *CI) const {
+  if (!EnableARMTailCalls)
+    return false;
+
+  if (!CI->isTailCall())
+    return false;
+
+  return !Subtarget->isThumb1Only();
+}
+
 // ConstantPool, JumpTable, GlobalAddress, and ExternalSymbol are lowered as
 // their target counterpart wrapped in the ARMISD::Wrapper node. Suppose N is
 // one of the above mentioned nodes. It has to be wrapped because otherwise

lib/Target/ARM/ARMISelLowering.h

@@ -457,6 +457,8 @@ namespace llvm {
 
     virtual bool isUsedByReturnOnly(SDNode *N) const;
 
+    virtual bool mayBeEmittedAsTailCall(CallInst *CI) const;
+
     SDValue getARMCmp(SDValue LHS, SDValue RHS, ISD::CondCode CC,
                       SDValue &ARMcc, SelectionDAG &DAG, DebugLoc dl) const;
     SDValue getVFPCmp(SDValue LHS, SDValue RHS,

lib/Target/X86/X86ISelLowering.cpp

@@ -45,6 +45,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/VectorExtras.h"
+#include "llvm/Support/CallSite.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Dwarf.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -1595,6 +1596,18 @@ static bool IsTailCallConvention(CallingConv::ID CC) {
   return (CC == CallingConv::Fast || CC == CallingConv::GHC);
 }
 
+bool X86TargetLowering::mayBeEmittedAsTailCall(CallInst *CI) const {
+  if (!CI->isTailCall())
+    return false;
+
+  CallSite CS(CI);
+  CallingConv::ID CalleeCC = CS.getCallingConv();
+  if (!IsTailCallConvention(CalleeCC) && CalleeCC != CallingConv::C)
+    return false;
+
+  return true;
+}
+
 /// FuncIsMadeTailCallSafe - Return true if the function is being made into
 /// a tailcall target by changing its ABI.
 static bool FuncIsMadeTailCallSafe(CallingConv::ID CC) {

lib/Target/X86/X86ISelLowering.h

@@ -843,6 +843,8 @@ namespace llvm {
 
     virtual bool isUsedByReturnOnly(SDNode *N) const;
 
+    virtual bool mayBeEmittedAsTailCall(CallInst *CI) const;
+
     virtual EVT
     getTypeForExtArgOrReturn(LLVMContext &Context, EVT VT,
                              ISD::NodeType ExtendKind) const;

lib/Transforms/Scalar/CodeGenPrepare.cpp

@@ -47,16 +47,17 @@ using namespace llvm;
 using namespace llvm::PatternMatch;
 
 STATISTIC(NumBlocksElim, "Number of blocks eliminated");
-STATISTIC(NumPHIsElim, "Number of trivial PHIs eliminated");
-STATISTIC(NumGEPsElim, "Number of GEPs converted to casts");
+STATISTIC(NumPHIsElim,   "Number of trivial PHIs eliminated");
+STATISTIC(NumGEPsElim,   "Number of GEPs converted to casts");
 STATISTIC(NumCmpUses, "Number of uses of Cmp expressions replaced with uses of "
                       "sunken Cmps");
 STATISTIC(NumCastUses, "Number of uses of Cast expressions replaced with uses "
                        "of sunken Casts");
 STATISTIC(NumMemoryInsts, "Number of memory instructions whose address "
                           "computations were sunk");
-STATISTIC(NumExtsMoved, "Number of [s|z]ext instructions combined with loads");
-STATISTIC(NumExtUses, "Number of uses of [s|z]ext instructions optimized");
+STATISTIC(NumExtsMoved,  "Number of [s|z]ext instructions combined with loads");
+STATISTIC(NumExtUses,    "Number of uses of [s|z]ext instructions optimized");
+STATISTIC(NumRetsDup,    "Number of return instructions duplicated");
 
 static cl::opt<bool> DisableBranchOpts(
   "disable-cgp-branch-opts", cl::Hidden, cl::init(false),
@@ -75,11 +76,15 @@ namespace {
     /// update it.
     BasicBlock::iterator CurInstIterator;
 
-    // Keeps track of non-local addresses that have been sunk into a block. This
-    // allows us to avoid inserting duplicate code for blocks with multiple
-    // load/stores of the same address.
+    /// Keeps track of non-local addresses that have been sunk into a block.
+    /// This allows us to avoid inserting duplicate code for blocks with
+    /// multiple load/stores of the same address.
     DenseMap<Value*, Value*> SunkAddrs;
 
+    /// UpdateDT - If CFG is modified in anyway, dominator tree may need to
+    /// be updated.
+    bool UpdateDT;
+
   public:
     static char ID; // Pass identification, replacement for typeid
     explicit CodeGenPrepare(const TargetLowering *tli = 0)
@@ -104,6 +109,7 @@ namespace {
     bool OptimizeCallInst(CallInst *CI);
     bool MoveExtToFormExtLoad(Instruction *I);
     bool OptimizeExtUses(Instruction *I);
+    bool DupRetToEnableTailCallOpts(ReturnInst *RI);
   };
 }
 
@@ -118,8 +124,10 @@ FunctionPass *llvm::createCodeGenPreparePass(const TargetLowering *TLI) {
 bool CodeGenPrepare::runOnFunction(Function &F) {
   bool EverMadeChange = false;
 
+  UpdateDT = false;
   DT = getAnalysisIfAvailable<DominatorTree>();
   PFI = getAnalysisIfAvailable<ProfileInfo>();
+
   // First pass, eliminate blocks that contain only PHI nodes and an
   // unconditional branch.
   EverMadeChange |= EliminateMostlyEmptyBlocks(F);
@@ -127,8 +135,10 @@ bool CodeGenPrepare::runOnFunction(Function &F) {
   bool MadeChange = true;
   while (MadeChange) {
     MadeChange = false;
-    for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
+    for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
+      BasicBlock *BB = I++;
       MadeChange |= OptimizeBlock(*BB);
+    }
     EverMadeChange |= MadeChange;
   }
 
@@ -139,11 +149,14 @@ bool CodeGenPrepare::runOnFunction(Function &F) {
     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
       MadeChange |= ConstantFoldTerminator(BB);
 
-    if (MadeChange && DT)
-      DT->DT->recalculate(F);
+    if (MadeChange)
+      UpdateDT = true;
     EverMadeChange |= MadeChange;
   }
 
+  if (UpdateDT && DT)
+    DT->DT->recalculate(F);
+
   return EverMadeChange;
 }
 
@@ -547,6 +560,102 @@ bool CodeGenPrepare::OptimizeCallInst(CallInst *CI) {
   return Simplifier.fold(CI, TD);
 }
 
+/// DupRetToEnableTailCallOpts - Look for opportunities to duplicate return
+/// instructions to the predecessor to enable tail call optimizations. The
+/// case it is currently looking for is:
+/// bb0:
+///   %tmp0 = tail call i32 @f0()
+///   br label %return
+/// bb1:
+///   %tmp1 = tail call i32 @f1()
+///   br label %return
+/// bb2:
+///   %tmp2 = tail call i32 @f2()
+///   br label %return
+/// return:
+///   %retval = phi i32 [ %tmp0, %bb0 ], [ %tmp1, %bb1 ], [ %tmp2, %bb2 ]
+///   ret i32 %retval
+///
+/// =>
+///
+/// bb0:
+///   %tmp0 = tail call i32 @f0()
+///   ret i32 %tmp0
+/// bb1:
+///   %tmp1 = tail call i32 @f1()
+///   ret i32 %tmp1
+/// bb2:
+///   %tmp2 = tail call i32 @f2()
+///   ret i32 %tmp2
+///
+bool CodeGenPrepare::DupRetToEnableTailCallOpts(ReturnInst *RI) {
+  Value *V = RI->getReturnValue();
+  if (!V)
+    return false;
+
+  if (PHINode *PN = dyn_cast<PHINode>(V)) {
+    BasicBlock *BB = RI->getParent();
+    if (PN->getParent() != BB)
+      return false;
+
+    // It's not safe to eliminate the sign / zero extension of the return value.
+    // See llvm::isInTailCallPosition().
+    const Function *F = BB->getParent();
+    unsigned CallerRetAttr = F->getAttributes().getRetAttributes();
+    if ((CallerRetAttr & Attribute::ZExt) || (CallerRetAttr & Attribute::SExt))
+      return false;
+
+    // Make sure there are no instructions between PHI and return.
+    BasicBlock::iterator BI = PN;
+    do { ++BI; } while (isa<DbgInfoIntrinsic>(BI));
+    if (&*BI != RI)
+      return false;
+
+    /// Only dup the ReturnInst if the CallInst is likely to be emitted as a
+    /// tail call.
+    SmallVector<CallInst*, 4> TailCalls;
+    for (unsigned I = 0, E = PN->getNumIncomingValues(); I != E; ++I) {
+      CallInst *CI = dyn_cast<CallInst>(PN->getIncomingValue(I));
+      // Make sure the phi value is indeed produced by the tail call.
+      if (CI && CI->hasOneUse() && CI->getParent() == PN->getIncomingBlock(I) &&
+          TLI->mayBeEmittedAsTailCall(CI))
+        TailCalls.push_back(CI);
+    }
+
+    bool Changed = false;
+    for (unsigned i = 0, e = TailCalls.size(); i != e; ++i) {
+      CallInst *CI = TailCalls[i];
+      CallSite CS(CI);
+
+      // Conservatively require the attributes of the call to match those of
+      // the return. Ignore noalias because it doesn't affect the call sequence.
+      unsigned CalleeRetAttr = CS.getAttributes().getRetAttributes();
+      if ((CalleeRetAttr ^ CallerRetAttr) & ~Attribute::NoAlias)
+        continue;
+
+      // Make sure the call instruction is followed by an unconditional branch
+      // to the return block.
+      BasicBlock *CallBB = CI->getParent();
+      BranchInst *BI = dyn_cast<BranchInst>(CallBB->getTerminator());
+      if (!BI || !BI->isUnconditional() || BI->getSuccessor(0) != BB)
+        continue;
+
+      // Duplicate the return into CallBB.
+      (void)FoldReturnIntoUncondBranch(RI, BB, CallBB);
+      UpdateDT = Changed = true;
+      ++NumRetsDup;
+    }
+
+    // If we eliminated all predecessors of the block, delete the block now.
+    if (Changed && pred_begin(BB) == pred_end(BB))
+      BB->eraseFromParent();
+
+    return Changed;
+  }
+
+  return false;
+}
+
 //===----------------------------------------------------------------------===//
 // Memory Optimization
 //===----------------------------------------------------------------------===//
@@ -970,6 +1079,9 @@ bool CodeGenPrepare::OptimizeInst(Instruction *I) {
   if (CallInst *CI = dyn_cast<CallInst>(I))
     return OptimizeCallInst(CI);
 
+  if (ReturnInst *RI = dyn_cast<ReturnInst>(I))
+    return DupRetToEnableTailCallOpts(RI);
+
   return false;
 }