Add a late optimization to BranchFolding that hoist common instruction sequences

at the start of basic blocks to their common predecessor. It's actually quite
common (e.g. about 50 times in JM/lencod) and has shown to be a nice code size
benefit. e.g.

        pushq   %rax
        testl   %edi, %edi
        jne     LBB0_2
## BB#1:
        xorb    %al, %al
        popq    %rdx
        ret
LBB0_2:
        xorb    %al, %al
        callq   _foo
        popq    %rdx
        ret

=>

        pushq   %rax
        xorb    %al, %al
        testl   %edi, %edi
        je      LBB0_2
## BB#1:
        callq   _foo
LBB0_2:
        popq    %rdx
        ret

rdar://9145558


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131172 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 261 insertions, 6 deletions

diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index 77043406bc..01aa550f5f 100644
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -41,6 +41,7 @@ using namespace llvm;
 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
 STATISTIC(NumBranchOpts, "Number of branches optimized");
 STATISTIC(NumTailMerge , "Number of block tails merged");
+STATISTIC(NumHoist     , "Number of times common instructions are hoisted");
 
 static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
                               cl::init(cl::BOU_UNSET), cl::Hidden);
@@ -65,7 +66,7 @@ namespace {
   public:
     static char ID;
     explicit BranchFolderPass(bool defaultEnableTailMerge)
-      : MachineFunctionPass(ID), BranchFolder(defaultEnableTailMerge) {}
+      : MachineFunctionPass(ID), BranchFolder(defaultEnableTailMerge, true) {}
 
     virtual bool runOnMachineFunction(MachineFunction &MF);
     virtual const char *getPassName() const { return "Control Flow Optimizer"; }
@@ -86,12 +87,14 @@ bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
 }
 
 
-BranchFolder::BranchFolder(bool defaultEnableTailMerge) {
+BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist) {
   switch (FlagEnableTailMerge) {
   case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
   case cl::BOU_TRUE: EnableTailMerge = true; break;
   case cl::BOU_FALSE: EnableTailMerge = false; break;
   }
+
+  EnableHoistCommonCode = CommonHoist;
 }
 
 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
@@ -186,9 +189,10 @@ bool BranchFolder::OptimizeFunction(MachineFunction &MF,
 
   bool MadeChangeThisIteration = true;
   while (MadeChangeThisIteration) {
-    MadeChangeThisIteration = false;
-    MadeChangeThisIteration |= TailMergeBlocks(MF);
-    MadeChangeThisIteration |= OptimizeBranches(MF);
+    MadeChangeThisIteration    = TailMergeBlocks(MF);
+    MadeChangeThisIteration   |= OptimizeBranches(MF);
+    if (EnableHoistCommonCode)
+      MadeChangeThisIteration |= HoistCommonCode(MF);
     MadeChange |= MadeChangeThisIteration;
   }
 
@@ -910,7 +914,8 @@ bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
   // Make sure blocks are numbered in order
   MF.RenumberBlocks();
 
-  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
+  for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
+       I != E; ) {
     MachineBasicBlock *MBB = I++;
     MadeChange |= OptimizeBlock(MBB);
 
@@ -1339,3 +1344,253 @@ ReoptimizeBlock:
 
   return MadeChange;
 }
+
+//===----------------------------------------------------------------------===//
+//  Hoist Common Code
+//===----------------------------------------------------------------------===//
+
+/// HoistCommonCode - Hoist common instruction sequences at the start of basic
+/// blocks to their common predecessor.
+/// NOTE: This optimization does not update live-in information so it must be
+/// run after all passes that require correct liveness information.
+bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
+  bool MadeChange = false;
+  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
+    MachineBasicBlock *MBB = I++;
+    MadeChange |= HoistCommonCodeInSuccs(MBB);
+  }
+
+  return MadeChange;
+}
+
+/// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
+/// its 'true' successor.
+static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
+                                         MachineBasicBlock *TrueBB) {
+  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
+         E = BB->succ_end(); SI != E; ++SI) {
+    MachineBasicBlock *SuccBB = *SI;
+    if (SuccBB != TrueBB)
+      return SuccBB;
+  }
+  return NULL;
+}
+
+/// findHoistingInsertPosAndDeps - Find the location to move common instructions
+/// in successors to. The location is ususally just before the terminator,
+/// however if the terminator is a conditional branch and its previous
+/// instruction is the flag setting instruction, the previous instruction is
+/// the preferred location. This function also gathers uses and defs of the
+/// instructions from the insertion point to the end of the block. The data is
+/// used by HoistCommonCodeInSuccs to ensure safety.
+static
+MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
+                                                  const TargetInstrInfo *TII,
+                                                  const TargetRegisterInfo *TRI,
+                                                  SmallSet<unsigned,4> &Uses,
+                                                  SmallSet<unsigned,4> &Defs) {
+  MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
+  if (!TII->isUnpredicatedTerminator(Loc))
+    return MBB->end();
+
+  for (unsigned i = 0, e = Loc->getNumOperands(); i != e; ++i) {
+    const MachineOperand &MO = Loc->getOperand(i);
+    if (!MO.isReg())
+      continue;
+    unsigned Reg = MO.getReg();
+    if (!Reg)
+      continue;
+    if (MO.isUse()) {
+      Uses.insert(Reg);
+      for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
+        Uses.insert(*AS);
+    } else if (!MO.isDead())
+      // Don't try to hoist code in the rare case the terminator defines a
+      // register that is later used.
+      return MBB->end();
+  }
+
+  if (Uses.empty())
+    return Loc;
+  if (Loc == MBB->begin())
+    return MBB->end();
+
+  // The terminator is probably a conditional branch, try not to separate the
+  // branch from condition setting instruction.
+  MachineBasicBlock::iterator PI = Loc;
+  --PI;
+  while (PI != MBB->begin() && Loc->isDebugValue())
+    --PI;
+
+  bool IsDef = false;
+  for (unsigned i = 0, e = PI->getNumOperands(); !IsDef && i != e; ++i) {
+    const MachineOperand &MO = PI->getOperand(i);
+    if (!MO.isReg() || MO.isUse())
+      continue;
+    unsigned Reg = MO.getReg();
+    if (!Reg)
+      continue;
+    if (Uses.count(Reg))
+      IsDef = true;
+  }
+  if (!IsDef)
+    // The condition setting instruction is not just before the conditional
+    // branch.
+    return Loc;
+
+  // Be conservative, don't insert instruction above something that may have
+  // side-effects. And since it's potentially bad to separate flag setting
+  // instruction from the conditional branch, just abort the optimization
+  // completely.
+  // Also avoid moving code above predicated instruction since it's hard to
+  // reason about register liveness with predicated instruction.
+  bool DontMoveAcrossStore = true;
+  if (!PI->isSafeToMove(TII, 0, DontMoveAcrossStore) ||
+      TII->isPredicated(PI))
+    return MBB->end();
+
+
+  // Find out what registers are live. Note this routine is ignoring other live
+  // registers which are only used by instructions in successor blocks.
+  for (unsigned i = 0, e = PI->getNumOperands(); i != e; ++i) {
+    const MachineOperand &MO = PI->getOperand(i);
+    if (!MO.isReg())
+      continue;
+    unsigned Reg = MO.getReg();
+    if (!Reg)
+      continue;
+    if (MO.isUse()) {
+      Uses.insert(Reg);
+      for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
+        Uses.insert(*AS);
+    } else {
+      if (Uses.count(Reg)) {
+        Uses.erase(Reg);
+        for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
+          Uses.erase(*SR); // Use getSubRegisters to be conservative
+        Defs.insert(Reg);
+        for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
+          Defs.insert(*AS);
+      }
+    }
+  }
+
+  return PI;
+}
+
+/// HoistCommonCodeInSuccs - If the successors of MBB has common instruction
+/// sequence at the start of the function, move the instructions before MBB
+/// terminator if it's legal.
+bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
+  MachineBasicBlock *TBB = 0, *FBB = 0;
+  SmallVector<MachineOperand, 4> Cond;
+  if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
+    return false;
+
+  if (!FBB) FBB = findFalseBlock(MBB, TBB);
+  if (!FBB)
+    // Malformed bcc? True and false blocks are the same?
+    return false;
+
+  // Restrict the optimization to cases where MBB is the only predecessor,
+  // it is an obvious win.
+  if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
+    return false;
+
+  // Find a suitable position to hoist the common instructions to. Also figure
+  // out which registers are used or defined by instructions from the insertion
+  // point to the end of the block.
+  SmallSet<unsigned, 4> Uses, Defs;
+  MachineBasicBlock::iterator Loc =
+    findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
+  if (Loc == MBB->end())
+    return false;
+
+  SmallSet<unsigned, 4> LocalDefs;
+  unsigned NumDups = 0;
+  MachineBasicBlock::iterator TIB = TBB->begin();
+  MachineBasicBlock::iterator FIB = FBB->begin();
+  MachineBasicBlock::iterator TIE = TBB->end();
+  MachineBasicBlock::iterator FIE = FBB->end();
+  while (TIB != TIE && FIB != FIE) {
+    // Skip dbg_value instructions. These do not count.
+    if (TIB->isDebugValue()) {
+      while (TIB != TIE && TIB->isDebugValue())
+        ++TIB;
+      if (TIB == TIE)
+        break;
+    }
+    if (FIB->isDebugValue()) {
+      while (FIB != FIE && FIB->isDebugValue())
+        ++FIB;
+      if (FIB == FIE)
+        break;
+    }
+    if (!TIB->isIdenticalTo(FIB))
+      break;
+
+    if (TII->isPredicated(TIB))
+      // Hard to reason about register liveness with predicated instruction.
+      break;
+
+    bool IsSafe = true;
+    for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
+      const MachineOperand &MO = TIB->getOperand(i);
+      if (!MO.isReg())
+        continue;
+      unsigned Reg = MO.getReg();
+      if (!Reg)
+        continue;
+      if (MO.isDef()) {
+        if (Uses.count(Reg)) {
+          // Avoid clobbering a register that's used by the instruction at
+          // the point of insertion.
+          IsSafe = false;
+          break;
+        }
+        if (!MO.isDead() && Defs.count(Reg)) {
+          // Don't hoist the instruction if the def would be clobber by the
+          // instruction at the point insertion. FIXME: This is overly
+          // conservative. It should be possible to hoist the instructions
+          // in BB2 in the following example:
+          // BB1:
+          // r1, eflag = op1 r2, r3
+          // brcc eflag
+          //
+          // BB2:
+          // r1 = op2, ...
+          //    = op3, r1<kill>
+          IsSafe = false;
+          break;
+        }
+        LocalDefs.insert(Reg);
+        for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
+          LocalDefs.insert(*SR);
+      } else if (!LocalDefs.count(Reg)) {
+        if (Defs.count(Reg)) {
+          // Use is defined by the instruction at the point of insertion.
+          IsSafe = false;
+          break;
+        }
+      }
+    }
+    if (!IsSafe)
+      break;
+
+    bool DontMoveAcrossStore = true;
+    if (!TIB->isSafeToMove(TII, 0, DontMoveAcrossStore))
+      break;
+
+    ++NumDups;
+    ++TIB;
+    ++FIB;
+  }
+
+  if (!NumDups)
+    return false;
+
+  MBB->splice(Loc, TBB, TBB->begin(), TIB);
+  FBB->erase(FBB->begin(), FIB);
+  ++NumHoist;
+  return true;
+}