It's not necessary to do rounding for alloca operations when the requested

alignment is equal to the stack alignment.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@40004 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 364 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/TailDuplication.cpp b/lib/Transforms/Scalar/TailDuplication.cpp
new file mode 100644
index 0000000..22d8157
--- /dev/null
+++ b/lib/Transforms/Scalar/TailDuplication.cpp
@@ -0,0 +1,364 @@
+//===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file was developed by the LLVM research group and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass performs a limited form of tail duplication, intended to simplify
+// CFGs by removing some unconditional branches.  This pass is necessary to
+// straighten out loops created by the C front-end, but also is capable of
+// making other code nicer.  After this pass is run, the CFG simplify pass
+// should be run to clean up the mess.
+//
+// This pass could be enhanced in the future to use profile information to be
+// more aggressive.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "tailduplicate"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Constant.h"
+#include "llvm/Function.h"
+#include "llvm/Instructions.h"
+#include "llvm/IntrinsicInst.h"
+#include "llvm/Pass.h"
+#include "llvm/Type.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/ADT/Statistic.h"
+using namespace llvm;
+
+STATISTIC(NumEliminated, "Number of unconditional branches eliminated");
+
+namespace {
+  cl::opt<unsigned>
+  Threshold("taildup-threshold", cl::desc("Max block size to tail duplicate"),
+            cl::init(6), cl::Hidden);
+  class VISIBILITY_HIDDEN TailDup : public FunctionPass {
+    bool runOnFunction(Function &F);
+  public:
+    static char ID; // Pass identification, replacement for typeid
+    TailDup() : FunctionPass((intptr_t)&ID) {}
+
+  private:
+    inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
+    inline void eliminateUnconditionalBranch(BranchInst *BI);
+  };
+  char TailDup::ID = 0;
+  RegisterPass<TailDup> X("tailduplicate", "Tail Duplication");
+}
+
+// Public interface to the Tail Duplication pass
+FunctionPass *llvm::createTailDuplicationPass() { return new TailDup(); }
+
+/// runOnFunction - Top level algorithm - Loop over each unconditional branch in
+/// the function, eliminating it if it looks attractive enough.
+///
+bool TailDup::runOnFunction(Function &F) {
+  bool Changed = false;
+  for (Function::iterator I = F.begin(), E = F.end(); I != E; )
+    if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
+      eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
+      Changed = true;
+    } else {
+      ++I;
+    }
+  return Changed;
+}
+
+/// shouldEliminateUnconditionalBranch - Return true if this branch looks
+/// attractive to eliminate.  We eliminate the branch if the destination basic
+/// block has <= 5 instructions in it, not counting PHI nodes.  In practice,
+/// since one of these is a terminator instruction, this means that we will add
+/// up to 4 instructions to the new block.
+///
+/// We don't count PHI nodes in the count since they will be removed when the
+/// contents of the block are copied over.
+///
+bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
+  BranchInst *BI = dyn_cast<BranchInst>(TI);
+  if (!BI || !BI->isUnconditional()) return false;  // Not an uncond branch!
+
+  BasicBlock *Dest = BI->getSuccessor(0);
+  if (Dest == BI->getParent()) return false;        // Do not loop infinitely!
+
+  // Do not inline a block if we will just get another branch to the same block!
+  TerminatorInst *DTI = Dest->getTerminator();
+  if (BranchInst *DBI = dyn_cast<BranchInst>(DTI))
+    if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
+      return false;                                 // Do not loop infinitely!
+
+  // FIXME: DemoteRegToStack cannot yet demote invoke instructions to the stack,
+  // because doing so would require breaking critical edges.  This should be
+  // fixed eventually.
+  if (!DTI->use_empty())
+    return false;
+
+  // Do not bother working on dead blocks...
+  pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
+  if (PI == PE && Dest != Dest->getParent()->begin())
+    return false;   // It's just a dead block, ignore it...
+
+  // Also, do not bother with blocks with only a single predecessor: simplify
+  // CFG will fold these two blocks together!
+  ++PI;
+  if (PI == PE) return false;  // Exactly one predecessor!
+
+  BasicBlock::iterator I = Dest->begin();
+  while (isa<PHINode>(*I)) ++I;
+
+  for (unsigned Size = 0; I != Dest->end(); ++I) {
+    if (Size == Threshold) return false;  // The block is too large.
+    // Only count instructions that are not debugger intrinsics.
+    if (!isa<DbgInfoIntrinsic>(I)) ++Size;
+  }
+
+  // Do not tail duplicate a block that has thousands of successors into a block
+  // with a single successor if the block has many other predecessors.  This can
+  // cause an N^2 explosion in CFG edges (and PHI node entries), as seen in
+  // cases that have a large number of indirect gotos.
+  unsigned NumSuccs = DTI->getNumSuccessors();
+  if (NumSuccs > 8) {
+    unsigned TooMany = 128;
+    if (NumSuccs >= TooMany) return false;
+    TooMany = TooMany/NumSuccs;
+    for (; PI != PE; ++PI)
+      if (TooMany-- == 0) return false;
+  }
+  
+  // Finally, if this unconditional branch is a fall-through, be careful about
+  // tail duplicating it.  In particular, we don't want to taildup it if the
+  // original block will still be there after taildup is completed: doing so
+  // would eliminate the fall-through, requiring unconditional branches.
+  Function::iterator DestI = Dest;
+  if (&*--DestI == BI->getParent()) {
+    // The uncond branch is a fall-through.  Tail duplication of the block is
+    // will eliminate the fall-through-ness and end up cloning the terminator
+    // at the end of the Dest block.  Since the original Dest block will
+    // continue to exist, this means that one or the other will not be able to
+    // fall through.  One typical example that this helps with is code like:
+    // if (a)
+    //   foo();
+    // if (b)
+    //   foo();
+    // Cloning the 'if b' block into the end of the first foo block is messy.
+    
+    // The messy case is when the fall-through block falls through to other
+    // blocks.  This is what we would be preventing if we cloned the block.
+    DestI = Dest;
+    if (++DestI != Dest->getParent()->end()) {
+      BasicBlock *DestSucc = DestI;
+      // If any of Dest's successors are fall-throughs, don't do this xform.
+      for (succ_iterator SI = succ_begin(Dest), SE = succ_end(Dest);
+           SI != SE; ++SI)
+        if (*SI == DestSucc)
+          return false;
+    }
+  }
+
+  return true;
+}
+
+/// FindObviousSharedDomOf - We know there is a branch from SrcBlock to
+/// DestBlock, and that SrcBlock is not the only predecessor of DstBlock.  If we
+/// can find a predecessor of SrcBlock that is a dominator of both SrcBlock and
+/// DstBlock, return it.
+static BasicBlock *FindObviousSharedDomOf(BasicBlock *SrcBlock,
+                                          BasicBlock *DstBlock) {
+  // SrcBlock must have a single predecessor.
+  pred_iterator PI = pred_begin(SrcBlock), PE = pred_end(SrcBlock);
+  if (PI == PE || ++PI != PE) return 0;
+
+  BasicBlock *SrcPred = *pred_begin(SrcBlock);
+
+  // Look at the predecessors of DstBlock.  One of them will be SrcBlock.  If
+  // there is only one other pred, get it, otherwise we can't handle it.
+  PI = pred_begin(DstBlock); PE = pred_end(DstBlock);
+  BasicBlock *DstOtherPred = 0;
+  if (*PI == SrcBlock) {
+    if (++PI == PE) return 0;
+    DstOtherPred = *PI;
+    if (++PI != PE) return 0;
+  } else {
+    DstOtherPred = *PI;
+    if (++PI == PE || *PI != SrcBlock || ++PI != PE) return 0;
+  }
+
+  // We can handle two situations here: "if then" and "if then else" blocks.  An
+  // 'if then' situation is just where DstOtherPred == SrcPred.
+  if (DstOtherPred == SrcPred)
+    return SrcPred;
+
+  // Check to see if we have an "if then else" situation, which means that
+  // DstOtherPred will have a single predecessor and it will be SrcPred.
+  PI = pred_begin(DstOtherPred); PE = pred_end(DstOtherPred);
+  if (PI != PE && *PI == SrcPred) {
+    if (++PI != PE) return 0;  // Not a single pred.
+    return SrcPred;  // Otherwise, it's an "if then" situation.  Return the if.
+  }
+
+  // Otherwise, this is something we can't handle.
+  return 0;
+}
+
+
+/// eliminateUnconditionalBranch - Clone the instructions from the destination
+/// block into the source block, eliminating the specified unconditional branch.
+/// If the destination block defines values used by successors of the dest
+/// block, we may need to insert PHI nodes.
+///
+void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) {
+  BasicBlock *SourceBlock = Branch->getParent();
+  BasicBlock *DestBlock = Branch->getSuccessor(0);
+  assert(SourceBlock != DestBlock && "Our predicate is broken!");
+
+  DOUT << "TailDuplication[" << SourceBlock->getParent()->getName()
+       << "]: Eliminating branch: " << *Branch;
+
+  // See if we can avoid duplicating code by moving it up to a dominator of both
+  // blocks.
+  if (BasicBlock *DomBlock = FindObviousSharedDomOf(SourceBlock, DestBlock)) {
+    DOUT << "Found shared dominator: " << DomBlock->getName() << "\n";
+
+    // If there are non-phi instructions in DestBlock that have no operands
+    // defined in DestBlock, and if the instruction has no side effects, we can
+    // move the instruction to DomBlock instead of duplicating it.
+    BasicBlock::iterator BBI = DestBlock->begin();
+    while (isa<PHINode>(BBI)) ++BBI;
+    while (!isa<TerminatorInst>(BBI)) {
+      Instruction *I = BBI++;
+
+      bool CanHoist = !I->isTrapping() && !I->mayWriteToMemory();
+      if (CanHoist) {
+        for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op)
+          if (Instruction *OpI = dyn_cast<Instruction>(I->getOperand(op)))
+            if (OpI->getParent() == DestBlock ||
+                (isa<InvokeInst>(OpI) && OpI->getParent() == DomBlock)) {
+              CanHoist = false;
+              break;
+            }
+        if (CanHoist) {
+          // Remove from DestBlock, move right before the term in DomBlock.
+          DestBlock->getInstList().remove(I);
+          DomBlock->getInstList().insert(DomBlock->getTerminator(), I);
+          DOUT << "Hoisted: " << *I;
+        }
+      }
+    }
+  }
+
+  // Tail duplication can not update SSA properties correctly if the values
+  // defined in the duplicated tail are used outside of the tail itself.  For
+  // this reason, we spill all values that are used outside of the tail to the
+  // stack.
+  for (BasicBlock::iterator I = DestBlock->begin(); I != DestBlock->end(); ++I)
+    for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
+         ++UI) {
+      bool ShouldDemote = false;
+      if (cast<Instruction>(*UI)->getParent() != DestBlock) {
+        // We must allow our successors to use tail values in their PHI nodes
+        // (if the incoming value corresponds to the tail block).
+        if (PHINode *PN = dyn_cast<PHINode>(*UI)) {
+          for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+            if (PN->getIncomingValue(i) == I &&
+                PN->getIncomingBlock(i) != DestBlock) {
+              ShouldDemote = true;
+              break;
+            }
+
+        } else {
+          ShouldDemote = true;
+        }
+      } else if (PHINode *PN = dyn_cast<PHINode>(cast<Instruction>(*UI))) {
+        // If the user of this instruction is a PHI node in the current block,
+        // which has an entry from another block using the value, spill it.
+        for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+          if (PN->getIncomingValue(i) == I &&
+              PN->getIncomingBlock(i) != DestBlock) {
+            ShouldDemote = true;
+            break;
+          }
+      }
+
+      if (ShouldDemote) {
+        // We found a use outside of the tail.  Create a new stack slot to
+        // break this inter-block usage pattern.
+        DemoteRegToStack(*I);
+        break;
+      }
+    }
+
+  // We are going to have to map operands from the original block B to the new
+  // copy of the block B'.  If there are PHI nodes in the DestBlock, these PHI
+  // nodes also define part of this mapping.  Loop over these PHI nodes, adding
+  // them to our mapping.
+  //
+  std::map<Value*, Value*> ValueMapping;
+
+  BasicBlock::iterator BI = DestBlock->begin();
+  bool HadPHINodes = isa<PHINode>(BI);
+  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
+    ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock);
+
+  // Clone the non-phi instructions of the dest block into the source block,
+  // keeping track of the mapping...
+  //
+  for (; BI != DestBlock->end(); ++BI) {
+    Instruction *New = BI->clone();
+    New->setName(BI->getName());
+    SourceBlock->getInstList().push_back(New);
+    ValueMapping[BI] = New;
+  }
+
+  // Now that we have built the mapping information and cloned all of the
+  // instructions (giving us a new terminator, among other things), walk the new
+  // instructions, rewriting references of old instructions to use new
+  // instructions.
+  //
+  BI = Branch; ++BI;  // Get an iterator to the first new instruction
+  for (; BI != SourceBlock->end(); ++BI)
+    for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i)
+      if (Value *Remapped = ValueMapping[BI->getOperand(i)])
+        BI->setOperand(i, Remapped);
+
+  // Next we check to see if any of the successors of DestBlock had PHI nodes.
+  // If so, we need to add entries to the PHI nodes for SourceBlock now.
+  for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock);
+       SI != SE; ++SI) {
+    BasicBlock *Succ = *SI;
+    for (BasicBlock::iterator PNI = Succ->begin(); isa<PHINode>(PNI); ++PNI) {
+      PHINode *PN = cast<PHINode>(PNI);
+      // Ok, we have a PHI node.  Figure out what the incoming value was for the
+      // DestBlock.
+      Value *IV = PN->getIncomingValueForBlock(DestBlock);
+
+      // Remap the value if necessary...
+      if (Value *MappedIV = ValueMapping[IV])
+        IV = MappedIV;
+      PN->addIncoming(IV, SourceBlock);
+    }
+  }
+
+  // Next, remove the old branch instruction, and any PHI node entries that we
+  // had.
+  BI = Branch; ++BI;  // Get an iterator to the first new instruction
+  DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
+  SourceBlock->getInstList().erase(Branch);  // Destroy the uncond branch...
+
+  // Final step: now that we have finished everything up, walk the cloned
+  // instructions one last time, constant propagating and DCE'ing them, because
+  // they may not be needed anymore.
+  //
+  if (HadPHINodes)
+    while (BI != SourceBlock->end())
+      if (!dceInstruction(BI) && !doConstantPropagation(BI))
+        ++BI;
+
+  ++NumEliminated;  // We just killed a branch!
+}