Optimize code placement in loop to eliminate unconditional branches or move unconditional branch to the outside of the loop. e.g.

/// A: /// ... /// <fallthrough to B> /// /// B: --> loop header /// ... /// jcc <cond> C, [exit] /// /// C: /// ... /// jmp B /// /// ==> /// /// A: /// ... /// jmp B /// /// C: --> new loop header /// ... /// <fallthough to B> /// /// B: /// ... /// jcc <cond> C, [exit] git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71209 91177308-0d34-0410-b5e6-96231b3b80d8
2025-09-17 05:25:47 +00:00 · 2009-05-08 06:34:09 +00:00
parent 4a228864f2
commit 45e0010e14
3 changed files with 371 additions and 17 deletions
--- a/lib/CodeGen/CodePlacementOpt.cpp
+++ b/lib/CodeGen/CodePlacementOpt.cpp
@@ -16,15 +16,40 @@
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/Passes.h"
+#include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetMachine.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/ADT/Statistic.h"
 using namespace llvm;

+static cl::opt<bool>
+OptLoopBBPlacement("opt-loop-bb-placement",
+                   cl::init(false), cl::Hidden,
+                   cl::desc("Optimize block placements in loops"));
+
+STATISTIC(NumHeaderAligned, "Number of loop header aligned");
+STATISTIC(NumIntraElim,     "Number of intra loop branches eliminated");
+STATISTIC(NumIntraMoved,    "Number of intra loop branches moved");
+
 namespace {
  class CodePlacementOpt : public MachineFunctionPass {
    const MachineLoopInfo *MLI;
+    const TargetInstrInfo *TII;
+    const TargetLowering  *TLI;
+
+    /// ChangedMBBs - BBs which are modified by OptimizeIntraLoopEdges.
+    SmallPtrSet<MachineBasicBlock*, 8> ChangedMBBs;
+
+    /// UncondJmpMBBs - A list of BBs which are in loops and end with
+    /// unconditional branches.
+    SmallVector<std::pair<MachineBasicBlock*,MachineBasicBlock*>, 4>
+    UncondJmpMBBs;
+
+    /// LoopHeaders - A list of BBs which are loop headers.
+    SmallVector<MachineBasicBlock*, 4> LoopHeaders;

  public:
    static char ID;
@@ -37,12 +62,12 @@ namespace {

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<MachineLoopInfo>();
-      AU.addPreserved<MachineLoopInfo>();
      AU.addPreservedID(MachineDominatorsID);
      MachineFunctionPass::getAnalysisUsage(AU);
    }

  private:
+    bool OptimizeIntraLoopEdges();
    bool AlignLoops(MachineFunction &MF);
  };

@@ -53,32 +78,199 @@ FunctionPass *llvm::createCodePlacementOptPass() {
  return new CodePlacementOpt();
 }

+/// OptimizeBackEdges - Place loop back edges to move unconditional branches
+/// out of the loop.
+///
+///       A:
+///       ...
+///       <fallthrough to B>
+///
+///       B:  --> loop header
+///       ...
+///       jcc <cond> C, [exit]
+///
+///       C:
+///       ...
+///       jmp B
+///
+/// ==>
+///
+///       A:
+///       ...
+///       jmp B
+///
+///       C:  --> new loop header
+///       ...
+///       <fallthough to B>
+///       
+///       B:
+///       ...
+///       jcc <cond> C, [exit]
+///
+bool CodePlacementOpt::OptimizeIntraLoopEdges() {
+  if (!OptLoopBBPlacement)
+    return false;
+
+  bool Changed = false;
+  for (unsigned i = 0, e = UncondJmpMBBs.size(); i != e; ++i) {
+    MachineBasicBlock *MBB = UncondJmpMBBs[i].first;
+    MachineBasicBlock *SuccMBB = UncondJmpMBBs[i].second;
+    MachineLoop *L = MLI->getLoopFor(MBB);
+    assert(L && "BB is expected to be in a loop!");
+
+    if (ChangedMBBs.count(MBB)) {
+      // BB has been modified, re-analyze.
+      MachineBasicBlock *TBB = 0, *FBB = 0;
+      SmallVector<MachineOperand, 4> Cond;
+      if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond) || !Cond.empty())
+        continue;
+      if (MLI->getLoopFor(TBB) != L || TBB->isLandingPad())
+        continue;
+      SuccMBB = TBB;
+    } else {
+      assert(MLI->getLoopFor(SuccMBB) == L &&
+             "Successor is not in the same loop!");
+    }
+
+    if (MBB->isLayoutSuccessor(SuccMBB)) {
+      // Successor is right after MBB, just eliminate the unconditional jmp.
+      // Can this happen?
+      TII->RemoveBranch(*MBB);
+      ChangedMBBs.insert(MBB);
+      ++NumIntraElim;
+      continue;
+    }
+
+    // Now check if the predecessor is fallthrough from any BB. If there is,
+    // that BB should be from outside the loop since edge will become a jmp.
+    bool OkToMove = true;
+    MachineBasicBlock *FtMBB = 0, *FtTBB = 0, *FtFBB = 0;
+    SmallVector<MachineOperand, 4> FtCond;    
+    for (MachineBasicBlock::pred_iterator PI = SuccMBB->pred_begin(),
+           PE = SuccMBB->pred_end(); PI != PE; ++PI) {
+      MachineBasicBlock *PredMBB = *PI;
+      if (PredMBB->isLayoutSuccessor(SuccMBB)) {
+        if (TII->AnalyzeBranch(*PredMBB, FtTBB, FtFBB, FtCond)) {
+          OkToMove = false;
+          break;
+        }
+        if (!FtTBB)
+          FtTBB = SuccMBB;
+        else if (!FtFBB) {
+          assert(FtFBB != SuccMBB && "Unexpected control flow!");
+          FtFBB = SuccMBB;
+        }
+        
+        // A fallthrough.
+        FtMBB = PredMBB;
+        MachineLoop *PL = MLI->getLoopFor(PredMBB);
+        if (PL && (PL == L || PL->getLoopDepth() >= L->getLoopDepth())) {
+          OkToMove = false;
+          break;
+        }
+      }
+    }
+
+    if (!OkToMove)
+      continue;
+
+    // Is it profitable? If SuccMBB can fallthrough itself, that can be changed
+    // into a jmp.
+    MachineBasicBlock *TBB = 0, *FBB = 0;
+    SmallVector<MachineOperand, 4> Cond;
+    if (TII->AnalyzeBranch(*SuccMBB, TBB, FBB, Cond))
+      continue;
+    if (!TBB && Cond.empty())
+      TBB = next(MachineFunction::iterator(SuccMBB));
+    else if (!FBB && !Cond.empty())
+      FBB = next(MachineFunction::iterator(SuccMBB));
+
+    // This calculate the cost of the transformation. Also, it finds the *only*
+    // intra-loop edge if there is one.
+    int Cost = 0;
+    bool HasOneIntraSucc = true;
+    MachineBasicBlock *IntraSucc = 0;
+    for (MachineBasicBlock::succ_iterator SI = SuccMBB->succ_begin(),
+           SE = SuccMBB->succ_end(); SI != SE; ++SI) {
+      MachineBasicBlock *SSMBB = *SI;
+      if (MLI->getLoopFor(SSMBB) == L) {
+        if (!IntraSucc)
+          IntraSucc = SSMBB;
+        else
+          HasOneIntraSucc = false;
+      }
+
+      if (SuccMBB->isLayoutSuccessor(SSMBB))
+        // This will become a jmp.
+        ++Cost;
+      else if (MBB->isLayoutSuccessor(SSMBB))
+        // One of the successor will become the new fallthrough.
+        if (SSMBB == FBB) {
+          FBB = 0;
+          --Cost;
+        } else if (!FBB && SSMBB == TBB && Cond.empty()) {
+          TBB = 0;
+          --Cost;
+        } else if (!TII->ReverseBranchCondition(Cond)) {
+          TBB = FBB;
+          FBB = 0;
+          --Cost;
+        }
+    }
+    if (Cost)
+      continue;
+
+    // Now, let's move the successor to below the BB to eliminate the jmp.
+    SuccMBB->moveAfter(MBB);
+    TII->RemoveBranch(*MBB);
+    TII->RemoveBranch(*SuccMBB);
+    if (TBB)
+      TII->InsertBranch(*SuccMBB, TBB, FBB, Cond);
+    ChangedMBBs.insert(MBB);
+    ChangedMBBs.insert(SuccMBB);
+    if (FtMBB) {
+      TII->RemoveBranch(*FtMBB);
+      TII->InsertBranch(*FtMBB, FtTBB, FtFBB, FtCond);
+      ChangedMBBs.insert(FtMBB);
+    }
+
+    // If BB is the loop latch, we may have a new loop headr.
+    if (MBB == L->getLoopLatch()) {
+      assert(MLI->isLoopHeader(SuccMBB) &&
+             "Only succ of loop latch is not the header?");
+      if (HasOneIntraSucc && IntraSucc)
+        std::replace(LoopHeaders.begin(),LoopHeaders.end(), SuccMBB, IntraSucc);
+    }
+  }
+
+  ++NumIntraMoved;
+  return Changed;
+}
+
 /// AlignLoops - Align loop headers to target preferred alignments.
 ///
 bool CodePlacementOpt::AlignLoops(MachineFunction &MF) {
-  const TargetLowering *TLI = MF.getTarget().getTargetLowering();
-  if (!TLI)
+  const Function *F = MF.getFunction();
+  if (F->hasFnAttr(Attribute::OptimizeForSize))
    return false;

  unsigned Align = TLI->getPrefLoopAlignment();
  if (!Align)
    return false;  // Don't care about loop alignment.

-  const Function *F = MF.getFunction();
-  if (F->hasFnAttr(Attribute::OptimizeForSize))
-    return false;
+  // Make sure blocks are numbered in order
+  MF.RenumberBlocks();

  bool Changed = false;
-  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
-    MachineBasicBlock *MBB = I;
-    if (MLI->isLoopHeader(MBB)) {
-      MachineBasicBlock *PredBB = prior(I);
-      if (MLI->getLoopFor(MBB) == MLI->getLoopFor(PredBB))
-        // If previously BB is in the same loop, don't align this BB. We want
-        // to prevent adding noop's inside a loop.
-        continue;
-      MBB->setAlignment(Align);
+  for (unsigned i = 0, e = LoopHeaders.size(); i != e; ++i) {
+    MachineBasicBlock *HeaderMBB = LoopHeaders[i];
+    MachineBasicBlock *PredMBB = prior(MachineFunction::iterator(HeaderMBB));
+    if (MLI->getLoopFor(HeaderMBB) != MLI->getLoopFor(PredMBB)) {
+      // If previously BB is in the same loop, don't align this BB. We want
+      // to prevent adding noop's inside a loop.
+      HeaderMBB->setAlignment(Align);
      Changed = true;
+      ++NumHeaderAligned;
    }
  }

@@ -90,8 +282,36 @@ bool CodePlacementOpt::runOnMachineFunction(MachineFunction &MF) {
  if (MLI->empty())
    return false;  // No loops.

-  bool Changed = false;
+  TLI = MF.getTarget().getTargetLowering();
+  TII = MF.getTarget().getInstrInfo();
+
+  // Analyze the BBs first and keep track of loop headers and BBs that
+  // end with an unconditional jmp to another block in the same loop.
+  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
+    MachineBasicBlock *MBB = I;
+    if (MBB->isLandingPad())
+      continue;
+    MachineLoop *L = MLI->getLoopFor(MBB);
+    if (!L)
+      continue;
+    if (MLI->isLoopHeader(MBB))
+      LoopHeaders.push_back(MBB);
+
+    MachineBasicBlock *TBB = 0, *FBB = 0;
+    SmallVector<MachineOperand, 4> Cond;
+    if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond) || !Cond.empty())
+      continue;
+    if (MLI->getLoopFor(TBB) == L && !TBB->isLandingPad())
+      UncondJmpMBBs.push_back(std::make_pair(MBB, TBB));
+  }
+
+  bool Changed = OptimizeIntraLoopEdges();
+
  Changed |= AlignLoops(MF);

+  ChangedMBBs.clear();
+  UncondJmpMBBs.clear();
+  LoopHeaders.clear();
+
  return Changed;
 }