Add an _embarassingly simple_ implementation of basic block layout. This is

more of a testcase for profiling information than anything that should reasonably
be used, but it's a starting point.  When I have more time I will whip this into
better shape.

llvm-svn: 11311

lib/Transforms/Scalar/BasicBlockPlacement.cpp

@@ -0,0 +1,141 @@
+//===-- BasicBlockPlacement.cpp - Basic Block Code Layout optimization ----===//
+// 
+//                     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 file implements a very simple profile guided basic block placement
+// algorithm.  The idea is to put frequently executed blocks together at the
+// start of the function, and hopefully increase the number of fall-through
+// conditional branches.  If there is no profile information for a particular
+// function, this pass basically orders blocks in depth-first order
+//
+// The algorithm implemented here is basically "Algo1" from "Profile Guided Code
+// Positioning" by Pettis and Hansen, except that it uses basic block counts
+// instead of edge counts.  This should be improved in many ways, but is very
+// simple for now.
+//
+// Basically we "place" the entry block, then loop over all successors in a DFO,
+// placing the most frequently executed successor until we run out of blocks.  I
+// told you this was _extremely_ simplistic. :) This is also much slower than it
+// could be.  When it becomes important, this pass will be rewritten to use a
+// better algorithm, and then we can worry about efficiency.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/ProfileInfo.h"
+#include "llvm/Function.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/CFG.h"
+#include "Support/Statistic.h"
+#include <set>
+using namespace llvm;
+
+namespace {
+  Statistic<> NumMoved("block-placement", "Number of basic blocks moved");
+  
+  struct BlockPlacement : public FunctionPass {
+    virtual bool runOnFunction(Function &F);
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.setPreservesCFG();
+      AU.addRequired<ProfileInfo>();
+      //AU.addPreserved<ProfileInfo>();  // Does this work?
+    }
+  private:
+    /// PI - The profile information that is guiding us.
+    ///
+    ProfileInfo *PI;
+
+    /// NumMovedBlocks - Every time we move a block, increment this counter.
+    ///
+    unsigned NumMovedBlocks;
+
+    /// PlacedBlocks - Every time we place a block, remember it so we don't get
+    /// into infinite loops.
+    std::set<BasicBlock*> PlacedBlocks;
+
+    /// InsertPos - This an iterator to the next place we want to insert a
+    /// block.
+    Function::iterator InsertPos;
+
+    /// PlaceBlocks - Recursively place the specified blocks and any unplaced
+    /// successors.
+    void PlaceBlocks(BasicBlock *BB);
+  };
+
+  RegisterOpt<BlockPlacement> X("block-placement",
+                                "Profile Guided Basic Block Placement");
+}
+
+bool BlockPlacement::runOnFunction(Function &F) {
+  PI = &getAnalysis<ProfileInfo>();
+
+  NumMovedBlocks = 0;
+  InsertPos = F.begin(); 
+
+  // Recursively place all blocks.
+  PlaceBlocks(F.begin());
+  
+  // If there are any unreachable blocks, move them to the end.
+
+  PlacedBlocks.clear();
+  NumMoved += NumMovedBlocks;
+  return NumMovedBlocks != 0;
+}
+
+
+/// PlaceBlocks - Recursively place the specified blocks and any unplaced
+/// successors.
+void BlockPlacement::PlaceBlocks(BasicBlock *BB) {
+  assert(!PlacedBlocks.count(BB) && "Already placed this block!");
+  PlacedBlocks.insert(BB);
+
+  // Place the specified block.
+  if (&*InsertPos != BB) {
+    // Use splice to move the block into the right place.  This avoids having to
+    // remove the block from the function then readd it, which causes a bunch of
+    // symbol table traffic that is entirely pointless.
+    Function::BasicBlockListType &Blocks = BB->getParent()->getBasicBlockList();
+    Blocks.splice(InsertPos, Blocks, BB);
+
+    ++NumMovedBlocks;
+  } else {
+    // This block is already in the right place, we don't have to do anything.
+    ++InsertPos;
+  }
+
+  // Keep placing successors until we run out of ones to place.  Note that this
+  // loop is very inefficient (N^2) for blocks with many successors, like switch
+  // statements.  FIXME!
+  while (1) {
+    // Okay, now place any unplaced successors.
+    succ_iterator SI = succ_begin(BB), E = succ_end(BB);
+    
+    // Scan for the first unplaced successor.
+    for (; SI != E && PlacedBlocks.count(*SI); ++SI)
+      /*empty*/;
+    if (SI == E) return;  // No more successors to place.
+    
+    unsigned MaxExecutionCount = PI->getExecutionCount(*SI);
+    BasicBlock *MaxSuccessor = *SI;
+
+    // Scan for more frequently executed successors
+    for (; SI != E; ++SI)
+      if (!PlacedBlocks.count(*SI)) {
+        unsigned Count = PI->getExecutionCount(*SI);
+        if (Count > MaxExecutionCount ||
+            // Prefer to not disturb the code.
+            (Count == MaxExecutionCount && *SI == &*InsertPos)) {
+          MaxExecutionCount = Count;
+          MaxSuccessor = *SI;
+        }
+      }
+
+    // Now that we picked the maximally executed successor, place it.
+    PlaceBlocks(MaxSuccessor);
+  }
+}