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//===-- BasicBlockPlacement.cpp - Basic Block Code Layout optimization ----===//
//
// The LLVM Compiler Infrastructure
//
// This file 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.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "block-placement"
#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ProfileInfo.h"
#include "llvm/Function.h"
#include "llvm/Pass.h"
#include "llvm/Support/CFG.h"
#include <set>
using namespace llvm;
STATISTIC(NumMoved, "Number of basic blocks moved");
namespace {
struct BlockPlacement : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
BlockPlacement() : FunctionPass(ID) {
initializeBlockPlacementPass(*PassRegistry::getPassRegistry());
}
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);
};
}
char BlockPlacement::ID = 0;
INITIALIZE_PASS_BEGIN(BlockPlacement, "block-placement",
"Profile Guided Basic Block Placement", false, false)
INITIALIZE_AG_DEPENDENCY(ProfileInfo)
INITIALIZE_PASS_END(BlockPlacement, "block-placement",
"Profile Guided Basic Block Placement", false, false)
FunctionPass *llvm::createBlockPlacementPass() { return new BlockPlacement(); }
bool BlockPlacement::runOnFunction(Function &F) {
PI = &getAnalysis<ProfileInfo>();
NumMovedBlocks = 0;
InsertPos = F.begin();
// Recursively place all blocks.
PlaceBlocks(F.begin());
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.
double MaxExecutionCount = PI->getExecutionCount(*SI);
BasicBlock *MaxSuccessor = *SI;
// Scan for more frequently executed successors
for (; SI != E; ++SI)
if (!PlacedBlocks.count(*SI)) {
double 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);
}
}
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