[llvm-commits] [llvm] r50002 - /llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp

[Chris Lattner]

Author: lattner
Date: Sun Apr 20 16:13:06 2008
New Revision: 50002

URL: http://llvm.org/viewvc/llvm-project?rev=50002&view=rev
Log:
improve comments, infrastructure, and add some validity checks for threading.
Add a cost function.

Modified:
    llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp?rev=50002&r1=50001&r2=50002&view=diff

==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp Sun Apr 20 16:13:06 2008
@@ -7,35 +7,51 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This pass performs 'jump threading', which looks at blocks that have multiple
-// predecessors and multiple successors.  If one or more of the predecessors of
-// the block can be proven to always jump to one of the successors, we forward
-// the edge from the predecessor to the successor by duplicating the contents of
-// this block.
+// This file implements the Jump Threading pass.
 //
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "jump-threading"
 #include "llvm/Transforms/Scalar.h"
+#include "llvm/IntrinsicInst.h"
 #include "llvm/Pass.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
 using namespace llvm;
 
 //STATISTIC(NumThreads, "Number of jumps threaded");
 
+static cl::opt<unsigned>
+Threshold("jump-threading-threshold", 
+          cl::desc("Max block size to duplicate for jump threading"),
+          cl::init(6), cl::Hidden);
+
 namespace {
-  cl::opt<unsigned>
-  Threshold("jump-threading-threshold", 
-            cl::desc("Max block size to duplicate for jump threading"),
-            cl::init(6), cl::Hidden);
+  /// This pass performs 'jump threading', which looks at blocks that have
+  /// multiple predecessors and multiple successors.  If one or more of the
+  /// predecessors of the block can be proven to always jump to one of the
+  /// successors, we forward the edge from the predecessor to the successor by
+  /// duplicating the contents of this block.
+  ///
+  /// An example of when this can occur is code like this:
+  ///
+  ///   if () { ...
+  ///     X = 4;
+  ///   }
+  ///   if (X < 3) {
+  ///
+  /// In this case, the unconditional branch at the end of the first if can be
+  /// revectored to the false side of the second if.
+  ///
   class VISIBILITY_HIDDEN JumpThreading : public FunctionPass {
   public:
     static char ID; // Pass identification
     JumpThreading() : FunctionPass((intptr_t)&ID) {}
 
     bool runOnFunction(Function &F);
+    bool ThreadBlock(BasicBlock &BB);
   };
   char JumpThreading::ID = 0;
   RegisterPass<JumpThreading> X("jump-threading", "Jump Threading");
@@ -47,6 +63,87 @@
 /// runOnFunction - Top level algorithm.
 ///
 bool JumpThreading::runOnFunction(Function &F) {
+  DOUT << "Jump threading on function '" << F.getNameStart() << "'\n";
   bool Changed = false;
+  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
+    Changed |= ThreadBlock(*I);
   return Changed;
 }
+
+/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
+/// thread across it.
+static unsigned getJumpThreadDuplicationCost(const BasicBlock &BB) {
+  BasicBlock::const_iterator I = BB.begin();
+  /// Ignore PHI nodes, these will be flattened when duplication happens.
+  while (isa<PHINode>(*I)) ++I;
+
+  // Sum up the cost of each instruction until we get to the terminator.  Don't
+  // include the terminator because the copy won't include it.
+  unsigned Size = 0;
+  for (; !isa<TerminatorInst>(I); ++I) {
+    // Debugger intrinsics don't incur code size.
+    if (isa<DbgInfoIntrinsic>(I)) continue;
+    
+    // If this is a pointer->pointer bitcast, it is free.
+    if (isa<BitCastInst>(I) && isa<PointerType>(I->getType()))
+      continue;
+    
+    // All other instructions count for at least one unit.
+    ++Size;
+    
+    // Calls are more expensive.  If they are non-intrinsic calls, we model them
+    // as having cost of 4.  If they are a non-vector intrinsic, we model them
+    // as having cost of 2 total, and if they are a vector intrinsic, we model
+    // them as having cost 1.
+    if (const CallInst *CI = dyn_cast<CallInst>(I)) {
+      if (!isa<IntrinsicInst>(CI))
+        Size += 3;
+      else if (isa<VectorType>(CI->getType()))
+        Size += 1;
+    }
+  }
+  
+  // Threading through a switch statement is particularly profitable.  If this
+  // block ends in a switch, decrease its cost to make it more likely to happen.
+  if (isa<SwitchInst>(I))
+    Size = Size > 6 ? Size-6 : 0;
+  
+  return Size;
+}
+
+
+/// ThreadBlock - If there are any predecessors whose control can be threaded
+/// through to a successor, transform them now.
+bool JumpThreading::ThreadBlock(BasicBlock &BB) {
+  // If there is only one predecessor or successor, then there is nothing to do.
+  if (BB.getTerminator()->getNumSuccessors() == 1 || BB.getSinglePredecessor())
+    return false;
+  
+  // See if this block ends with a branch of switch.  If so, see if the
+  // condition is a phi node.  If so, and if an entry of the phi node is a
+  // constant, we can thread the block.
+  Value *Condition;
+  if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator()))
+    Condition = BI->getCondition();
+  else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator()))
+    Condition = SI->getCondition();
+  else
+    return false; // Must be an invoke.
+
+  // See if this is a phi node in the current block.
+  PHINode *PN = dyn_cast<PHINode>(Condition);
+  if (!PN || PN->getParent() != &BB) return false;
+  
+  // See if the cost of duplicating this block is low enough.
+  unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
+  if (JumpThreadCost > Threshold) {
+    DOUT << "  Not threading BB '" << BB.getNameStart()
+         << "': Cost is too high: " << JumpThreadCost << "\n";
+    return false;
+  }
+
+  DOUT << "  Threading BB '" << BB.getNameStart() << "'.  Cost is : "
+       << JumpThreadCost << "\n";
+  
+  return false;
+}