Fix a quadratic algorithm in MachineBranchProbabilityInfo.

The getSumForBlock function was quadratic in the number of successors
because getSuccWeight would perform a linear search for an already known
iterator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161460 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineBasicBlock.h

@@ -572,7 +572,7 @@ private:
   /// getSuccWeight - Return weight of the edge from this block to MBB. This
   /// method should NOT be called directly, but by using getEdgeWeight method
   /// from MachineBranchProbabilityInfo class.
-  uint32_t getSuccWeight(const MachineBasicBlock *succ) const;
+  uint32_t getSuccWeight(const_succ_iterator Succ) const;
 
 
   // Methods used to maintain doubly linked list of blocks...

include/llvm/CodeGen/MachineBranchProbabilityInfo.h

@@ -16,14 +16,12 @@
 #define LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
 
 #include "llvm/Pass.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/Support/BranchProbability.h"
 #include <climits>
 
 namespace llvm {
 
-class raw_ostream;
-class MachineBasicBlock;
-
 class MachineBranchProbabilityInfo : public ImmutablePass {
   virtual void anchor();
 
@@ -52,6 +50,11 @@ public:
   uint32_t getEdgeWeight(const MachineBasicBlock *Src,
                          const MachineBasicBlock *Dst) const;
 
+  // Same thing, but using a const_succ_iterator from Src. This is faster when
+  // the iterator is already available.
+  uint32_t getEdgeWeight(const MachineBasicBlock *Src,
+                         MachineBasicBlock::const_succ_iterator Dst) const;
+
   // Get sum of the block successors' weights, potentially scaling them to fit
   // within 32-bits. If scaling is required, sets Scale based on the necessary
   // adjustment. Any edge weights used with the sum should be divided by Scale.

lib/CodeGen/MachineBasicBlock.cpp

@@ -912,12 +912,11 @@ MachineBasicBlock::findDebugLoc(instr_iterator MBBI) {
 
 /// getSuccWeight - Return weight of the edge from this block to MBB.
 ///
-uint32_t MachineBasicBlock::getSuccWeight(const MachineBasicBlock *succ) const {
+uint32_t MachineBasicBlock::getSuccWeight(const_succ_iterator Succ) const {
   if (Weights.empty())
     return 0;
 
-  const_succ_iterator I = std::find(Successors.begin(), Successors.end(), succ);
+  return *getWeightIterator(Succ);
-  return *getWeightIterator(I);
 }
 
 /// getWeightIterator - Return wight iterator corresonding to the I successor

lib/CodeGen/MachineBranchProbabilityInfo.cpp

@@ -38,7 +38,7 @@ getSumForBlock(const MachineBasicBlock *MBB, uint32_t &Scale) const {
   Scale = 1;
   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
        E = MBB->succ_end(); I != E; ++I) {
-    uint32_t Weight = getEdgeWeight(MBB, *I);
+    uint32_t Weight = getEdgeWeight(MBB, I);
     Sum += Weight;
   }
 
@@ -53,22 +53,30 @@ getSumForBlock(const MachineBasicBlock *MBB, uint32_t &Scale) const {
   Sum = 0;
   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
        E = MBB->succ_end(); I != E; ++I) {
-    uint32_t Weight = getEdgeWeight(MBB, *I);
+    uint32_t Weight = getEdgeWeight(MBB, I);
     Sum += Weight / Scale;
   }
   assert(Sum <= UINT32_MAX);
   return Sum;
 }
 
-uint32_t
+uint32_t MachineBranchProbabilityInfo::
-MachineBranchProbabilityInfo::getEdgeWeight(const MachineBasicBlock *Src,
+getEdgeWeight(const MachineBasicBlock *Src,
-                                            const MachineBasicBlock *Dst) const {
+              MachineBasicBlock::const_succ_iterator Dst) const {
   uint32_t Weight = Src->getSuccWeight(Dst);
   if (!Weight)
     return DEFAULT_WEIGHT;
   return Weight;
 }
 
+uint32_t MachineBranchProbabilityInfo::
+getEdgeWeight(const MachineBasicBlock *Src,
+              const MachineBasicBlock *Dst) const {
+  // This is a linear search. Try to use the const_succ_iterator version when
+  // possible.
+  return getEdgeWeight(Src, std::find(Src->succ_begin(), Src->succ_end(), Dst));
+}
+
 bool MachineBranchProbabilityInfo::isEdgeHot(MachineBasicBlock *Src,
                                              MachineBasicBlock *Dst) const {
   // Hot probability is at least 4/5 = 80%
@@ -82,7 +90,7 @@ MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
   MachineBasicBlock *MaxSucc = 0;
   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
        E = MBB->succ_end(); I != E; ++I) {
-    uint32_t Weight = getEdgeWeight(MBB, *I);
+    uint32_t Weight = getEdgeWeight(MBB, I);
     if (Weight > MaxWeight) {
       MaxWeight = Weight;
       MaxSucc = *I;