Code cleanups and added comments.

llvm-svn: 80781

llvm/lib/Transforms/Instrumentation/OptimalEdgeProfiling.cpp

@@ -14,14 +14,11 @@
 //===----------------------------------------------------------------------===//
 #define DEBUG_TYPE "insert-optimal-edge-profiling"
 #include "ProfilingUtils.h"
-#include "llvm/Constants.h"
-#include "llvm/Function.h"
-#include "llvm/LLVMContext.h"
 #include "llvm/Module.h"
 #include "llvm/Pass.h"
 #include "llvm/Analysis/Passes.h"
-#include "llvm/Analysis/ProfileInfo.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Instrumentation.h"
@@ -75,8 +72,19 @@ bool OptimalEdgeProfiler::runOnModule(Module &M) {
     return false;  // No main, no instrumentation!
   }
 
+  // BlocksToInstrument stores all blocks that are in the function prior to
+  // instrumenting, since the spliting of critical edges adds new blocks (which
+  // have not to be instrumented), we have to remember them for later.
   std::set<BasicBlock*> BlocksToInstrument;
+
+  // NumEdges counts all the edges that may be instrumented. Later on its
+  // decided which edges to actually instrument, to achieve optimal profiling.
+  // For the entry block a virtual edge (0,entry) is reserved, for each block
+  // with no successors an edge (BB,0) is reserved. These edges are necessary
+  // to calculate a truly optimal maximum spanning tree and thus an optimal
+  // instrumentation.
   unsigned NumEdges = 0;
+
   for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
     if (F->isDeclaration()) continue;
     // Reserve space for (0,entry) edge.
@@ -95,6 +103,13 @@ bool OptimalEdgeProfiler::runOnModule(Module &M) {
     }
   }
 
+  // In the profiling output a counter for each edge is reserved, but only few
+  // are used. This is done to be able to read back in the profile without
+  // calulating the maximum spanning tree again, instead each edge counter that
+  // is not used is initialised with -1 to signal that this edge counter has to
+  // be calculated from other edge counters on reading the profile info back
+  // in.
+
   const Type *Int32 = Type::getInt32Ty(M.getContext());
   const ArrayType *ATy = ArrayType::get(Int32, NumEdges);
   GlobalVariable *Counters =
@@ -112,13 +127,23 @@ bool OptimalEdgeProfiler::runOnModule(Module &M) {
     if (F->isDeclaration()) continue;
     DEBUG(errs()<<"Working on "<<F->getNameStr()<<"\n");
 
-    PI = &getAnalysisID<ProfileInfo>(ProfileEstimatorPassID,*F);
-    MaximumSpanningTree MST = MaximumSpanningTree(&(*F),PI,true);
+    // Calculate a Maximum Spanning Tree with the edge weights determined by
+    // ProfileEstimator. ProfileEstimator also assign weights to the virtual
+    // edges (0,entry) and (BB,0) (for blocks with no successors) and this
+    // edges also participate in the maximum spanning tree calculation. 
+    // The third parameter of MaximumSpanningTree() has the effect that not the
+    // actual MST is returned but the edges _not_ in the MST.
+
+    PI = &getAnalysisID<ProfileInfo>(ProfileEstimatorPassID, *F);
+    MaximumSpanningTree MST = MaximumSpanningTree(&(*F), PI, true);
+
+    // Check if (0,entry) not in the MST. If not, instrument edge
+    // (IncrementCounterInBlock()) and set the counter initially to zero, if
+    // the edge is in the MST the counter is initialised to -1.
 
-    // Create counter for (0,entry) edge.
     BasicBlock *entry = &(F->getEntryBlock());
     ProfileInfo::Edge edge = ProfileInfo::getEdge(0,entry);
-    if (std::binary_search(MST.begin(),MST.end(),edge)) {
+    if (std::binary_search(MST.begin(), MST.end(), edge)) {
       printEdgeCounter(edge,entry,i);
       IncrementCounterInBlock(entry, i, Counters); NumEdgesInserted++;
       Initializer[i++] = (zeroc);
@@ -127,15 +152,17 @@ bool OptimalEdgeProfiler::runOnModule(Module &M) {
     }
 
     for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
-      if (!BlocksToInstrument.count(BB)) continue; // Don't count new blocks
+      // Do not count blocks that where introduced by spliting critical edges.
+      if (!BlocksToInstrument.count(BB)) continue;
+
       // Okay, we have to add a counter of each outgoing edge not in MST. If
       // the outgoing edge is not critical don't split it, just insert the
-      // counter in the source or destination of the edge.
+      // counter in the source or destination of the edge. Also, if the block
+      // has no successors, the virtual edge (BB,0) is processed.
       TerminatorInst *TI = BB->getTerminator();
       if (TI->getNumSuccessors() == 0) {
-        // Create counter for (BB,0), edge.
         ProfileInfo::Edge edge = ProfileInfo::getEdge(BB,0);
-        if (std::binary_search(MST.begin(),MST.end(),edge)) {
+        if (std::binary_search(MST.begin(), MST.end(), edge)) {
           printEdgeCounter(edge,BB,i);
           IncrementCounterInBlock(BB, i, Counters); NumEdgesInserted++;
           Initializer[i++] = (zeroc);
@@ -146,14 +173,14 @@ bool OptimalEdgeProfiler::runOnModule(Module &M) {
       for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s) {
         BasicBlock *Succ = TI->getSuccessor(s);
         ProfileInfo::Edge edge = ProfileInfo::getEdge(BB,Succ);
-        if (std::binary_search(MST.begin(),MST.end(),edge)) {
+        if (std::binary_search(MST.begin(), MST.end(), edge)) {
 
           // If the edge is critical, split it.
           SplitCriticalEdge(TI,s,this);
           Succ = TI->getSuccessor(s);
 
-          // Okay, we are guaranteed that the edge is no longer critical.  If we
-          // only have a single successor, insert the counter in this block,
+          // Okay, we are guaranteed that the edge is no longer critical.  If
+          // we only have a single successor, insert the counter in this block,
           // otherwise insert it in the successor block.
           if (TI->getNumSuccessors() == 1) {
             // Insert counter at the start of the block
@@ -172,10 +199,11 @@ bool OptimalEdgeProfiler::runOnModule(Module &M) {
     }
   }
 
-  // check if indeed all counters have been used
+  // Check if the number of edges counted at first was the number of edges we
+  // considered for instrumentation.
   assert(i==NumEdges && "the number of edges in counting array is wrong");
 
-  // assign initialiser to array
+  // Assing the now completely defined initialiser to the array.
   Constant *init = ConstantArray::get(ATy, Initializer);
   Counters->setInitializer(init);