Add a stat to show how often the MaxLookupSearchDepth limit is hit

[wmi]

Summary

As http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150413/271370.html described, GEP depth can directly affect the precision of BasicAA. Add a statistic use to count how often we hit the limit.

[qcolombet]

Hi Wei,

Aside from bike shedding with comments and variable names.
LGTM.

Thanks,
-Quentin

lib/Analysis/BasicAliasAnalysis.cpp

51

Number of times the limit to decompose GEPs is reached.

52

Number of times a GEP is decomposed.

lib/Analysis/BasicAliasAnalysis.cpp

 // Alias Analysis interface that implements identities (two different
 // globals cannot alias, etc), but does no stateful analysis.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/Passes.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/AssumptionCache.h"
 #include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/CaptureTracking.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"

 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Operator.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <algorithm>
 using namespace llvm;
 
+/// SearchLimitReached / SearchTimes shows how often the limit of
+/// MaxLookupSearchDepth is hit. It will affect the precision
+/// of basic alias analysis.
+#define DEBUG_TYPE "basicaa"
+STATISTIC(SearchLimitReached, "Number of times the limit of "
+                              "MaxLookupSearchDepth is reached");
+STATISTIC(SearchTimes, "Number of times the gep expression is searched")
+
 /// Cutoff after which to stop analysing a set of phi nodes potentially involved
 /// in a cycle. Because we are analysing 'through' phi nodes we need to be
 /// careful with value equivalence. We use reachability to make sure a value
 /// cannot be involved in a cycle.
 const unsigned MaxNumPhiBBsValueReachabilityCheck = 20;
 
 // The max limit of the search depth in DecomposeGEPExpression() and
 // GetUnderlyingObject(), both functions need to use the same search

 static const Value *
 DecomposeGEPExpression(const Value *V, int64_t &BaseOffs,
                        SmallVectorImpl<VariableGEPIndex> &VarIndices,
                        bool &MaxLookupReached, const DataLayout &DL,
                        AssumptionCache *AC, DominatorTree *DT) {
   // Limit recursion depth to limit compile time in crazy cases.
   unsigned MaxLookup = MaxLookupSearchDepth;
   MaxLookupReached = false;
+  SearchTimes++;
 
   BaseOffs = 0;
   do {
     // See if this is a bitcast or GEP.
     const Operator *Op = dyn_cast<Operator>(V);
     if (!Op) {
       // The only non-operator case we can handle are GlobalAliases.
       if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
do {
     }
 
     // Analyze the base pointer next.
     V = GEPOp->getOperand(0);
   } while (--MaxLookup);
 
   // If the chain of expressions is too deep, just return early.
   MaxLookupReached = true;
+  SearchLimitReached++;
   return V;
 }
 
 //===----------------------------------------------------------------------===//
 // BasicAliasAnalysis Pass
 //===----------------------------------------------------------------------===//
 
 #ifndef NDEBUG