[asan] Optimize accesses to global arrays with constant index

Summary:
Given a global array G[N], which is declared in this CU and has static initializer
avoid instrumenting accesses like G[i], where 'i' is a constant and 0<=i<N.
Also add a bit of stats.

This eliminates ~1% of instrumentations on SPEC2006
and also partially helps when asan is being run together with coverage.

Reviewers: samsonov

Reviewed By: samsonov

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1947

llvm-svn: 192794

1 files changed, 33 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
index 7ced56bedd2..de0a43bfd94 100644
--- a/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -23,6 +23,7 @@
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/DIBuilder.h"
@@ -193,6 +194,13 @@ static cl::opt<int> ClDebugMin("asan-debug-min", cl::desc("Debug min inst"),
 static cl::opt<int> ClDebugMax("asan-debug-max", cl::desc("Debug man inst"),
                                cl::Hidden, cl::init(-1));
 
+STATISTIC(NumInstrumentedReads, "Number of instrumented reads");
+STATISTIC(NumInstrumentedWrites, "Number of instrumented writes");
+STATISTIC(NumOptimizedAccessesToGlobalArray,
+          "Number of optimized accesses to global arrays");
+STATISTIC(NumOptimizedAccessesToGlobalVar,
+          "Number of optimized accesses to global vars");
+
 namespace {
 /// A set of dynamically initialized globals extracted from metadata.
 class SetOfDynamicallyInitializedGlobals {
@@ -315,6 +323,7 @@ struct AddressSanitizer : public FunctionPass {
   bool ShouldInstrumentGlobal(GlobalVariable *G);
   bool LooksLikeCodeInBug11395(Instruction *I);
   void FindDynamicInitializers(Module &M);
+  bool GlobalIsLinkerInitialized(GlobalVariable *G);
 
   bool CheckInitOrder;
   bool CheckUseAfterReturn;
@@ -655,6 +664,13 @@ static Value *isInterestingMemoryAccess(Instruction *I, bool *IsWrite) {
   return NULL;
 }
 
+bool AddressSanitizer::GlobalIsLinkerInitialized(GlobalVariable *G) {
+  // If a global variable does not have dynamic initialization we don't
+  // have to instrument it.  However, if a global does not have initializer
+  // at all, we assume it has dynamic initializer (in other TU).
+  return G->hasInitializer() && !DynamicallyInitializedGlobals.Contains(G);
+}
+
 void AddressSanitizer::instrumentMop(Instruction *I) {
   bool IsWrite = false;
   Value *Addr = isInterestingMemoryAccess(I, &IsWrite);
@@ -663,13 +679,19 @@ void AddressSanitizer::instrumentMop(Instruction *I) {
     if (GlobalVariable *G = dyn_cast<GlobalVariable>(Addr)) {
       // If initialization order checking is disabled, a simple access to a
       // dynamically initialized global is always valid.
-      if (!CheckInitOrder)
-        return;
-      // If a global variable does not have dynamic initialization we don't
-      // have to instrument it.  However, if a global does not have initailizer
-      // at all, we assume it has dynamic initializer (in other TU).
-      if (G->hasInitializer() && !DynamicallyInitializedGlobals.Contains(G))
+      if (!CheckInitOrder || GlobalIsLinkerInitialized(G)) {
+        NumOptimizedAccessesToGlobalVar++;
         return;
+      }
+    }
+    ConstantExpr *CE = dyn_cast<ConstantExpr>(Addr);
+    if (CE && CE->isGEPWithNoNotionalOverIndexing()) {
+      if (GlobalVariable *G = dyn_cast<GlobalVariable>(CE->getOperand(0))) {
+        if (CE->getOperand(1)->isNullValue() && GlobalIsLinkerInitialized(G)) {
+          NumOptimizedAccessesToGlobalArray++;
+          return;
+        }
+      }
     }
   }
 
@@ -681,6 +703,11 @@ void AddressSanitizer::instrumentMop(Instruction *I) {
 
   assert((TypeSize % 8) == 0);
 
+  if (IsWrite)
+    NumInstrumentedWrites++;
+  else
+    NumInstrumentedReads++;
+
   // Instrument a 1-, 2-, 4-, 8-, or 16- byte access with one check.
   if (TypeSize == 8  || TypeSize == 16 ||
       TypeSize == 32 || TypeSize == 64 || TypeSize == 128)