1 files changed, 55 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
index 8c56e8709e2..8f0bb460502 100644
--- a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
+++ b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
@@ -80,6 +80,16 @@ static cl::opt<bool>
                                    "callbacks at every basic block"),
                           cl::Hidden, cl::init(false));
 
+// Experimental 8-bit counters used as an additional search heuristic during
+// coverage-guided fuzzing.
+// The counters are not thread-friendly:
+//   - contention on these counters may cause significant slowdown;
+//   - the counter updates are racy and the results may be inaccurate.
+// They are also inaccurate due to 8-bit integer overflow.
+static cl::opt<bool> ClUse8bitCounters("sanitizer-coverage-8bit-counters",
+                                       cl::desc("Experimental 8-bit counters"),
+                                       cl::Hidden, cl::init(false));
+
 namespace {
 
 class SanitizerCoverageModule : public ModulePass {
@@ -114,6 +124,7 @@ class SanitizerCoverageModule : public ModulePass {
   LLVMContext *C;
 
   GlobalVariable *GuardArray;
+  GlobalVariable *EightBitCounterArray;
 
   int CoverageLevel;
 };
@@ -152,9 +163,9 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
       M.getOrInsertFunction(kSanCovWithCheckName, VoidTy, Int32PtrTy, nullptr));
   SanCovIndirCallFunction = checkInterfaceFunction(M.getOrInsertFunction(
       kSanCovIndirCallName, VoidTy, IntptrTy, IntptrTy, nullptr));
-  SanCovModuleInit = checkInterfaceFunction(
-      M.getOrInsertFunction(kSanCovModuleInitName, Type::getVoidTy(*C),
-                            Int32PtrTy, IntptrTy, Int8PtrTy, nullptr));
+  SanCovModuleInit = checkInterfaceFunction(M.getOrInsertFunction(
+      kSanCovModuleInitName, Type::getVoidTy(*C), Int32PtrTy, IntptrTy,
+      Int8PtrTy, Int8PtrTy, nullptr));
   SanCovModuleInit->setLinkage(Function::ExternalLinkage);
   // We insert an empty inline asm after cov callbacks to avoid callback merge.
   EmptyAsm = InlineAsm::get(FunctionType::get(IRB.getVoidTy(), false),
@@ -171,9 +182,15 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
   // At this point we create a dummy array of guards because we don't
   // know how many elements we will need.
   Type *Int32Ty = IRB.getInt32Ty();
+  Type *Int8Ty = IRB.getInt8Ty();
+
   GuardArray =
       new GlobalVariable(M, Int32Ty, false, GlobalValue::ExternalLinkage,
                          nullptr, "__sancov_gen_cov_tmp");
+  if (ClUse8bitCounters)
+    EightBitCounterArray =
+        new GlobalVariable(M, Int8Ty, false, GlobalVariable::ExternalLinkage,
+                           nullptr, "__sancov_gen_cov_tmp");
 
   for (auto &F : M)
     runOnFunction(F);
@@ -186,11 +203,28 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
       M, Int32ArrayNTy, false, GlobalValue::PrivateLinkage,
       Constant::getNullValue(Int32ArrayNTy), "__sancov_gen_cov");
 
+
   // Replace the dummy array with the real one.
   GuardArray->replaceAllUsesWith(
       IRB.CreatePointerCast(RealGuardArray, Int32PtrTy));
   GuardArray->eraseFromParent();
 
+  GlobalVariable *RealEightBitCounterArray;
+  if (ClUse8bitCounters) {
+    // Make sure the array is 16-aligned.
+    static const int kCounterAlignment = 16;
+    Type *Int8ArrayNTy =
+        ArrayType::get(Int8Ty, RoundUpToAlignment(SanCovFunction->getNumUses(),
+                                                  kCounterAlignment));
+    RealEightBitCounterArray = new GlobalVariable(
+        M, Int8ArrayNTy, false, GlobalValue::PrivateLinkage,
+        Constant::getNullValue(Int8ArrayNTy), "__sancov_gen_cov_counter");
+    RealEightBitCounterArray->setAlignment(kCounterAlignment);
+    EightBitCounterArray->replaceAllUsesWith(
+        IRB.CreatePointerCast(RealEightBitCounterArray, Int8PtrTy));
+    EightBitCounterArray->eraseFromParent();
+  }
+
   // Create variable for module (compilation unit) name
   Constant *ModNameStrConst =
       ConstantDataArray::getString(M.getContext(), M.getName(), true);
@@ -200,10 +234,13 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
 
   // Call __sanitizer_cov_module_init
   IRB.SetInsertPoint(CtorFunc->getEntryBlock().getTerminator());
-  IRB.CreateCall3(SanCovModuleInit,
-                  IRB.CreatePointerCast(RealGuardArray, Int32PtrTy),
-                  ConstantInt::get(IntptrTy, SanCovFunction->getNumUses()),
-                  IRB.CreatePointerCast(ModuleName, Int8PtrTy));
+  IRB.CreateCall4(
+      SanCovModuleInit, IRB.CreatePointerCast(RealGuardArray, Int32PtrTy),
+      ConstantInt::get(IntptrTy, SanCovFunction->getNumUses()),
+      ClUse8bitCounters
+          ? IRB.CreatePointerCast(RealEightBitCounterArray, Int8PtrTy)
+          : Constant::getNullValue(Int8PtrTy),
+      IRB.CreatePointerCast(ModuleName, Int8PtrTy));
   return true;
 }
 
@@ -314,6 +351,17 @@ void SanitizerCoverageModule::InjectCoverageAtBlock(Function &F, BasicBlock &BB,
     IRB.CreateCall(EmptyAsm);  // Avoids callback merge.
   }
 
+  if(ClUse8bitCounters) {
+    IRB.SetInsertPoint(IP);
+    Value *P = IRB.CreateAdd(
+        IRB.CreatePointerCast(EightBitCounterArray, IntptrTy),
+        ConstantInt::get(IntptrTy, SanCovFunction->getNumUses() - 1));
+    P = IRB.CreateIntToPtr(P, IRB.getInt8PtrTy());
+    Value *LI = IRB.CreateLoad(P);
+    Value *Inc = IRB.CreateAdd(LI, ConstantInt::get(IRB.getInt8Ty(), 1));
+    IRB.CreateStore(Inc, P);
+  }
+
   if (ClExperimentalTracing) {
     // Experimental support for tracing.
     // Insert a callback with the same guard variable as used for coverage.