fix string comparison

This patch fixes strcmp by reimplementing it as a IRBuilder method and
unrolling the loop based on the literal string size instead of
STRING_SIZE.

Fixes: https://github.com/iovisor/bpftrace/issues/14

src/ast/codegen_llvm.cpp

@@ -617,33 +617,41 @@ void CodegenLLVM::visit(Binop &binop)
     return;
   }
 
-  Value *lhs, *rhs;
-  binop.left->accept(*this);
-  lhs = expr_;
-  binop.right->accept(*this);
-  rhs = expr_;
-
   Type &type = binop.left->type.type;
   if (type == Type::string)
   {
-    Function *strcmp_func = module_->getFunction("strcmp");
+    Value *val;
+    std::string string_literal("");
+    if (binop.right->is_literal) {
+      binop.left->accept(*this);
+      val = expr_;
+      string_literal = reinterpret_cast<String*>(binop.right)->str;
+    } else {
+      binop.right->accept(*this);
+      val = expr_;
+      string_literal = reinterpret_cast<String*>(binop.left)->str;
+    }
+
     switch (binop.op) {
       case bpftrace::Parser::token::EQ:
-        expr_ = b_.CreateCall(strcmp_func, {lhs, rhs}, "strcmp");
+        expr_ = b_.CreateStrcmp(val, string_literal);
         break;
       case bpftrace::Parser::token::NE:
-        expr_ = b_.CreateNot(b_.CreateCall(strcmp_func, {lhs, rhs}, "strcmp"));
+        expr_ = b_.CreateStrcmp(val, string_literal, true);
         break;
       default:
         abort();
     }
-    if (!binop.left->is_variable)
-      b_.CreateLifetimeEnd(lhs);
-    if (!binop.right->is_variable)
-      b_.CreateLifetimeEnd(rhs);
+    b_.CreateLifetimeEnd(val);
   }
   else
   {
+    Value *lhs, *rhs;
+    binop.left->accept(*this);
+    lhs = expr_;
+    binop.right->accept(*this);
+    rhs = expr_;
+
     switch (binop.op) {
       case bpftrace::Parser::token::EQ:    expr_ = b_.CreateICmpEQ (lhs, rhs); break;
       case bpftrace::Parser::token::NE:    expr_ = b_.CreateICmpNE (lhs, rhs); break;
@@ -1276,52 +1284,10 @@ void CodegenLLVM::createLinearFunction()
   b_.CreateRet(b_.CreateLoad(result_alloc));
 }
 
-void CodegenLLVM::createStrcmpFunction()
-{
-  // Returns 1 if strings match, 0 otherwise
-  // i1 strcmp(const char *s1, const char *s2)
-  // {
-  //   for (int i=0; i<STRING_SIZE; i++)
-  //   {
-  //     if (s1[i] != s2[i]) return 0;
-  //   }
-  //   return 1;
-  // }
-
-  FunctionType *strcmp_func_type = FunctionType::get(b_.getInt1Ty(), {b_.getInt8PtrTy(), b_.getInt8PtrTy()}, false);
-  Function *strcmp_func = Function::Create(strcmp_func_type, Function::InternalLinkage, "strcmp", module_.get());
-  strcmp_func->addFnAttr(Attribute::AlwaysInline);
-  strcmp_func->setSection("helpers");
-  BasicBlock *entry = BasicBlock::Create(module_->getContext(), "strcmp.entry", strcmp_func);
-  BasicBlock *not_equal_block = BasicBlock::Create(module_->getContext(), "strcmp.not_equal", strcmp_func);
-  b_.SetInsertPoint(entry);
-
-  Value *s1 = strcmp_func->arg_begin();
-  Value *s2 = strcmp_func->arg_begin()+1;
-
-  for (int i=0; i<STRING_SIZE; i++)
-  {
-    Value *s1_char = b_.CreateGEP(s1, {b_.getInt64(i)});
-    Value *s2_char = b_.CreateGEP(s2, {b_.getInt64(i)});
-
-    BasicBlock *continue_block = BasicBlock::Create(module_->getContext(), "strcmp.continue", strcmp_func);
-
-    Value *cmp = b_.CreateICmpNE(b_.CreateLoad(s1_char), b_.CreateLoad(s2_char));
-    b_.CreateCondBr(cmp, not_equal_block, continue_block);
-
-    b_.SetInsertPoint(continue_block);
-  }
-  b_.CreateRet(b_.getInt1(1));
-
-  b_.SetInsertPoint(not_equal_block);
-  b_.CreateRet(b_.getInt1(0));
-}
-
 std::unique_ptr<BpfOrc> CodegenLLVM::compile(DebugLevel debug, std::ostream &out)
 {
   createLog2Function();
   createLinearFunction();
-  createStrcmpFunction();
   root_->accept(*this);
 
   LLVMInitializeBPFTargetInfo();

src/ast/codegen_llvm.h

@@ -54,7 +54,6 @@ public:
 
   void createLog2Function();
   void createLinearFunction();
-  void createStrcmpFunction();
   std::unique_ptr<BpfOrc> compile(DebugLevel debug=DebugLevel::kNone, std::ostream &out=std::cerr);
 
 private:

src/ast/irbuilderbpf.cpp

@@ -323,6 +323,39 @@ Value *IRBuilderBPF::CreateUSDTReadArgument(Value *ctx, AttachPoint *attach_poin
   return result;
 }
 
+Value *IRBuilderBPF::CreateStrcmp(Value* val, std::string str, bool inverse) {
+  Function *parent = GetInsertBlock()->getParent();
+  BasicBlock *str_ne = BasicBlock::Create(module_.getContext(), "strcmp.false", parent);
+  AllocaInst *store = CreateAllocaBPF(getInt8Ty(), "strcmp.result");
+
+  CreateStore(getInt1(inverse), store);
+
+  const char *c_str = str.c_str();
+  for (int i = 0; i < strlen(c_str) + 1; i++)
+  {
+    BasicBlock *char_eq = BasicBlock::Create(module_.getContext(), "strcmp.loop", parent);
+    AllocaInst *val_char = CreateAllocaBPF(getInt8Ty(), "strcmp.char");
+    Value *ptr = CreateAdd(
+        val,
+        getInt64(i));
+    CreateProbeRead(val_char, 8, ptr);
+
+    Value *l = CreateLoad(getInt8Ty(), val_char);
+    CreateLifetimeEnd(store);
+    Value *r = getInt8(c_str[i]);
+    Value *cmp = CreateICmpNE(l, r, "strcmp.cmp");
+    CreateCondBr(cmp, str_ne, char_eq);
+    SetInsertPoint(char_eq);
+  }
+  CreateStore(getInt1(!inverse), store);
+  CreateBr(str_ne);
+
+  SetInsertPoint(str_ne);
+  Value *result = CreateLoad(store);
+  CreateLifetimeEnd(store);
+  return result;
+}
+
 CallInst *IRBuilderBPF::CreateGetNs()
 {
   // u64 ktime_get_ns()

src/ast/irbuilderbpf.h

@@ -34,6 +34,7 @@ public:
   CallInst   *CreateProbeReadStr(AllocaInst *dst, size_t size, Value *src);
   CallInst   *CreateProbeReadStr(Value *dst, size_t size, Value *src);
   Value      *CreateUSDTReadArgument(Value *ctx, AttachPoint *attach_point, int arg_name, Builtin &builtin);
+  Value      *CreateStrcmp(Value* val, std::string str, bool inverse=false);
   CallInst   *CreateGetNs();
   CallInst   *CreateGetPidTgid();
   CallInst   *CreateGetUidGid();

src/ast/semantic_analyser.cpp

@@ -382,6 +382,10 @@ void SemanticAnalyser::visit(Binop &binop)
       err_ << "comparing '" << lhs << "' ";
       err_ << "with '" << rhs << "'" << std::endl;
     }
+    else if (lhs == Type::string && !(binop.left->is_literal || binop.right->is_literal)) {
+      err_ << "Comparison between two variables of ";
+      err_ << "type string is not allowed" << std::endl;
+    }
     else if (lhs != Type::integer &&
              binop.op != Parser::token::EQ &&
              binop.op != Parser::token::NE) {