GCDAntipatternChecker.cpp

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//===- GCDAntipatternChecker.cpp ---------------------------------*- C++ -*-==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines GCDAntipatternChecker which checks against a common
// antipattern when synchronous API is emulated from asynchronous callbacks
// using a semaphore:
//
//   dispatch_semaphore_t sema = dispatch_semaphore_create(0);
//
//   AnyCFunctionCall(^{
//     // code…
//     dispatch_semaphore_signal(sema);
//   })
//   dispatch_semaphore_wait(sema, *)
//
// Such code is a common performance problem, due to inability of GCD to
// properly handle QoS when a combination of queues and semaphores is used.
// Good code would either use asynchronous API (when available), or perform
// the necessary action in asynchronous callback.
//
// Currently, the check is performed using a simple heuristical AST pattern
// matching.
//
//===----------------------------------------------------------------------===//
 
#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
#include "llvm/Support/Debug.h"
 
using namespace clang;
using namespace ento;
using namespace ast_matchers;
 
namespace {
 
// ID of a node at which the diagnostic would be emitted.
const char *WarnAtNode = "waitcall";
 
class GCDAntipatternChecker : public Checker<check::ASTCodeBody> {
public:
  void checkASTCodeBody(const Decl *D,
                        AnalysisManager &AM,
                        BugReporter &BR) const;
};
 
decltype(auto) callsName(const char *FunctionName) {
  return callee(functionDecl(hasName(FunctionName)));
}
 
decltype(auto) equalsBoundArgDecl(int ArgIdx, const char *DeclName) {
  return hasArgument(ArgIdx, ignoringParenCasts(declRefExpr(
                                 to(varDecl(equalsBoundNode(DeclName))))));
}
 
decltype(auto) bindAssignmentToDecl(const char *DeclName) {
  return hasLHS(ignoringParenImpCasts(
                         declRefExpr(to(varDecl().bind(DeclName)))));
}
 
/// The pattern is very common in tests, and it is OK to use it there.
/// We have to heuristics for detecting tests: method name starts with "test"
/// (used in XCTest), and a class name contains "mock" or "test" (used in
/// helpers which are not tests themselves, but used exclusively in tests).
static bool isTest(const Decl *D) {
  if (const auto* ND = dyn_cast<NamedDecl>(D)) {
    std::string DeclName = ND->getNameAsString();
    if (StringRef(DeclName).starts_with("test"))
      return true;
  }
  if (const auto *OD = dyn_cast<ObjCMethodDecl>(D)) {
    if (const auto *CD = dyn_cast<ObjCContainerDecl>(OD->getParent())) {
      std::string ContainerName = CD->getNameAsString();
      StringRef CN(ContainerName);
      if (CN.contains_insensitive("test") || CN.contains_insensitive("mock"))
        return true;
    }
  }
  return false;
}
 
static auto findGCDAntiPatternWithSemaphore() -> decltype(compoundStmt()) {
 
  const char *SemaphoreBinding = "semaphore_name";
  auto SemaphoreCreateM = callExpr(allOf(
      callsName("dispatch_semaphore_create"),
      hasArgument(0, ignoringParenCasts(integerLiteral(equals(0))))));
 
  auto SemaphoreBindingM = anyOf(
      forEachDescendant(
          varDecl(hasDescendant(SemaphoreCreateM)).bind(SemaphoreBinding)),
      forEachDescendant(binaryOperator(bindAssignmentToDecl(SemaphoreBinding),
                     hasRHS(SemaphoreCreateM))));
 
  auto HasBlockArgumentM = hasAnyArgument(hasType(
            hasCanonicalType(blockPointerType())
            ));
 
  auto ArgCallsSignalM = hasAnyArgument(stmt(hasDescendant(callExpr(
          allOf(
              callsName("dispatch_semaphore_signal"),
              equalsBoundArgDecl(0, SemaphoreBinding)
              )))));
 
  auto HasBlockAndCallsSignalM = allOf(HasBlockArgumentM, ArgCallsSignalM);
 
  auto HasBlockCallingSignalM =
    forEachDescendant(
      stmt(anyOf(
        callExpr(HasBlockAndCallsSignalM),
        objcMessageExpr(HasBlockAndCallsSignalM)
           )));
 
  auto SemaphoreWaitM = forEachDescendant(
    callExpr(
      allOf(
        callsName("dispatch_semaphore_wait"),
        equalsBoundArgDecl(0, SemaphoreBinding)
      )
    ).bind(WarnAtNode));
 
  return compoundStmt(
      SemaphoreBindingM, HasBlockCallingSignalM, SemaphoreWaitM);
}
 
static auto findGCDAntiPatternWithGroup() -> decltype(compoundStmt()) {
 
  const char *GroupBinding = "group_name";
  auto DispatchGroupCreateM = callExpr(callsName("dispatch_group_create"));
 
  auto GroupBindingM = anyOf(
      forEachDescendant(
          varDecl(hasDescendant(DispatchGroupCreateM)).bind(GroupBinding)),
      forEachDescendant(binaryOperator(bindAssignmentToDecl(GroupBinding),
                     hasRHS(DispatchGroupCreateM))));
 
  auto GroupEnterM = forEachDescendant(
      stmt(callExpr(allOf(callsName("dispatch_group_enter"),
                          equalsBoundArgDecl(0, GroupBinding)))));
 
  auto HasBlockArgumentM = hasAnyArgument(hasType(
            hasCanonicalType(blockPointerType())
            ));
 
  auto ArgCallsSignalM = hasAnyArgument(stmt(hasDescendant(callExpr(
          allOf(
              callsName("dispatch_group_leave"),
              equalsBoundArgDecl(0, GroupBinding)
              )))));
 
  auto HasBlockAndCallsLeaveM = allOf(HasBlockArgumentM, ArgCallsSignalM);
 
  auto AcceptsBlockM =
    forEachDescendant(
      stmt(anyOf(
        callExpr(HasBlockAndCallsLeaveM),
        objcMessageExpr(HasBlockAndCallsLeaveM)
           )));
 
  auto GroupWaitM = forEachDescendant(
    callExpr(
      allOf(
        callsName("dispatch_group_wait"),
        equalsBoundArgDecl(0, GroupBinding)
      )
    ).bind(WarnAtNode));
 
  return compoundStmt(GroupBindingM, GroupEnterM, AcceptsBlockM, GroupWaitM);
}
 
static void emitDiagnostics(const BoundNodes &Nodes,
                            const char* Type,
                            BugReporter &BR,
                            AnalysisDeclContext *ADC,
                            const GCDAntipatternChecker *Checker) {
  const auto *SW = Nodes.getNodeAs<CallExpr>(WarnAtNode);
  assert(SW);
 
  std::string Diagnostics;
  llvm::raw_string_ostream OS(Diagnostics);
  OS << "Waiting on a callback using a " << Type << " creates useless threads "
     << "and is subject to priority inversion; consider "
     << "using a synchronous API or changing the caller to be asynchronous";
 
  BR.EmitBasicReport(
    ADC->getDecl(),
    Checker,
    /*Name=*/"GCD performance anti-pattern",
    /*BugCategory=*/"Performance",
    OS.str(),
    PathDiagnosticLocation::createBegin(SW, BR.getSourceManager(), ADC),
    SW->getSourceRange());
}
 
void GCDAntipatternChecker::checkASTCodeBody(const Decl *D,
                                             AnalysisManager &AM,
                                             BugReporter &BR) const {
  if (isTest(D))
    return;
 
  AnalysisDeclContext *ADC = AM.getAnalysisDeclContext(D);
 
  auto SemaphoreMatcherM = findGCDAntiPatternWithSemaphore();
  auto Matches = match(SemaphoreMatcherM, *D->getBody(), AM.getASTContext());
  for (BoundNodes Match : Matches)
    emitDiagnostics(Match, "semaphore", BR, ADC, this);
 
  auto GroupMatcherM = findGCDAntiPatternWithGroup();
  Matches = match(GroupMatcherM, *D->getBody(), AM.getASTContext());
  for (BoundNodes Match : Matches)
    emitDiagnostics(Match, "group", BR, ADC, this);
}
 
} // end of anonymous namespace
 
void ento::registerGCDAntipattern(CheckerManager &Mgr) {
  Mgr.registerChecker<GCDAntipatternChecker>();
}
 
bool ento::shouldRegisterGCDAntipattern(const CheckerManager &mgr) {
  return true;
}