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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).startswith("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_lower("test") || CN.contains_lower("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;
}
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