DESCRIPTION ------------------------------------------------------------------- gcmole is a simple static analysis tool used to find possible evaluation order dependent GC-unsafe places in the V8 codebase. For example the following code is GC-unsafe: Handle Foo(); // Assume Foo can trigger a GC. void Bar(Object*, Object*); Handle baz; baz->Qux(*Foo()); // (a) Bar(*Foo(), *baz); // (b) Both in cases (a) and (b) compiler is free to evaluate call arguments (that includes receiver) in any order. That means it can dereference baz before calling to Foo and save a raw pointer to a heap object in the register or on the stack. PREREQUISITES ----------------------------------------------------------------- (1) Install Lua 5.1 $ sudo apt-get install lua5.1 (2) Get LLVM 8.0 and Clang 8.0 sources and build them. Follow the instructions on http://clang.llvm.org/get_started.html. Make sure to pass -DCMAKE_BUILD_TYPE=Release to cmake to get Release build instead of a Debug one. (3) Build gcmole Clang plugin (libgcmole.so) In the tools/gcmole directory execute the following command: $ BUILD_ROOT= LLVM_SRC_ROOT= CLANG_SRC_ROOT= make (*) Note that steps (2) and (3) can also be achieved by just using the included bootstrapping script in this directory: $ ./tools/gcmole/bootstrap.sh This will use "third_party/llvm+clang-build" as a build directory and checkout required sources in the "third_party" directory. USING GCMOLE ------------------------------------------------------------------ gcmole consists of driver script written in Lua and Clang plugin that does C++ AST processing. Plugin (libgcmole.so) is expected to be in the same folder as driver (gcmole.lua). To start analysis cd into the root of v8 checkout and execute the following command: CLANG_BIN= lua tools/gcmole/gcmole.lua [] where arch should be one of architectures supported by V8 (arm, ia32, x64). Analysis will be performed in 2 stages: - on the first stage driver will parse all files and build a global callgraph approximation to find all functions that might potentially cause GC, list of this functions will be written into gcsuspects file. - on the second stage driver will parse all files again and will locate all callsites that might be GC-unsafe based on the list of functions causing GC. Such places are marked with a "Possible problem with evaluation order." warning. Messages "Failed to resolve v8::internal::Object" are benign and can be ignored. If any errors were found driver exits with non-zero status. PACKAGING --------------------------------------------------------------------- gcmole is deployed on V8's buildbot infrastructure to run it as part of the continuous integration. A pre-built package of gcmole together with Clang is hosted on Google Cloud Storage for this purpose. To update this package to a newer version, use the provided packaging script: $ ./tools/gcmole/package.sh This will create a new "tools/gcmole/gcmole-tools.tar.gz" package with the corresponding SHA1 sum suitable to be used for this purpose. It assumes that Clang was built in "third_party/llvm+clang-build" (e.g. by the bootstrapping script "bootstrap.sh" mentioned above). TROUBLESHOOTING --------------------------------------------------------------- gcmole is tighly coupled with the AST structure that Clang produces. Therefore when upgrading to a newer Clang version, it might start producing bogus output or completely stop outputting warnings. In such occasion, one might start the debugging process by checking weather a new AST node type is introduced which is currently not supported by gcmole. Insert the following code at the end of the FunctionAnalyzer::VisitExpr method to see the unsupported AST class(es) and the source position which generates them: if (expr) { clang::Stmt::StmtClass stmtClass = expr->getStmtClass(); d_.Report(clang::FullSourceLoc(expr->getExprLoc(), sm_), d_.getCustomDiagID(clang::DiagnosticsEngine::Remark, "%0")) << stmtClass; } For instance, gcmole currently doesn't support AtomicExprClass statements introduced for atomic operations. A convenient way to observe the AST generated by Clang is to pass the following flags when invoking clang++ -Xclang -ast-dump -fsyntax-only