The -Wlifetime warnings implemented in clang

This project contains clang's experimental implementation of the lifetime safety profile of the C++ core guidelines.

You can try this implementation on Compiler Explorer by choosing the clang version experimental -Wlifetime.

Warning flags

• -Wdangling-gsl: Statement-local analysis (enabled by default)
• -Wreturn-stack-address: Statement-local analysis (enabled by default)
• -Wlifetime: Main flow-sensitive analysis
• -Wno-lifetime-null: Disable flow-sensitive warnings about nullness of Pointers
• -Wlifetime-global: Flow-sensitive analysis that requires global variables of Pointer type to always point to variables with static lifetime
• -Wlifetime-disabled: Get warnings when the flow-sensitive analysis is disabled on a function due to forbidden constructs (reinterpret_cast or pointer arithmetic)

Suppress Warnings

• The [[gsl::suppress("lifetime")]] attribute can be used to suppress lifetime warnings for functions. See clang/test/Sema/attr-lifetime-suppress.cpp for an example.

Build

You can either try it on Compiler Explorer, or build clang yourself:

mkdir build
cd build 
cmake ../llvm -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_ENABLE_PROJECTS=clang
make
# Run the tests (optional)
make check-clang 
# Use clang
./bin/clang++ <some-source> <warning-flags>

Further information

The specification

CppCon 2019 Talk, focusing on statement local analysis

EuroLLVM 2019 Talk, focusing on flow-sensitive analysis

CppCon 2018 Talk, focusing on flow-sensitive analysis

CppCon 2018 Keynote, by Herb Sutter

CppCon 2015 Keynote, by Herb Sutter, first introduction

Development

Tests:

• clang/test/Sema/warn-lifetime-analysis-nocfg.cpp Test statement-local analysis
• clang/test/Sema/warn-lifetime-analysis.cpp: Test all diagnostics emitted by -Wlifetime
• clang/test/Sema/attr-psets.cpp: The main tests of the flow-sensitive analysis. They verify how psets are propagated through all types of C++ expressions.
• clang/test/Sema/warn-lifetime-analysis-nonull.cpp: Test -Wno-lifetime-null
• clang/test/Sema/attr-psets-annotation.cpp: Test for the function attributes to give pre- and post-conditions to the psets of parameters and the return value
• clang/test/Sema/warn-lifetime-analysis-nocfg-disabled.cpp Test disabling the statement-local analysis (because it's enabled by default)
• lifetime-integration-tests/: Integration tests that verify the analysis against various standard libraries and versions

Implementation:

• Entry point into flow-sensitive analysis, checking the warning flag and calling runAnalysis: https://github.com/mgehre/llvm-project/blob/lifetime/clang/lib/Sema/AnalysisBasedWarnings.cpp#L2277
• Implementation of runAnalysis https://github.com/mgehre/llvm-project/blob/lifetime/clang/lib/Analysis/Lifetime.cpp#L294, delegating to LifetimeContext.
• Builds a CFG (Control flow graph) of the AST (Abstract syntax tree) nodes and then iterates over the basic blocks until the pset converge. Calls VisitBlock https://github.com/mgehre/llvm-project/blob/lifetime/clang/lib/Analysis/Lifetime.cpp#L253 to compute the psets of a BasicBlock.
• VisitBlock https://github.com/mgehre/llvm-project/blob/lifetime/clang/lib/Analysis/LifetimePsetBuilder.cpp#L1255 uses a PSetsBuilder to visit each (sub-)expression in basic block.
• This dispatches to the various Visit* member functions, e.g. https://github.com/mgehre/llvm-project/blob/lifetime/clang/lib/Analysis/LifetimePsetBuilder.cpp#L1719, to compute the pset of the current expression (if it has one - i.e. it's an lvalue or an prvalue of Pointer type) based on the psets of its subexpressions (which have been computed earlier).