[cfe-commits] r119288 - /cfe/trunk/lib/Checker/README.txt

Mon Nov 15 15:19:25 PST 2010

Author: wan
Date: Mon Nov 15 17:19:25 2010
New Revision: 119288

URL: http://llvm.org/viewvc/llvm-project?rev=119288&view=rev
Log:
Adds a README for the Clang static analyzer to document its design and work flow.  The contents are taken from email notes by kremenek and xuzhongxing.

Added:
    cfe/trunk/lib/Checker/README.txt

Added: cfe/trunk/lib/Checker/README.txt
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Checker/README.txt?rev=119288&view=auto
==============================================================================

--- cfe/trunk/lib/Checker/README.txt (added)
+++ cfe/trunk/lib/Checker/README.txt Mon Nov 15 17:19:25 2010
@@ -0,0 +1,117 @@
+//===----------------------------------------------------------------------===//
+// Clang Static Analyzer
+//===----------------------------------------------------------------------===//
+
+= Library Structure =
+
+The analyzer library has two layers: a (low-level) static analysis
+engine (GRExprEngine.cpp and friends), and some static checkers
+(*Checker.cpp).  The latter are built on top of the former via the
+Checker and CheckerVisitor interfaces (Checker.h and
+CheckerVisitor.h).  The Checker interface is designed to be minimal
+and simple for checker writers, and attempts to isolate them from much
+of the gore of the internal analysis engine.
+
+= How It Works =
+
+The analyzer is inspired by several foundational research papers ([1],
+[2]).  (FIXME: kremenek to add more links)
+
+In a nutshell, the analyzer is basically a source code simulator that
+traces out possible paths of execution.  The state of the program
+(values of variables and expressions) is encapsulated by the state
+(GRState).  A location in the program is called a program point
+(ProgramPoint), and the combination of state and program point is a
+node in an exploded graph (ExplodedGraph).  The term "exploded" comes
+from exploding the control-flow edges in the control-flow graph (CFG).
+
+Conceptually the analyzer does a reachability analysis through the
+ExplodedGraph.  We start at a root node, which has the entry program
+point and initial state, and then simulate transitions by analyzing
+individual expressions.  The analysis of an expression can cause the
+state to change, resulting in a new node in the ExplodedGraph with an
+updated program point and an updated state.  A bug is found by hitting
+a node that satisfies some "bug condition" (basically a violation of a
+checking invariant).
+
+The analyzer traces out multiple paths by reasoning about branches and
+then bifurcating the state: on the true branch the conditions of the
+branch are assumed to be true and on the false branch the conditions
+of the branch are assumed to be false.  Such "assumptions" create
+constraints on the values of the program, and those constraints are
+recorded in the GRState object (and are manipulated by the
+ConstraintManager).  If assuming the conditions of a branch would
+cause the constraints to be unsatisfiable, the branch is considered
+infeasible and that path is not taken.  This is how we get
+path-sensitivity.  We reduce exponential blow-up by caching nodes.  If
+a new node with the same state and program point as an existing node
+would get generated, the path "caches out" and we simply reuse the
+existing node.  Thus the ExplodedGraph is not a DAG; it can contain
+cycles as paths loop back onto each other and cache out.
+
+GRState and ExplodedNodes are basically immutable once created.  Once
+one creates a GRState, you need to create a new one to get a new
+GRState.  This immutability is key since the ExplodedGraph represents
+the behavior of the analyzed program from the entry point.  To
+represent these efficiently, we use functional data structures (e.g.,
+ImmutableMaps) which share data between instances.
+
+Finally, individual Checkers work by also manipulating the analysis
+state.  The analyzer engine talks to them via a visitor interface.
+For example, the PreVisitCallExpr() method is called by GRExprEngine
+to tell the Checker that we are about to analyze a CallExpr, and the
+checker is asked to check for any preconditions that might not be
+satisfied.  The checker can do nothing, or it can generate a new
+GRState and ExplodedNode which contains updated checker state.  If it
+finds a bug, it can tell the BugReporter object about the bug,
+providing it an ExplodedNode which is the last node in the path that
+triggered the problem.
+
+= Working on the Analyzer =
+
+If you are interested in bringing up support for C++ expressions, the
+best place to look is the visitation logic in GRExprEngine, which
+handles the simulation of individual expressions.  There are plenty of
+examples there of how other expressions are handled.
+
+If you are interested in writing checkers, look at the Checker and
+CheckerVisitor interfaces (Checker.h and CheckerVisitor.h).  Also look
+at the files named *Checker.cpp for examples on how you can implement
+these interfaces.
+
+= Debugging the Analyzer =
+
+There are some useful command-line options for debugging.  For example:
+
+$ clang -cc1 -help | grep analyze
+ -analyze-function <value>
+ -analyzer-display-progress
+ -analyzer-viz-egraph-graphviz
+ ...
+
+The first allows you to specify only analyzing a specific function.
+The second prints to the console what function is being analyzed.  The
+third generates a graphviz dot file of the ExplodedGraph.  This is
+extremely useful when debugging the analyzer and viewing the
+simulation results.
+
+Of course, viewing the CFG (Control-Flow Graph) is also useful:
+
+$ clang -cc1 -help | grep cfg
+ -cfg-add-implicit-dtors Add C++ implicit destructors to CFGs for all analyses
+ -cfg-add-initializers   Add C++ initializers to CFGs for all analyses
+ -cfg-dump               Display Control-Flow Graphs
+ -cfg-view               View Control-Flow Graphs using GraphViz
+ -unoptimized-cfg        Generate unoptimized CFGs for all analyses
+
+-cfg-dump dumps a textual representation of the CFG to the console,
+and -cfg-view creates a GraphViz representation.
+
+= References =
+
+[1] Precise interprocedural dataflow analysis via graph reachability,
+    T Reps, S Horwitz, and M Sagiv, POPL '95,
+    http://portal.acm.org/citation.cfm?id=199462
+
+[2] A memory model for static analysis of C programs, Z Xu, T
+    Kremenek, and J Zhang, http://lcs.ios.ac.cn/~xzx/memmodel.pdf



