Inlining
========

  -analyzer-ipa=none - All inlining is disabled. This is the only mode available
     in LLVM 3.1 and earlier and in Xcode 4.3 and earlier.

  -analyzer-ipa=basic-inlining - Turns on inlining for C functions, C++ static
     member functions, and blocks -- essentially, the calls that behave like
     simple C function calls. This is essentially the mode used in Xcode 4.4.

  -analyzer-ipa=inlining - Turns on inlining when we can confidently find the
    function/method body corresponding to the call. (C functions, static
    functions, devirtualized C++ methods, Objective-C class methods, Objective-C
    instance methods when ExprEngine is confident about the dynamic type of the
    instance).

  -analyzer-ipa=dynamic - Inline instance methods for which the type is
   determined at runtime and we are not 100% sure that our type info is
   correct. For virtual calls, inline the most plausible definition.

  -analyzer-ipa=dynamic-bifurcate - Same as -analyzer-ipa=dynamic, but the path
   is split. We inline on one branch and do not inline on the other. This mode
   does not drop the coverage in cases when the parent class has code that is
   only exercised when some of its methods are overriden.

Currently, -analyzer-ipa=basic-inlining is the default mode.

Basics of Implementation
-----------------------

The low-level mechanism of inlining a function is handled in
ExprEngine::inlineCall and ExprEngine::processCallExit.

If the conditions are right for inlining, a CallEnter node is created and added
to the analysis work list. The CallEnter node marks the change to a new
LocationContext representing the called function, and its state includes the
contents of the new stack frame. When the CallEnter node is actually processed,
its single successor will be a edge to the first CFG block in the function.

Exiting an inlined function is a bit more work, fortunately broken up into
reasonable steps:

1. The CoreEngine realizes we're at the end of an inlined call and generates a
   CallExitBegin node.

2. ExprEngine takes over (in processCallExit) and finds the return value of the
   function, if it has one. This is bound to the expression that triggered the
   call. (In the case of calls without origin expressions, such as destructors,
   this step is skipped.)

3. Dead symbols and bindings are cleaned out from the state, including any local
   bindings.

4. A CallExitEnd node is generated, which marks the transition back to the
   caller's LocationContext.

5. Custom post-call checks are processed and the final nodes are pushed back
   onto the work list, so that evaluation of the caller can continue.

Retry Without Inlining
----------------------

In some cases, we would like to retry analyzes without inlining the particular
call.

Currently, we use this technique to recover the coverage in case we stop
analyzing a path due to exceeding the maximum block count inside an inlined
function.

When this situation is detected, we walk up the path to find the first node
before inlining was started and enqueue it on the WorkList with a special
ReplayWithoutInlining bit added to it (ExprEngine::replayWithoutInlining).  The
path is then re-analyzed from that point without inlining that particular call.

Deciding When to Inline
-----------------------

In general, the analyzer attempts to inline as much as possible, since it
provides a better summary of what actually happens in the program.  There are
some cases, however, where the analyzer chooses not to inline:

- If there is no definition available for the called function or method.  In
  this case, there is no opportunity to inline.

- If we the CFG cannot be constructed for a called function, or the liveness
  cannot be computed.  These are prerequisites for analyzing a function body,
  with or without inlining.

- If the LocationContext chain for a given ExplodedNode reaches a maximum cutoff
  depth.  This prevents unbounded analysis due to infinite recursion, but also
  serves as a useful cutoff for performance reasons.

- If the function is variadic.  This is not a hard limitation, but an engineering
  limitation.

  Tracked by: <rdar://problem/12147064> Support inlining of variadic functions

- In C++, ExprEngine does not inline constructors unless the destructor is
  guaranteed to be inlined as well.

  **TMK/COMMENT** This needs to be a bit more precise.  How do we know the
                  destructor is guaranteed to be inlined?

- In C++, ExprEngine does not inline custom implementations of operator 'new'
  implementations).  This is due to a lack of complete handling of destructors.

- Calls resulting in "dynamic dispatch" are specially handled.  See more below.

- Engine::FunctionSummaries map stores additional information about
  declarations, some of which is collected at runtime based on previous analyzes
  of the function. We do not inline functions which were not profitable to
  inline in a different context (for example, if the maximum block count was
  exceeded, see Retry Without Inlining).


Dynamic Calls and Devirtualization
----------------------------------

"Dynamic" calls are those that are resolved at runtime, such as C++ virtual
method calls and Objective-C message sends. Due to the path-sensitive nature of
the analyzer, the analyzer may be able to reason about the dynamic type of the
object whose method is being called and thus "devirtualize" the call. 

This path-sensitive devirtualization occurs when the analyzer can determine what
method would actually be called at runtime.  This is possible when the type
information is constrained enough for a simulated C++/Objective-C object in
order to make such a decision.

 == RuntimeDefinition ==

The basis of this devirtualization is CallEvent's getRuntimeDefinition() method,
which returns a RuntimeDefinition object.  The "runtime" + "defintion"
corresponds to the definition of the called method as would be computed at
runtime.  In the case of no dynamic dispatch, this object resolves to a Decl*
for the called function.  In the case of dynamic dispatch, the RuntimeDefinition
object also includes an optional MemRegion* corresponding to the object being
called (i.e., the "receiver" in Objective-C parlance).  This information is
later consulted by ExprEngine (along with tracked dynamic type information) to
potentially resolve the called method.

 == DynamicTypeInfo ==

In addition to RuntimeDefinition, the analyzer needs to track the potential
runtime type of a simulated C++/Objective-C object.  As the analyzer analyzes a
path, it may accrue more information to refine the knowledge about the type of
an object.  This can then be used to make better decisions about the target
method of a call.

Such type information is tracked as DynamicTypeInfo.  This is path-sensitive
data that is stored in ProgramState, which defines a mapping from MemRegions to
an (optional) DynamicTypeInfo.

If no DynamicTypeInfo has been explicitly set for a MemRegion, it will be lazily
inferred from the region's type or associated symbol. Information from symbolic
regions is weaker than from true typed regions.

  EXAMPLE: A C++ object declared "A obj" is known to have the class 'A', but a
           reference "A &ref" may dynamically be a subclass of 'A'.

The DynamicTypePropagation checker gathers and propagates DynamicTypeInfo,
updating it as information is observed along a path that can refine that type
information for a region.

  WARNING: Not all of the existing analyzer code has been retrofitted to use
           DynamicTypeInfo, nor is it universally appropriate. In particular,
           DynamicTypeInfo always applies to a region with all casts stripped
           off, but sometimes the information provided by casts can be useful.)


When asked to provide a definition, the CallEvents for dynamic calls will use
the DynamicTypeInfo in their ProgramState to provide the best definition of the
method to be called. In some cases this devirtualization can be perfect or
near-perfect, and the analyzer can inline the definition as usual. In other
cases ExprEngine can make a guess, but report that our guess may not be the
method actually called at runtime.

  **TMK/COMMENT**: what does it mean to "report" that our guess may not be the
                   method actually called?

The -analyzer-ipa option has four different modes: none, inlining, dynamic, and
dynamic-bifurcate. Under -analyzer-ipa=dynamic, all dynamic calls are inlined,
whether we are certain or not that this will actually be the definition used at
runtime. Under -analyzer-ipa=inlining, only "near-perfect" devirtualized calls
are inlined*, and other dynamic calls are evaluated conservatively (as if no
definition were available).

* Currently, no Objective-C messages are not inlined under
  -analyzer-ipa=inlining, even if we are reasonably confident of the type of the
  receiver. We plan to enable this once we have tested our heuristics more
  thoroughly.

The last option, -analyzer-ipa=dynamic-bifurcate, behaves similarly to
"dynamic", but performs a conservative invalidation in the general virtual case
in *addition* to inlining. The details of this are discussed below.

Bifurcation
-----------

ExprEngine::BifurcateCall implements the -analyzer-ipa=dynamic-bifurcate
mode.

When a call is made on a region with imprecise dynamic type information 
(RuntimeDefinition::mayHaveOtherDefinitions() evaluates to TRUE), ExprEngine
bifurcates the path and marks the MemRegion (derived from a RuntimeDefinition
object) with a path-sensitive "mode" in the ProgramState.

Currently, there are 2 modes: 

 DynamicDispatchModeInlined - Models the case where the dynamic type information
   of the receiver (MemoryRegion) is assumed to be perfectly constrained so 
   that a given definition of a method is expected to be the code actually 
   called. When this mode is set, ExprEngine uses the Decl from 
   RuntimeDefinition to inline any dynamically dispatched call sent to this 
   receiver because the function definition is considered to be fully resolved.

 DynamicDispatchModeConservative - Models the case where the dynamic type
   information is assumed to be incorrect, for example, implies that the method 
   definition is overriden in a subclass. In such cases, ExprEngine does not 
   inline the methods sent to the receiver (MemoryRegion), even if a candidate 
   definition is available. This mode is conservative about simulating the 
   effects of a call.

Going forward along the symbolic execution path, ExprEngine consults the mode 
of the receiver's MemRegion to make decisions on whether the calls should be 
inlined or not, which ensures that there is at most one split per region.

At a high level, "bifurcation mode" allows for increased semantic coverage in
cases where the parent method contains code which is only executed when the
class is subclassed. The disadvantages of this mode are a (considerable?)
performance hit and the possibility of false positives on the path where the
conservative mode is used.

Objective-C Message Heuristics
------------------------------

ExprEngine relies on a set of heuristics to partition the set of Objective-C 
method calls into those that require bifurcation and those that do not. Below 
are the cases when the DynamicTypeInfo of the object is considered precise
(cannot be a subclass):

 - If the object was created with +alloc or +new and initialized with an -init
   method.

 - If the calls are property accesses using dot syntax. This is based on the
   assumption that children rarely override properties, or do so in an
   essentially compatible way.

 - If the class interface is declared inside the main source file. In this case
   it is unlikely that it will be subclassed.

 - If the method is not declared outside of main source file, either by the
   receiver's class or by any superclasses.

C++ Inlining Caveats
--------------------

C++11 [class.cdtor]p4 describes how the vtable of an object is modified as it is
being constructed or destructed; that is, the type of the object depends on
which base constructors have been completed. This is tracked using
DynamicTypeInfo in the DynamicTypePropagation checker.

There are several limitations in the current implementation:

- Temporaries are poorly modelled right now because we're not confident in the
  placement

- 'new' is poorly modelled due to some nasty CFG/design issues.  This is tracked
  in PR12014.  'delete' is not modelled at all.

- Arrays of objects are modeled very poorly right now.  ExprEngine currently
  only simualtes the first constructor and first destructor. Because of this,
  ExprEngine does not inline any constructors or destructors for arrays.

CallEvent
---------

A CallEvent represents a specific call to a function, method, or other body of
code. It is path-sensitive, containing both the current state (ProgramStateRef)
and stack space (LocationContext), and provides uniform access to the argument
values and return type of a call, no matter how the call is written in the
source or what sort of code body is being invoked.

  NOTE: For those familiar with Cocoa, CallEvent is roughly equivalent to
        NSInvocation.

CallEvent should be used whenever there is logic dealing with function calls
that does not care how the call occurred.

Examples include checking that arguments satisfy preconditions (such as
__attribute__((nonnull))), and attempting to inline a call.

CallEvents are reference-counted objects managed by a CallEventManager. While
there is no inherent issue with persisting them (say, in a ProgramState's GDM),
they are intended for short-lived use, and can be recreated from CFGElements or
StackFrameContexts fairly easily.
