forked from OSchip/llvm-project
387 lines
17 KiB
Plaintext
387 lines
17 KiB
Plaintext
Inlining
|
|
========
|
|
|
|
There are several options that control which calls the analyzer will consider for
|
|
inlining. The major one is -analyzer-config ipa:
|
|
|
|
-analyzer-config 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-config 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-config 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-config 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-config ipa=dynamic-bifurcate - Same as -analyzer-config 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 overridden.
|
|
|
|
Currently, -analyzer-config ipa=dynamic-bifurcate is the default mode.
|
|
|
|
While -analyzer-config ipa determines in general how aggressively the analyzer
|
|
will try to inline functions, several additional options control which types of
|
|
functions can inlined, in an all-or-nothing way. These options use the
|
|
analyzer's configuration table, so they are all specified as follows:
|
|
|
|
-analyzer-config OPTION=VALUE
|
|
|
|
### c++-inlining ###
|
|
|
|
This option controls which C++ member functions may be inlined.
|
|
|
|
-analyzer-config c++-inlining=[none | methods | constructors | destructors]
|
|
|
|
Each of these modes implies that all the previous member function kinds will be
|
|
inlined as well; it doesn't make sense to inline destructors without inlining
|
|
constructors, for example.
|
|
|
|
The default c++-inlining mode is 'destructors', meaning that all member
|
|
functions with visible definitions will be considered for inlining. In some
|
|
cases the analyzer may still choose not to inline the function.
|
|
|
|
Note that under 'constructors', constructors for types with non-trivial
|
|
destructors will not be inlined. Additionally, no C++ member functions will be
|
|
inlined under -analyzer-config ipa=none or -analyzer-config ipa=basic-inlining,
|
|
regardless of the setting of the c++-inlining mode.
|
|
|
|
### c++-template-inlining ###
|
|
|
|
This option controls whether C++ templated functions may be inlined.
|
|
|
|
-analyzer-config c++-template-inlining=[true | false]
|
|
|
|
Currently, template functions are considered for inlining by default.
|
|
|
|
The motivation behind this option is that very generic code can be a source
|
|
of false positives, either by considering paths that the caller considers
|
|
impossible (by some unstated precondition), or by inlining some but not all
|
|
of a deep implementation of a function.
|
|
|
|
### c++-stdlib-inlining ###
|
|
|
|
This option controls whether functions from the C++ standard library, including
|
|
methods of the container classes in the Standard Template Library, should be
|
|
considered for inlining.
|
|
|
|
-analyzer-config c++-stdlib-inlining=[true | false]
|
|
|
|
Currently, C++ standard library functions are considered for inlining by
|
|
default.
|
|
|
|
The standard library functions and the STL in particular are used ubiquitously
|
|
enough that our tolerance for false positives is even lower here. A false
|
|
positive due to poor modeling of the STL leads to a poor user experience, since
|
|
most users would not be comfortable adding assertions to system headers in order
|
|
to silence analyzer warnings.
|
|
|
|
### c++-container-inlining ###
|
|
|
|
This option controls whether constructors and destructors of "container" types
|
|
should be considered for inlining.
|
|
|
|
-analyzer-config c++-container-inlining=[true | false]
|
|
|
|
Currently, these constructors and destructors are NOT considered for inlining
|
|
by default.
|
|
|
|
The current implementation of this setting checks whether a type has a member
|
|
named 'iterator' or a member named 'begin'; these names are idiomatic in C++,
|
|
with the latter specified in the C++11 standard. The analyzer currently does a
|
|
fairly poor job of modeling certain data structure invariants of container-like
|
|
objects. For example, these three expressions should be equivalent:
|
|
|
|
std::distance(c.begin(), c.end()) == 0
|
|
c.begin() == c.end()
|
|
c.empty())
|
|
|
|
Many of these issues are avoided if containers always have unknown, symbolic
|
|
state, which is what happens when their constructors are treated as opaque.
|
|
In the future, we may decide specific containers are "safe" to model through
|
|
inlining, or choose to model them directly using checkers instead.
|
|
|
|
|
|
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 analysis without inlining a particular
|
|
call.
|
|
|
|
Currently, we use this technique to recover 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 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++, constructors are not inlined unless the destructor call will be
|
|
processed by the ExprEngine. Thus, if the CFG was built without nodes for
|
|
implicit destructors, or if the destructors for the given object are not
|
|
represented in the CFG, the constructor will not be inlined. (As an exception,
|
|
constructors for objects with trivial constructors can still be inlined.)
|
|
See "C++ Caveats" below.
|
|
|
|
- In C++, ExprEngine does not inline custom implementations of operator 'new'
|
|
or operator 'delete', nor does it inline the constructors and destructors
|
|
associated with these. See "C++ Caveats" below.
|
|
|
|
- Calls resulting in "dynamic dispatch" are specially handled. See more below.
|
|
|
|
- The FunctionSummaries map stores additional information about declarations,
|
|
some of which is collected at runtime based on previous analyses.
|
|
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 analysis, 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 that
|
|
the analyzer can make such a decision.
|
|
|
|
== DynamicTypeInfo ==
|
|
|
|
As the analyzer analyzes a path, it may accrue 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.
|
|
|
|
|
|
== RuntimeDefinition ==
|
|
|
|
The basis of devirtualization is CallEvent's getRuntimeDefinition() method,
|
|
which returns a RuntimeDefinition object. When asked to provide a definition,
|
|
the CallEvents for dynamic calls will use the DynamicTypeInfo in their
|
|
ProgramState to attempt to devirtualize the call. In the case of no dynamic
|
|
dispatch, or perfectly constrained devirtualization, the resulting
|
|
RuntimeDefinition contains a Decl corresponding to the definition of the called
|
|
function, and RuntimeDefinition::mayHaveOtherDefinitions will return FALSE.
|
|
|
|
In the case of dynamic dispatch where our information is not perfect, CallEvent
|
|
can make a guess, but RuntimeDefinition::mayHaveOtherDefinitions will return
|
|
TRUE. The RuntimeDefinition object will then also include a MemRegion
|
|
corresponding to the object being called (i.e., the "receiver" in Objective-C
|
|
parlance), which ExprEngine uses to decide whether or not the call should be
|
|
inlined.
|
|
|
|
== Inlining Dynamic Calls ==
|
|
|
|
The -analyzer-config ipa option has five different modes: none, basic-inlining,
|
|
inlining, dynamic, and dynamic-bifurcate. Under -analyzer-config 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-config 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-config 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-config 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.
|
|
|
|
As stated above, -analyzer-config ipa=basic-inlining does not inline any C++
|
|
member functions or Objective-C method calls, even if they are non-virtual or
|
|
can be safely devirtualized.
|
|
|
|
|
|
Bifurcation
|
|
-----------
|
|
|
|
ExprEngine::BifurcateCall implements the -analyzer-config ipa=dynamic-bifurcate
|
|
mode.
|
|
|
|
When a call is made on an object with imprecise dynamic type information
|
|
(RuntimeDefinition::mayHaveOtherDefinitions() evaluates to TRUE), ExprEngine
|
|
bifurcates the path and marks the object's region (retrieved from the
|
|
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++ 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 modeled right now because we're not confident in the
|
|
placement of their destructors in the CFG. We currently won't inline their
|
|
constructors unless the destructor is trivial, and don't process their
|
|
destructors at all, not even to invalidate the region.
|
|
|
|
- 'new' is poorly modeled due to some nasty CFG/design issues. This is tracked
|
|
in PR12014. 'delete' is not modeled at all.
|
|
|
|
- Arrays of objects are modeled very poorly right now. ExprEngine currently
|
|
only simulates 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
|
|
non-top-level StackFrameContexts fairly easily.
|