1================================== 2LLVM Alias Analysis Infrastructure 3================================== 4 5.. contents:: 6 :local: 7 8Introduction 9============ 10 11Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt to 12determine whether or not two pointers ever can point to the same object in 13memory. There are many different algorithms for alias analysis and many 14different ways of classifying them: flow-sensitive vs. flow-insensitive, 15context-sensitive vs. context-insensitive, field-sensitive 16vs. field-insensitive, unification-based vs. subset-based, etc. Traditionally, 17alias analyses respond to a query with a `Must, May, or No`_ alias response, 18indicating that two pointers always point to the same object, might point to the 19same object, or are known to never point to the same object. 20 21The LLVM `AliasAnalysis 22<http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`__ class is the 23primary interface used by clients and implementations of alias analyses in the 24LLVM system. This class is the common interface between clients of alias 25analysis information and the implementations providing it, and is designed to 26support a wide range of implementations and clients (but currently all clients 27are assumed to be flow-insensitive). In addition to simple alias analysis 28information, this class exposes Mod/Ref information from those implementations 29which can provide it, allowing for powerful analyses and transformations to work 30well together. 31 32This document contains information necessary to successfully implement this 33interface, use it, and to test both sides. It also explains some of the finer 34points about what exactly results mean. If you feel that something is unclear 35or should be added, please `let me know <mailto:sabre@nondot.org>`_. 36 37``AliasAnalysis`` Class Overview 38================================ 39 40The `AliasAnalysis <http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`__ 41class defines the interface that the various alias analysis implementations 42should support. This class exports two important enums: ``AliasResult`` and 43``ModRefResult`` which represent the result of an alias query or a mod/ref 44query, respectively. 45 46The ``AliasAnalysis`` interface exposes information about memory, represented in 47several different ways. In particular, memory objects are represented as a 48starting address and size, and function calls are represented as the actual 49``call`` or ``invoke`` instructions that performs the call. The 50``AliasAnalysis`` interface also exposes some helper methods which allow you to 51get mod/ref information for arbitrary instructions. 52 53All ``AliasAnalysis`` interfaces require that in queries involving multiple 54values, values which are not :ref:`constants <constants>` are all 55defined within the same function. 56 57Representation of Pointers 58-------------------------- 59 60Most importantly, the ``AliasAnalysis`` class provides several methods which are 61used to query whether or not two memory objects alias, whether function calls 62can modify or read a memory object, etc. For all of these queries, memory 63objects are represented as a pair of their starting address (a symbolic LLVM 64``Value*``) and a static size. 65 66Representing memory objects as a starting address and a size is critically 67important for correct Alias Analyses. For example, consider this (silly, but 68possible) C code: 69 70.. code-block:: c++ 71 72 int i; 73 char C[2]; 74 char A[10]; 75 /* ... */ 76 for (i = 0; i != 10; ++i) { 77 C[0] = A[i]; /* One byte store */ 78 C[1] = A[9-i]; /* One byte store */ 79 } 80 81In this case, the ``basicaa`` pass will disambiguate the stores to ``C[0]`` and 82``C[1]`` because they are accesses to two distinct locations one byte apart, and 83the accesses are each one byte. In this case, the Loop Invariant Code Motion 84(LICM) pass can use store motion to remove the stores from the loop. In 85constrast, the following code: 86 87.. code-block:: c++ 88 89 int i; 90 char C[2]; 91 char A[10]; 92 /* ... */ 93 for (i = 0; i != 10; ++i) { 94 ((short*)C)[0] = A[i]; /* Two byte store! */ 95 C[1] = A[9-i]; /* One byte store */ 96 } 97 98In this case, the two stores to C do alias each other, because the access to the 99``&C[0]`` element is a two byte access. If size information wasn't available in 100the query, even the first case would have to conservatively assume that the 101accesses alias. 102 103.. _alias: 104 105The ``alias`` method 106-------------------- 107 108The ``alias`` method is the primary interface used to determine whether or not 109two memory objects alias each other. It takes two memory objects as input and 110returns MustAlias, PartialAlias, MayAlias, or NoAlias as appropriate. 111 112Like all ``AliasAnalysis`` interfaces, the ``alias`` method requires that either 113the two pointer values be defined within the same function, or at least one of 114the values is a :ref:`constant <constants>`. 115 116.. _Must, May, or No: 117 118Must, May, and No Alias Responses 119^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 120 121The ``NoAlias`` response may be used when there is never an immediate dependence 122between any memory reference *based* on one pointer and any memory reference 123*based* the other. The most obvious example is when the two pointers point to 124non-overlapping memory ranges. Another is when the two pointers are only ever 125used for reading memory. Another is when the memory is freed and reallocated 126between accesses through one pointer and accesses through the other --- in this 127case, there is a dependence, but it's mediated by the free and reallocation. 128 129As an exception to this is with the :ref:`noalias <noalias>` keyword; 130the "irrelevant" dependencies are ignored. 131 132The ``MayAlias`` response is used whenever the two pointers might refer to the 133same object. 134 135The ``PartialAlias`` response is used when the two memory objects are known to 136be overlapping in some way, but do not start at the same address. 137 138The ``MustAlias`` response may only be returned if the two memory objects are 139guaranteed to always start at exactly the same location. A ``MustAlias`` 140response implies that the pointers compare equal. 141 142The ``getModRefInfo`` methods 143----------------------------- 144 145The ``getModRefInfo`` methods return information about whether the execution of 146an instruction can read or modify a memory location. Mod/Ref information is 147always conservative: if an instruction **might** read or write a location, 148``ModRef`` is returned. 149 150The ``AliasAnalysis`` class also provides a ``getModRefInfo`` method for testing 151dependencies between function calls. This method takes two call sites (``CS1`` 152& ``CS2``), returns ``NoModRef`` if neither call writes to memory read or 153written by the other, ``Ref`` if ``CS1`` reads memory written by ``CS2``, 154``Mod`` if ``CS1`` writes to memory read or written by ``CS2``, or ``ModRef`` if 155``CS1`` might read or write memory written to by ``CS2``. Note that this 156relation is not commutative. 157 158Other useful ``AliasAnalysis`` methods 159-------------------------------------- 160 161Several other tidbits of information are often collected by various alias 162analysis implementations and can be put to good use by various clients. 163 164The ``pointsToConstantMemory`` method 165^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 166 167The ``pointsToConstantMemory`` method returns true if and only if the analysis 168can prove that the pointer only points to unchanging memory locations 169(functions, constant global variables, and the null pointer). This information 170can be used to refine mod/ref information: it is impossible for an unchanging 171memory location to be modified. 172 173.. _never access memory or only read memory: 174 175The ``doesNotAccessMemory`` and ``onlyReadsMemory`` methods 176^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 177 178These methods are used to provide very simple mod/ref information for function 179calls. The ``doesNotAccessMemory`` method returns true for a function if the 180analysis can prove that the function never reads or writes to memory, or if the 181function only reads from constant memory. Functions with this property are 182side-effect free and only depend on their input arguments, allowing them to be 183eliminated if they form common subexpressions or be hoisted out of loops. Many 184common functions behave this way (e.g., ``sin`` and ``cos``) but many others do 185not (e.g., ``acos``, which modifies the ``errno`` variable). 186 187The ``onlyReadsMemory`` method returns true for a function if analysis can prove 188that (at most) the function only reads from non-volatile memory. Functions with 189this property are side-effect free, only depending on their input arguments and 190the state of memory when they are called. This property allows calls to these 191functions to be eliminated and moved around, as long as there is no store 192instruction that changes the contents of memory. Note that all functions that 193satisfy the ``doesNotAccessMemory`` method also satisfies ``onlyReadsMemory``. 194 195Writing a new ``AliasAnalysis`` Implementation 196============================================== 197 198Writing a new alias analysis implementation for LLVM is quite straight-forward. 199There are already several implementations that you can use for examples, and the 200following information should help fill in any details. For a examples, take a 201look at the `various alias analysis implementations`_ included with LLVM. 202 203Different Pass styles 204--------------------- 205 206The first step to determining what type of :doc:`LLVM pass <WritingAnLLVMPass>` 207you need to use for your Alias Analysis. As is the case with most other 208analyses and transformations, the answer should be fairly obvious from what type 209of problem you are trying to solve: 210 211#. If you require interprocedural analysis, it should be a ``Pass``. 212#. If you are a function-local analysis, subclass ``FunctionPass``. 213#. If you don't need to look at the program at all, subclass ``ImmutablePass``. 214 215In addition to the pass that you subclass, you should also inherit from the 216``AliasAnalysis`` interface, of course, and use the ``RegisterAnalysisGroup`` 217template to register as an implementation of ``AliasAnalysis``. 218 219Required initialization calls 220----------------------------- 221 222Your subclass of ``AliasAnalysis`` is required to invoke two methods on the 223``AliasAnalysis`` base class: ``getAnalysisUsage`` and 224``InitializeAliasAnalysis``. In particular, your implementation of 225``getAnalysisUsage`` should explicitly call into the 226``AliasAnalysis::getAnalysisUsage`` method in addition to doing any declaring 227any pass dependencies your pass has. Thus you should have something like this: 228 229.. code-block:: c++ 230 231 void getAnalysisUsage(AnalysisUsage &AU) const { 232 AliasAnalysis::getAnalysisUsage(AU); 233 // declare your dependencies here. 234 } 235 236Additionally, your must invoke the ``InitializeAliasAnalysis`` method from your 237analysis run method (``run`` for a ``Pass``, ``runOnFunction`` for a 238``FunctionPass``, or ``InitializePass`` for an ``ImmutablePass``). For example 239(as part of a ``Pass``): 240 241.. code-block:: c++ 242 243 bool run(Module &M) { 244 InitializeAliasAnalysis(this); 245 // Perform analysis here... 246 return false; 247 } 248 249Required methods to override 250---------------------------- 251 252You must override the ``getAdjustedAnalysisPointer`` method on all subclasses 253of ``AliasAnalysis``. An example implementation of this method would look like: 254 255.. code-block:: c++ 256 257 void *getAdjustedAnalysisPointer(const void* ID) override { 258 if (ID == &AliasAnalysis::ID) 259 return (AliasAnalysis*)this; 260 return this; 261 } 262 263Interfaces which may be specified 264--------------------------------- 265 266All of the `AliasAnalysis 267<http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`__ virtual methods 268default to providing :ref:`chaining <aliasanalysis-chaining>` to another alias 269analysis implementation, which ends up returning conservatively correct 270information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries 271respectively). Depending on the capabilities of the analysis you are 272implementing, you just override the interfaces you can improve. 273 274.. _aliasanalysis-chaining: 275 276``AliasAnalysis`` chaining behavior 277----------------------------------- 278 279With only one special exception (the :ref:`-no-aa <aliasanalysis-no-aa>` pass) 280every alias analysis pass chains to another alias analysis implementation (for 281example, the user can specify "``-basicaa -ds-aa -licm``" to get the maximum 282benefit from both alias analyses). The alias analysis class automatically 283takes care of most of this for methods that you don't override. For methods 284that you do override, in code paths that return a conservative MayAlias or 285Mod/Ref result, simply return whatever the superclass computes. For example: 286 287.. code-block:: c++ 288 289 AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size, 290 const Value *V2, unsigned V2Size) { 291 if (...) 292 return NoAlias; 293 ... 294 295 // Couldn't determine a must or no-alias result. 296 return AliasAnalysis::alias(V1, V1Size, V2, V2Size); 297 } 298 299In addition to analysis queries, you must make sure to unconditionally pass LLVM 300`update notification`_ methods to the superclass as well if you override them, 301which allows all alias analyses in a change to be updated. 302 303.. _update notification: 304 305Updating analysis results for transformations 306--------------------------------------------- 307 308Alias analysis information is initially computed for a static snapshot of the 309program, but clients will use this information to make transformations to the 310code. All but the most trivial forms of alias analysis will need to have their 311analysis results updated to reflect the changes made by these transformations. 312 313The ``AliasAnalysis`` interface exposes four methods which are used to 314communicate program changes from the clients to the analysis implementations. 315Various alias analysis implementations should use these methods to ensure that 316their internal data structures are kept up-to-date as the program changes (for 317example, when an instruction is deleted), and clients of alias analysis must be 318sure to call these interfaces appropriately. 319 320The ``deleteValue`` method 321^^^^^^^^^^^^^^^^^^^^^^^^^^ 322 323The ``deleteValue`` method is called by transformations when they remove an 324instruction or any other value from the program (including values that do not 325use pointers). Typically alias analyses keep data structures that have entries 326for each value in the program. When this method is called, they should remove 327any entries for the specified value, if they exist. 328 329The ``copyValue`` method 330^^^^^^^^^^^^^^^^^^^^^^^^ 331 332The ``copyValue`` method is used when a new value is introduced into the 333program. There is no way to introduce a value into the program that did not 334exist before (this doesn't make sense for a safe compiler transformation), so 335this is the only way to introduce a new value. This method indicates that the 336new value has exactly the same properties as the value being copied. 337 338The ``replaceWithNewValue`` method 339^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 340 341This method is a simple helper method that is provided to make clients easier to 342use. It is implemented by copying the old analysis information to the new 343value, then deleting the old value. This method cannot be overridden by alias 344analysis implementations. 345 346The ``addEscapingUse`` method 347^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 348 349The ``addEscapingUse`` method is used when the uses of a pointer value have 350changed in ways that may invalidate precomputed analysis information. 351Implementations may either use this callback to provide conservative responses 352for points whose uses have change since analysis time, or may recompute some or 353all of their internal state to continue providing accurate responses. 354 355In general, any new use of a pointer value is considered an escaping use, and 356must be reported through this callback, *except* for the uses below: 357 358* A ``bitcast`` or ``getelementptr`` of the pointer 359* A ``store`` through the pointer (but not a ``store`` *of* the pointer) 360* A ``load`` through the pointer 361 362Efficiency Issues 363----------------- 364 365From the LLVM perspective, the only thing you need to do to provide an efficient 366alias analysis is to make sure that alias analysis **queries** are serviced 367quickly. The actual calculation of the alias analysis results (the "run" 368method) is only performed once, but many (perhaps duplicate) queries may be 369performed. Because of this, try to move as much computation to the run method 370as possible (within reason). 371 372Limitations 373----------- 374 375The AliasAnalysis infrastructure has several limitations which make writing a 376new ``AliasAnalysis`` implementation difficult. 377 378There is no way to override the default alias analysis. It would be very useful 379to be able to do something like "``opt -my-aa -O2``" and have it use ``-my-aa`` 380for all passes which need AliasAnalysis, but there is currently no support for 381that, short of changing the source code and recompiling. Similarly, there is 382also no way of setting a chain of analyses as the default. 383 384There is no way for transform passes to declare that they preserve 385``AliasAnalysis`` implementations. The ``AliasAnalysis`` interface includes 386``deleteValue`` and ``copyValue`` methods which are intended to allow a pass to 387keep an AliasAnalysis consistent, however there's no way for a pass to declare 388in its ``getAnalysisUsage`` that it does so. Some passes attempt to use 389``AU.addPreserved<AliasAnalysis>``, however this doesn't actually have any 390effect. 391 392``AliasAnalysisCounter`` (``-count-aa``) and ``AliasDebugger`` (``-debug-aa``) 393are implemented as ``ModulePass`` classes, so if your alias analysis uses 394``FunctionPass``, it won't be able to use these utilities. If you try to use 395them, the pass manager will silently route alias analysis queries directly to 396``BasicAliasAnalysis`` instead. 397 398Similarly, the ``opt -p`` option introduces ``ModulePass`` passes between each 399pass, which prevents the use of ``FunctionPass`` alias analysis passes. 400 401The ``AliasAnalysis`` API does have functions for notifying implementations when 402values are deleted or copied, however these aren't sufficient. There are many 403other ways that LLVM IR can be modified which could be relevant to 404``AliasAnalysis`` implementations which can not be expressed. 405 406The ``AliasAnalysisDebugger`` utility seems to suggest that ``AliasAnalysis`` 407implementations can expect that they will be informed of any relevant ``Value`` 408before it appears in an alias query. However, popular clients such as ``GVN`` 409don't support this, and are known to trigger errors when run with the 410``AliasAnalysisDebugger``. 411 412Due to several of the above limitations, the most obvious use for the 413``AliasAnalysisCounter`` utility, collecting stats on all alias queries in a 414compilation, doesn't work, even if the ``AliasAnalysis`` implementations don't 415use ``FunctionPass``. There's no way to set a default, much less a default 416sequence, and there's no way to preserve it. 417 418The ``AliasSetTracker`` class (which is used by ``LICM``) makes a 419non-deterministic number of alias queries. This can cause stats collected by 420``AliasAnalysisCounter`` to have fluctuations among identical runs, for 421example. Another consequence is that debugging techniques involving pausing 422execution after a predetermined number of queries can be unreliable. 423 424Many alias queries can be reformulated in terms of other alias queries. When 425multiple ``AliasAnalysis`` queries are chained together, it would make sense to 426start those queries from the beginning of the chain, with care taken to avoid 427infinite looping, however currently an implementation which wants to do this can 428only start such queries from itself. 429 430Using alias analysis results 431============================ 432 433There are several different ways to use alias analysis results. In order of 434preference, these are: 435 436Using the ``MemoryDependenceAnalysis`` Pass 437------------------------------------------- 438 439The ``memdep`` pass uses alias analysis to provide high-level dependence 440information about memory-using instructions. This will tell you which store 441feeds into a load, for example. It uses caching and other techniques to be 442efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations. 443 444.. _AliasSetTracker: 445 446Using the ``AliasSetTracker`` class 447----------------------------------- 448 449Many transformations need information about alias **sets** that are active in 450some scope, rather than information about pairwise aliasing. The 451`AliasSetTracker <http://llvm.org/doxygen/classllvm_1_1AliasSetTracker.html>`__ 452class is used to efficiently build these Alias Sets from the pairwise alias 453analysis information provided by the ``AliasAnalysis`` interface. 454 455First you initialize the AliasSetTracker by using the "``add``" methods to add 456information about various potentially aliasing instructions in the scope you are 457interested in. Once all of the alias sets are completed, your pass should 458simply iterate through the constructed alias sets, using the ``AliasSetTracker`` 459``begin()``/``end()`` methods. 460 461The ``AliasSet``\s formed by the ``AliasSetTracker`` are guaranteed to be 462disjoint, calculate mod/ref information and volatility for the set, and keep 463track of whether or not all of the pointers in the set are Must aliases. The 464AliasSetTracker also makes sure that sets are properly folded due to call 465instructions, and can provide a list of pointers in each set. 466 467As an example user of this, the `Loop Invariant Code Motion 468<doxygen/structLICM.html>`_ pass uses ``AliasSetTracker``\s to calculate alias 469sets for each loop nest. If an ``AliasSet`` in a loop is not modified, then all 470load instructions from that set may be hoisted out of the loop. If any alias 471sets are stored to **and** are must alias sets, then the stores may be sunk 472to outside of the loop, promoting the memory location to a register for the 473duration of the loop nest. Both of these transformations only apply if the 474pointer argument is loop-invariant. 475 476The AliasSetTracker implementation 477^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 478 479The AliasSetTracker class is implemented to be as efficient as possible. It 480uses the union-find algorithm to efficiently merge AliasSets when a pointer is 481inserted into the AliasSetTracker that aliases multiple sets. The primary data 482structure is a hash table mapping pointers to the AliasSet they are in. 483 484The AliasSetTracker class must maintain a list of all of the LLVM ``Value*``\s 485that are in each AliasSet. Since the hash table already has entries for each 486LLVM ``Value*`` of interest, the AliasesSets thread the linked list through 487these hash-table nodes to avoid having to allocate memory unnecessarily, and to 488make merging alias sets extremely efficient (the linked list merge is constant 489time). 490 491You shouldn't need to understand these details if you are just a client of the 492AliasSetTracker, but if you look at the code, hopefully this brief description 493will help make sense of why things are designed the way they are. 494 495Using the ``AliasAnalysis`` interface directly 496---------------------------------------------- 497 498If neither of these utility class are what your pass needs, you should use the 499interfaces exposed by the ``AliasAnalysis`` class directly. Try to use the 500higher-level methods when possible (e.g., use mod/ref information instead of the 501`alias`_ method directly if possible) to get the best precision and efficiency. 502 503Existing alias analysis implementations and clients 504=================================================== 505 506If you're going to be working with the LLVM alias analysis infrastructure, you 507should know what clients and implementations of alias analysis are available. 508In particular, if you are implementing an alias analysis, you should be aware of 509the `the clients`_ that are useful for monitoring and evaluating different 510implementations. 511 512.. _various alias analysis implementations: 513 514Available ``AliasAnalysis`` implementations 515------------------------------------------- 516 517This section lists the various implementations of the ``AliasAnalysis`` 518interface. With the exception of the :ref:`-no-aa <aliasanalysis-no-aa>` 519implementation, all of these :ref:`chain <aliasanalysis-chaining>` to other 520alias analysis implementations. 521 522.. _aliasanalysis-no-aa: 523 524The ``-no-aa`` pass 525^^^^^^^^^^^^^^^^^^^ 526 527The ``-no-aa`` pass is just like what it sounds: an alias analysis that never 528returns any useful information. This pass can be useful if you think that alias 529analysis is doing something wrong and are trying to narrow down a problem. 530 531The ``-basicaa`` pass 532^^^^^^^^^^^^^^^^^^^^^ 533 534The ``-basicaa`` pass is an aggressive local analysis that *knows* many 535important facts: 536 537* Distinct globals, stack allocations, and heap allocations can never alias. 538* Globals, stack allocations, and heap allocations never alias the null pointer. 539* Different fields of a structure do not alias. 540* Indexes into arrays with statically differing subscripts cannot alias. 541* Many common standard C library functions `never access memory or only read 542 memory`_. 543* Pointers that obviously point to constant globals "``pointToConstantMemory``". 544* Function calls can not modify or references stack allocations if they never 545 escape from the function that allocates them (a common case for automatic 546 arrays). 547 548The ``-globalsmodref-aa`` pass 549^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 550 551This pass implements a simple context-sensitive mod/ref and alias analysis for 552internal global variables that don't "have their address taken". If a global 553does not have its address taken, the pass knows that no pointers alias the 554global. This pass also keeps track of functions that it knows never access 555memory or never read memory. This allows certain optimizations (e.g. GVN) to 556eliminate call instructions entirely. 557 558The real power of this pass is that it provides context-sensitive mod/ref 559information for call instructions. This allows the optimizer to know that calls 560to a function do not clobber or read the value of the global, allowing loads and 561stores to be eliminated. 562 563.. note:: 564 565 This pass is somewhat limited in its scope (only support non-address taken 566 globals), but is very quick analysis. 567 568The ``-steens-aa`` pass 569^^^^^^^^^^^^^^^^^^^^^^^ 570 571The ``-steens-aa`` pass implements a variation on the well-known "Steensgaard's 572algorithm" for interprocedural alias analysis. Steensgaard's algorithm is a 573unification-based, flow-insensitive, context-insensitive, and field-insensitive 574alias analysis that is also very scalable (effectively linear time). 575 576The LLVM ``-steens-aa`` pass implements a "speculatively field-**sensitive**" 577version of Steensgaard's algorithm using the Data Structure Analysis framework. 578This gives it substantially more precision than the standard algorithm while 579maintaining excellent analysis scalability. 580 581.. note:: 582 583 ``-steens-aa`` is available in the optional "poolalloc" module. It is not part 584 of the LLVM core. 585 586The ``-ds-aa`` pass 587^^^^^^^^^^^^^^^^^^^ 588 589The ``-ds-aa`` pass implements the full Data Structure Analysis algorithm. Data 590Structure Analysis is a modular unification-based, flow-insensitive, 591context-**sensitive**, and speculatively field-**sensitive** alias 592analysis that is also quite scalable, usually at ``O(n * log(n))``. 593 594This algorithm is capable of responding to a full variety of alias analysis 595queries, and can provide context-sensitive mod/ref information as well. The 596only major facility not implemented so far is support for must-alias 597information. 598 599.. note:: 600 601 ``-ds-aa`` is available in the optional "poolalloc" module. It is not part of 602 the LLVM core. 603 604The ``-scev-aa`` pass 605^^^^^^^^^^^^^^^^^^^^^ 606 607The ``-scev-aa`` pass implements AliasAnalysis queries by translating them into 608ScalarEvolution queries. This gives it a more complete understanding of 609``getelementptr`` instructions and loop induction variables than other alias 610analyses have. 611 612Alias analysis driven transformations 613------------------------------------- 614 615LLVM includes several alias-analysis driven transformations which can be used 616with any of the implementations above. 617 618The ``-adce`` pass 619^^^^^^^^^^^^^^^^^^ 620 621The ``-adce`` pass, which implements Aggressive Dead Code Elimination uses the 622``AliasAnalysis`` interface to delete calls to functions that do not have 623side-effects and are not used. 624 625The ``-licm`` pass 626^^^^^^^^^^^^^^^^^^ 627 628The ``-licm`` pass implements various Loop Invariant Code Motion related 629transformations. It uses the ``AliasAnalysis`` interface for several different 630transformations: 631 632* It uses mod/ref information to hoist or sink load instructions out of loops if 633 there are no instructions in the loop that modifies the memory loaded. 634 635* It uses mod/ref information to hoist function calls out of loops that do not 636 write to memory and are loop-invariant. 637 638* If uses alias information to promote memory objects that are loaded and stored 639 to in loops to live in a register instead. It can do this if there are no may 640 aliases to the loaded/stored memory location. 641 642The ``-argpromotion`` pass 643^^^^^^^^^^^^^^^^^^^^^^^^^^ 644 645The ``-argpromotion`` pass promotes by-reference arguments to be passed in 646by-value instead. In particular, if pointer arguments are only loaded from it 647passes in the value loaded instead of the address to the function. This pass 648uses alias information to make sure that the value loaded from the argument 649pointer is not modified between the entry of the function and any load of the 650pointer. 651 652The ``-gvn``, ``-memcpyopt``, and ``-dse`` passes 653^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 654 655These passes use AliasAnalysis information to reason about loads and stores. 656 657.. _the clients: 658 659Clients for debugging and evaluation of implementations 660------------------------------------------------------- 661 662These passes are useful for evaluating the various alias analysis 663implementations. You can use them with commands like: 664 665.. code-block:: bash 666 667 % opt -ds-aa -aa-eval foo.bc -disable-output -stats 668 669The ``-print-alias-sets`` pass 670^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 671 672The ``-print-alias-sets`` pass is exposed as part of the ``opt`` tool to print 673out the Alias Sets formed by the `AliasSetTracker`_ class. This is useful if 674you're using the ``AliasSetTracker`` class. To use it, use something like: 675 676.. code-block:: bash 677 678 % opt -ds-aa -print-alias-sets -disable-output 679 680The ``-count-aa`` pass 681^^^^^^^^^^^^^^^^^^^^^^ 682 683The ``-count-aa`` pass is useful to see how many queries a particular pass is 684making and what responses are returned by the alias analysis. As an example: 685 686.. code-block:: bash 687 688 % opt -basicaa -count-aa -ds-aa -count-aa -licm 689 690will print out how many queries (and what responses are returned) by the 691``-licm`` pass (of the ``-ds-aa`` pass) and how many queries are made of the 692``-basicaa`` pass by the ``-ds-aa`` pass. This can be useful when debugging a 693transformation or an alias analysis implementation. 694 695The ``-aa-eval`` pass 696^^^^^^^^^^^^^^^^^^^^^ 697 698The ``-aa-eval`` pass simply iterates through all pairs of pointers in a 699function and asks an alias analysis whether or not the pointers alias. This 700gives an indication of the precision of the alias analysis. Statistics are 701printed indicating the percent of no/may/must aliases found (a more precise 702algorithm will have a lower number of may aliases). 703 704Memory Dependence Analysis 705========================== 706 707If you're just looking to be a client of alias analysis information, consider 708using the Memory Dependence Analysis interface instead. MemDep is a lazy, 709caching layer on top of alias analysis that is able to answer the question of 710what preceding memory operations a given instruction depends on, either at an 711intra- or inter-block level. Because of its laziness and caching policy, using 712MemDep can be a significant performance win over accessing alias analysis 713directly. 714