1==================================== 2LLVM's Analysis and Transform Passes 3==================================== 4 5.. contents:: 6 :local: 7 8.. toctree:: 9 :hidden: 10 11 KernelInfo 12 13Introduction 14============ 15.. warning:: This document is not updated frequently, and the list of passes 16 is most likely incomplete. It is possible to list passes known by the opt 17 tool using ``opt -print-passes``. 18 19This document serves as a high level summary of the optimization features that 20LLVM provides. Optimizations are implemented as Passes that traverse some 21portion of a program to either collect information or transform the program. 22The table below divides the passes that LLVM provides into three categories. 23Analysis passes compute information that other passes can use or for debugging 24or program visualization purposes. Transform passes can use (or invalidate) 25the analysis passes. Transform passes all mutate the program in some way. 26Utility passes provides some utility but don't otherwise fit categorization. 27For example passes to extract functions to bitcode or write a module to bitcode 28are neither analysis nor transform passes. The table of contents above 29provides a quick summary of each pass and links to the more complete pass 30description later in the document. 31 32Analysis Passes 33=============== 34 35This section describes the LLVM Analysis Passes. 36 37``aa-eval``: Exhaustive Alias Analysis Precision Evaluator 38---------------------------------------------------------- 39 40This is a simple N^2 alias analysis accuracy evaluator. Basically, for each 41function in the program, it simply queries to see how the alias analysis 42implementation answers alias queries between each pair of pointers in the 43function. 44 45This is inspired and adapted from code by: Naveen Neelakantam, Francesco 46Spadini, and Wojciech Stryjewski. 47 48``basic-aa``: Basic Alias Analysis (stateless AA impl) 49------------------------------------------------------ 50 51A basic alias analysis pass that implements identities (two different globals 52cannot alias, etc), but does no stateful analysis. 53 54``basiccg``: Basic CallGraph Construction 55----------------------------------------- 56 57Yet to be written. 58 59.. _passes-da: 60 61``da``: Dependence Analysis 62--------------------------- 63 64Dependence analysis framework, which is used to detect dependences in memory 65accesses. 66 67``domfrontier``: Dominance Frontier Construction 68------------------------------------------------ 69 70This pass is a simple dominator construction algorithm for finding forward 71dominator frontiers. 72 73``domtree``: Dominator Tree Construction 74---------------------------------------- 75 76This pass is a simple dominator construction algorithm for finding forward 77dominators. 78 79 80``dot-callgraph``: Print Call Graph to "dot" file 81------------------------------------------------- 82 83This pass, only available in ``opt``, prints the call graph into a ``.dot`` 84graph. This graph can then be processed with the "dot" tool to convert it to 85postscript or some other suitable format. 86 87``dot-cfg``: Print CFG of function to "dot" file 88------------------------------------------------ 89 90This pass, only available in ``opt``, prints the control flow graph into a 91``.dot`` graph. This graph can then be processed with the :program:`dot` tool 92to convert it to postscript or some other suitable format. 93Additionally the ``-cfg-func-name=<substring>`` option can be used to filter the 94functions that are printed. All functions that contain the specified substring 95will be printed. 96 97``dot-cfg-only``: Print CFG of function to "dot" file (with no function bodies) 98------------------------------------------------------------------------------- 99 100This pass, only available in ``opt``, prints the control flow graph into a 101``.dot`` graph, omitting the function bodies. This graph can then be processed 102with the :program:`dot` tool to convert it to postscript or some other suitable 103format. 104Additionally the ``-cfg-func-name=<substring>`` option can be used to filter the 105functions that are printed. All functions that contain the specified substring 106will be printed. 107 108``dot-dom``: Print dominance tree of function to "dot" file 109----------------------------------------------------------- 110 111This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` 112graph. This graph can then be processed with the :program:`dot` tool to 113convert it to postscript or some other suitable format. 114 115``dot-dom-only``: Print dominance tree of function to "dot" file (with no function bodies) 116------------------------------------------------------------------------------------------ 117 118This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` 119graph, omitting the function bodies. This graph can then be processed with the 120:program:`dot` tool to convert it to postscript or some other suitable format. 121 122``dot-post-dom``: Print postdominance tree of function to "dot" file 123-------------------------------------------------------------------- 124 125This pass, only available in ``opt``, prints the post dominator tree into a 126``.dot`` graph. This graph can then be processed with the :program:`dot` tool 127to convert it to postscript or some other suitable format. 128 129``dot-post-dom-only``: Print postdominance tree of function to "dot" file (with no function bodies) 130--------------------------------------------------------------------------------------------------- 131 132This pass, only available in ``opt``, prints the post dominator tree into a 133``.dot`` graph, omitting the function bodies. This graph can then be processed 134with the :program:`dot` tool to convert it to postscript or some other suitable 135format. 136 137``globals-aa``: Simple mod/ref analysis for globals 138--------------------------------------------------- 139 140This simple pass provides alias and mod/ref information for global values that 141do not have their address taken, and keeps track of whether functions read or 142write memory (are "pure"). For this simple (but very common) case, we can 143provide pretty accurate and useful information. 144 145``instcount``: Counts the various types of ``Instruction``\ s 146------------------------------------------------------------- 147 148This pass collects the count of all instructions and reports them. 149 150``iv-users``: Induction Variable Users 151-------------------------------------- 152 153Bookkeeping for "interesting" users of expressions computed from induction 154variables. 155 156``kernel-info``: GPU Kernel Info 157-------------------------------- 158 159Reports various statistics for codes compiled for GPUs. This pass is 160:doc:`documented separately<KernelInfo>`. 161 162``lazy-value-info``: Lazy Value Information Analysis 163---------------------------------------------------- 164 165Interface for lazy computation of value constraint information. 166 167``lint``: Statically lint-checks LLVM IR 168---------------------------------------- 169 170This pass statically checks for common and easily-identified constructs which 171produce undefined or likely unintended behavior in LLVM IR. 172 173It is not a guarantee of correctness, in two ways. First, it isn't 174comprehensive. There are checks which could be done statically which are not 175yet implemented. Some of these are indicated by TODO comments, but those 176aren't comprehensive either. Second, many conditions cannot be checked 177statically. This pass does no dynamic instrumentation, so it can't check for 178all possible problems. 179 180Another limitation is that it assumes all code will be executed. A store 181through a null pointer in a basic block which is never reached is harmless, but 182this pass will warn about it anyway. 183 184Optimization passes may make conditions that this pass checks for more or less 185obvious. If an optimization pass appears to be introducing a warning, it may 186be that the optimization pass is merely exposing an existing condition in the 187code. 188 189This code may be run before :ref:`instcombine <passes-instcombine>`. In many 190cases, instcombine checks for the same kinds of things and turns instructions 191with undefined behavior into unreachable (or equivalent). Because of this, 192this pass makes some effort to look through bitcasts and so on. 193 194``loops``: Natural Loop Information 195----------------------------------- 196 197This analysis is used to identify natural loops and determine the loop depth of 198various nodes of the CFG. Note that the loops identified may actually be 199several natural loops that share the same header node... not just a single 200natural loop. 201 202``memdep``: Memory Dependence Analysis 203-------------------------------------- 204 205An analysis that determines, for a given memory operation, what preceding 206memory operations it depends on. It builds on alias analysis information, and 207tries to provide a lazy, caching interface to a common kind of alias 208information query. 209 210``print<module-debuginfo>``: Decodes module-level debug info 211------------------------------------------------------------ 212 213This pass decodes the debug info metadata in a module and prints it to standard output in a 214(sufficiently-prepared-) human-readable form. 215 216``postdomtree``: Post-Dominator Tree Construction 217------------------------------------------------- 218 219This pass is a simple post-dominator construction algorithm for finding 220post-dominators. 221 222``print-alias-sets``: Alias Set Printer 223--------------------------------------- 224 225Yet to be written. 226 227``print-callgraph``: Print a call graph 228--------------------------------------- 229 230This pass, only available in ``opt``, prints the call graph to standard error 231in a human-readable form. 232 233``print-callgraph-sccs``: Print SCCs of the Call Graph 234------------------------------------------------------ 235 236This pass, only available in ``opt``, prints the SCCs of the call graph to 237standard error in a human-readable form. 238 239``print-cfg-sccs``: Print SCCs of each function CFG 240--------------------------------------------------- 241 242This pass, only available in ``opt``, prints the SCCs of each function CFG to 243standard error in a human-readable fom. 244 245``function(print)``: Print function to stderr 246--------------------------------------------- 247 248The ``PrintFunctionPass`` class is designed to be pipelined with other 249``FunctionPasses``, and prints out the functions of the module as they are 250processed. 251 252``module(print)``: Print module to stderr 253----------------------------------------- 254 255This pass simply prints out the entire module when it is executed. 256 257``regions``: Detect single entry single exit regions 258---------------------------------------------------- 259 260The ``RegionInfo`` pass detects single entry single exit regions in a function, 261where a region is defined as any subgraph that is connected to the remaining 262graph at only two spots. Furthermore, a hierarchical region tree is built. 263 264.. _passes-scalar-evolution: 265 266``scalar-evolution``: Scalar Evolution Analysis 267----------------------------------------------- 268 269The ``ScalarEvolution`` analysis can be used to analyze and categorize scalar 270expressions in loops. It specializes in recognizing general induction 271variables, representing them with the abstract and opaque ``SCEV`` class. 272Given this analysis, trip counts of loops and other important properties can be 273obtained. 274 275This analysis is primarily useful for induction variable substitution and 276strength reduction. 277 278``scev-aa``: ScalarEvolution-based Alias Analysis 279------------------------------------------------- 280 281Simple alias analysis implemented in terms of ``ScalarEvolution`` queries. 282 283This differs from traditional loop dependence analysis in that it tests for 284dependencies within a single iteration of a loop, rather than dependencies 285between different iterations. 286 287``ScalarEvolution`` has a more complete understanding of pointer arithmetic 288than ``BasicAliasAnalysis``' collection of ad-hoc analyses. 289 290``stack-safety``: Stack Safety Analysis 291--------------------------------------- 292 293The ``StackSafety`` analysis can be used to determine if stack allocated 294variables can be considered safe from memory access bugs. 295 296This analysis' primary purpose is to be used by sanitizers to avoid unnecessary 297instrumentation of safe variables. 298 299Transform Passes 300================ 301 302This section describes the LLVM Transform Passes. 303 304``adce``: Aggressive Dead Code Elimination 305------------------------------------------ 306 307ADCE aggressively tries to eliminate code. This pass is similar to :ref:`DCE 308<passes-dce>` but it assumes that values are dead until proven otherwise. This 309is similar to :ref:`SCCP <passes-sccp>`, except applied to the liveness of 310values. 311 312``always-inline``: Inliner for ``always_inline`` functions 313---------------------------------------------------------- 314 315A custom inliner that handles only functions that are marked as "always 316inline". 317 318``argpromotion``: Promote 'by reference' arguments to scalars 319------------------------------------------------------------- 320 321This pass promotes "by reference" arguments to be "by value" arguments. In 322practice, this means looking for internal functions that have pointer 323arguments. If it can prove, through the use of alias analysis, that an 324argument is *only* loaded, then it can pass the value into the function instead 325of the address of the value. This can cause recursive simplification of code 326and lead to the elimination of allocas (especially in C++ template code like 327the STL). 328 329This pass also handles aggregate arguments that are passed into a function, 330scalarizing them if the elements of the aggregate are only loaded. Note that 331it refuses to scalarize aggregates which would require passing in more than 332three operands to the function, because passing thousands of operands for a 333large array or structure is unprofitable! 334 335Note that this transformation could also be done for arguments that are only 336stored to (returning the value instead), but does not currently. This case 337would be best handled when and if LLVM starts supporting multiple return values 338from functions. 339 340``block-placement``: Profile Guided Basic Block Placement 341--------------------------------------------------------- 342 343This pass is a very simple profile guided basic block placement algorithm. The 344idea is to put frequently executed blocks together at the start of the function 345and hopefully increase the number of fall-through conditional branches. If 346there is no profile information for a particular function, this pass basically 347orders blocks in depth-first order. 348 349``break-crit-edges``: Break critical edges in CFG 350------------------------------------------------- 351 352Break all of the critical edges in the CFG by inserting a dummy basic block. 353It may be "required" by passes that cannot deal with critical edges. This 354transformation obviously invalidates the CFG, but can update forward dominator 355(set, immediate dominators, tree, and frontier) information. 356 357``codegenprepare``: Optimize for code generation 358------------------------------------------------ 359 360This pass munges the code in the input function to better prepare it for 361SelectionDAG-based code generation. This works around limitations in its 362basic-block-at-a-time approach. It should eventually be removed. 363 364``constmerge``: Merge Duplicate Global Constants 365------------------------------------------------ 366 367Merges duplicate global constants together into a single constant that is 368shared. This is useful because some passes (i.e., TraceValues) insert a lot of 369string constants into the program, regardless of whether or not an existing 370string is available. 371 372.. _passes-dce: 373 374``dce``: Dead Code Elimination 375------------------------------ 376 377Dead code elimination is similar to dead instruction elimination, but it 378rechecks instructions that were used by removed instructions to see if they 379are newly dead. 380 381``deadargelim``: Dead Argument Elimination 382------------------------------------------ 383 384This pass deletes dead arguments from internal functions. Dead argument 385elimination removes arguments which are directly dead, as well as arguments 386only passed into function calls as dead arguments of other functions. This 387pass also deletes dead arguments in a similar way. 388 389This pass is often useful as a cleanup pass to run after aggressive 390interprocedural passes, which add possibly-dead arguments. 391 392``dse``: Dead Store Elimination 393------------------------------- 394 395A trivial dead store elimination that only considers basic-block local 396redundant stores. 397 398.. _passes-function-attrs: 399 400``function-attrs``: Deduce function attributes 401---------------------------------------------- 402 403A simple interprocedural pass which walks the call-graph, looking for functions 404which do not access or only read non-local memory, and marking them 405``readnone``/``readonly``. In addition, it marks function arguments (of 406pointer type) "``nocapture``" if a call to the function does not create any 407copies of the pointer value that outlive the call. This more or less means 408that the pointer is only dereferenced, and not returned from the function or 409stored in a global. This pass is implemented as a bottom-up traversal of the 410call-graph. 411 412``globaldce``: Dead Global Elimination 413-------------------------------------- 414 415This transform is designed to eliminate unreachable internal globals from the 416program. It uses an aggressive algorithm, searching out globals that are known 417to be alive. After it finds all of the globals which are needed, it deletes 418whatever is left over. This allows it to delete recursive chunks of the 419program which are unreachable. 420 421``globalopt``: Global Variable Optimizer 422---------------------------------------- 423 424This pass transforms simple global variables that never have their address 425taken. If obviously true, it marks read/write globals as constant, deletes 426variables only stored to, etc. 427 428``gvn``: Global Value Numbering 429------------------------------- 430 431This pass performs global value numbering to eliminate fully and partially 432redundant instructions. It also performs redundant load elimination. 433 434.. _passes-indvars: 435 436``indvars``: Canonicalize Induction Variables 437--------------------------------------------- 438 439This transformation analyzes and transforms the induction variables (and 440computations derived from them) into simpler forms suitable for subsequent 441analysis and transformation. 442 443This transformation makes the following changes to each loop with an 444identifiable induction variable: 445 446* All loops are transformed to have a *single* canonical induction variable 447 which starts at zero and steps by one. 448* The canonical induction variable is guaranteed to be the first PHI node in 449 the loop header block. 450* Any pointer arithmetic recurrences are raised to use array subscripts. 451 452If the trip count of a loop is computable, this pass also makes the following 453changes: 454 455* The exit condition for the loop is canonicalized to compare the induction 456 value against the exit value. This turns loops like: 457 458 .. code-block:: c++ 459 460 for (i = 7; i*i < 1000; ++i) 461 462 into 463 464 .. code-block:: c++ 465 466 for (i = 0; i != 25; ++i) 467 468* Any use outside of the loop of an expression derived from the indvar is 469 changed to compute the derived value outside of the loop, eliminating the 470 dependence on the exit value of the induction variable. If the only purpose 471 of the loop is to compute the exit value of some derived expression, this 472 transformation will make the loop dead. 473 474This transformation should be followed by strength reduction after all of the 475desired loop transformations have been performed. Additionally, on targets 476where it is profitable, the loop could be transformed to count down to zero 477(the "do loop" optimization). 478 479``inline``: Function Integration/Inlining 480----------------------------------------- 481 482Bottom-up inlining of functions into callees. 483 484.. _passes-instcombine: 485 486``instcombine``: Combine redundant instructions 487----------------------------------------------- 488 489Combine instructions to form fewer, simple instructions. This pass does not 490modify the CFG. This pass is where algebraic simplification happens. 491 492This pass combines things like: 493 494.. code-block:: llvm 495 496 %Y = add i32 %X, 1 497 %Z = add i32 %Y, 1 498 499into: 500 501.. code-block:: llvm 502 503 %Z = add i32 %X, 2 504 505This is a simple worklist driven algorithm. 506 507This pass guarantees that the following canonicalizations are performed on the 508program: 509 510#. If a binary operator has a constant operand, it is moved to the right-hand 511 side. 512#. Bitwise operators with constant operands are always grouped so that shifts 513 are performed first, then ``or``\ s, then ``and``\ s, then ``xor``\ s. 514#. Compare instructions are converted from ``<``, ``>``, ``≤``, or ``≥`` to 515 ``=`` or ``≠`` if possible. 516#. All ``cmp`` instructions on boolean values are replaced with logical 517 operations. 518#. ``add X, X`` is represented as ``mul X, 2`` ⇒ ``shl X, 1`` 519#. Multiplies with a constant power-of-two argument are transformed into 520 shifts. 521#. … etc. 522 523This pass can also simplify calls to specific well-known function calls (e.g. 524runtime library functions). For example, a call ``exit(3)`` that occurs within 525the ``main()`` function can be transformed into simply ``return 3``. Whether or 526not library calls are simplified is controlled by the 527:ref:`-function-attrs <passes-function-attrs>` pass and LLVM's knowledge of 528library calls on different targets. 529 530.. _passes-aggressive-instcombine: 531 532``aggressive-instcombine``: Combine expression patterns 533-------------------------------------------------------- 534 535Combine expression patterns to form expressions with fewer, simple instructions. 536 537For example, this pass reduce width of expressions post-dominated by TruncInst 538into smaller width when applicable. 539 540It differs from instcombine pass in that it can modify CFG and contains pattern 541optimization that requires higher complexity than the O(1), thus, it should run fewer 542times than instcombine pass. 543 544``internalize``: Internalize Global Symbols 545------------------------------------------- 546 547This pass loops over all of the functions in the input module, looking for a 548main function. If a main function is found, all other functions and all global 549variables with initializers are marked as internal. 550 551``ipsccp``: Interprocedural Sparse Conditional Constant Propagation 552------------------------------------------------------------------- 553 554An interprocedural variant of :ref:`Sparse Conditional Constant Propagation 555<passes-sccp>`. 556 557``ir-normalizer``: Transforms IR into a normal form that's easier to diff 558---------------------------------------------------------------------------- 559 560This pass aims to transform LLVM Modules into a normal form by reordering and 561renaming instructions while preserving the same semantics. The normalizer makes 562it easier to spot semantic differences while diffing two modules which have 563undergone two different passes. 564 565``jump-threading``: Jump Threading 566---------------------------------- 567 568Jump threading tries to find distinct threads of control flow running through a 569basic block. This pass looks at blocks that have multiple predecessors and 570multiple successors. If one or more of the predecessors of the block can be 571proven to always cause a jump to one of the successors, we forward the edge 572from the predecessor to the successor by duplicating the contents of this 573block. 574 575An example of when this can occur is code like this: 576 577.. code-block:: c++ 578 579 if () { ... 580 X = 4; 581 } 582 if (X < 3) { 583 584In this case, the unconditional branch at the end of the first if can be 585revectored to the false side of the second if. 586 587.. _passes-lcssa: 588 589``lcssa``: Loop-Closed SSA Form Pass 590------------------------------------ 591 592This pass transforms loops by placing phi nodes at the end of the loops for all 593values that are live across the loop boundary. For example, it turns the left 594into the right code: 595 596.. code-block:: c++ 597 598 for (...) for (...) 599 if (c) if (c) 600 X1 = ... X1 = ... 601 else else 602 X2 = ... X2 = ... 603 X3 = phi(X1, X2) X3 = phi(X1, X2) 604 ... = X3 + 4 X4 = phi(X3) 605 ... = X4 + 4 606 607This is still valid LLVM; the extra phi nodes are purely redundant, and will be 608trivially eliminated by ``InstCombine``. The major benefit of this 609transformation is that it makes many other loop optimizations, such as 610``LoopUnswitch``\ ing, simpler. You can read more in the 611:ref:`loop terminology section for the LCSSA form <loop-terminology-lcssa>`. 612 613.. _passes-licm: 614 615``licm``: Loop Invariant Code Motion 616------------------------------------ 617 618This pass performs loop invariant code motion, attempting to remove as much 619code from the body of a loop as possible. It does this by either hoisting code 620into the preheader block, or by sinking code to the exit blocks if it is safe. 621This pass also promotes must-aliased memory locations in the loop to live in 622registers, thus hoisting and sinking "invariant" loads and stores. 623 624Hoisting operations out of loops is a canonicalization transform. It enables 625and simplifies subsequent optimizations in the middle-end. Rematerialization 626of hoisted instructions to reduce register pressure is the responsibility of 627the back-end, which has more accurate information about register pressure and 628also handles other optimizations than LICM that increase live-ranges. 629 630This pass uses alias analysis for two purposes: 631 632#. Moving loop invariant loads and calls out of loops. If we can determine 633 that a load or call inside of a loop never aliases anything stored to, we 634 can hoist it or sink it like any other instruction. 635 636#. Scalar Promotion of Memory. If there is a store instruction inside of the 637 loop, we try to move the store to happen AFTER the loop instead of inside of 638 the loop. This can only happen if a few conditions are true: 639 640 #. The pointer stored through is loop invariant. 641 #. There are no stores or loads in the loop which *may* alias the pointer. 642 There are no calls in the loop which mod/ref the pointer. 643 644 If these conditions are true, we can promote the loads and stores in the 645 loop of the pointer to use a temporary alloca'd variable. We then use the 646 :ref:`mem2reg <passes-mem2reg>` functionality to construct the appropriate 647 SSA form for the variable. 648 649``loop-deletion``: Delete dead loops 650------------------------------------ 651 652This file implements the Dead Loop Deletion Pass. This pass is responsible for 653eliminating loops with non-infinite computable trip counts that have no side 654effects or volatile instructions, and do not contribute to the computation of 655the function's return value. 656 657.. _passes-loop-extract: 658 659``loop-extract``: Extract loops into new functions 660-------------------------------------------------- 661 662A pass wrapper around the ``ExtractLoop()`` scalar transformation to extract 663each top-level loop into its own new function. If the loop is the *only* loop 664in a given function, it is not touched. This is a pass most useful for 665debugging via bugpoint. 666 667``loop-reduce``: Loop Strength Reduction 668---------------------------------------- 669 670This pass performs a strength reduction on array references inside loops that 671have as one or more of their components the loop induction variable. This is 672accomplished by creating a new value to hold the initial value of the array 673access for the first iteration, and then creating a new GEP instruction in the 674loop to increment the value by the appropriate amount. 675 676.. _passes-loop-rotate: 677 678``loop-rotate``: Rotate Loops 679----------------------------- 680 681A simple loop rotation transformation. A summary of it can be found in 682:ref:`Loop Terminology for Rotated Loops <loop-terminology-loop-rotate>`. 683 684 685.. _passes-loop-simplify: 686 687``loop-simplify``: Canonicalize natural loops 688--------------------------------------------- 689 690This pass performs several transformations to transform natural loops into a 691simpler form, which makes subsequent analyses and transformations simpler and 692more effective. A summary of it can be found in 693:ref:`Loop Terminology, Loop Simplify Form <loop-terminology-loop-simplify>`. 694 695Loop pre-header insertion guarantees that there is a single, non-critical entry 696edge from outside of the loop to the loop header. This simplifies a number of 697analyses and transformations, such as :ref:`LICM <passes-licm>`. 698 699Loop exit-block insertion guarantees that all exit blocks from the loop (blocks 700which are outside of the loop that have predecessors inside of the loop) only 701have predecessors from inside of the loop (and are thus dominated by the loop 702header). This simplifies transformations such as store-sinking that are built 703into LICM. 704 705This pass also guarantees that loops will have exactly one backedge. 706 707Note that the :ref:`simplifycfg <passes-simplifycfg>` pass will clean up blocks 708which are split out but end up being unnecessary, so usage of this pass should 709not pessimize generated code. 710 711This pass obviously modifies the CFG, but updates loop information and 712dominator information. 713 714``loop-unroll``: Unroll loops 715----------------------------- 716 717This pass implements a simple loop unroller. It works best when loops have 718been canonicalized by the :ref:`indvars <passes-indvars>` pass, allowing it to 719determine the trip counts of loops easily. 720 721``loop-unroll-and-jam``: Unroll and Jam loops 722--------------------------------------------- 723 724This pass implements a simple unroll and jam classical loop optimisation pass. 725It transforms loop from: 726 727.. code-block:: c++ 728 729 for i.. i+= 1 for i.. i+= 4 730 for j.. for j.. 731 code(i, j) code(i, j) 732 code(i+1, j) 733 code(i+2, j) 734 code(i+3, j) 735 remainder loop 736 737Which can be seen as unrolling the outer loop and "jamming" (fusing) the inner 738loops into one. When variables or loads can be shared in the new inner loop, this 739can lead to significant performance improvements. It uses 740:ref:`Dependence Analysis <passes-da>` for proving the transformations are safe. 741 742``lower-global-dtors``: Lower global destructors 743------------------------------------------------ 744 745This pass lowers global module destructors (``llvm.global_dtors``) by creating 746wrapper functions that are registered as global constructors in 747``llvm.global_ctors`` and which contain a call to ``__cxa_atexit`` to register 748their destructor functions. 749 750``lower-atomic``: Lower atomic intrinsics to non-atomic form 751------------------------------------------------------------ 752 753This pass lowers atomic intrinsics to non-atomic form for use in a known 754non-preemptible environment. 755 756The pass does not verify that the environment is non-preemptible (in general 757this would require knowledge of the entire call graph of the program including 758any libraries which may not be available in bitcode form); it simply lowers 759every atomic intrinsic. 760 761``lower-invoke``: Lower invokes to calls, for unwindless code generators 762------------------------------------------------------------------------ 763 764This transformation is designed for use by code generators which do not yet 765support stack unwinding. This pass converts ``invoke`` instructions to 766``call`` instructions, so that any exception-handling ``landingpad`` blocks 767become dead code (which can be removed by running the ``-simplifycfg`` pass 768afterwards). 769 770``lower-switch``: Lower ``SwitchInst``\ s to branches 771----------------------------------------------------- 772 773Rewrites switch instructions with a sequence of branches, which allows targets 774to get away with not implementing the switch instruction until it is 775convenient. 776 777.. _passes-mem2reg: 778 779``mem2reg``: Promote Memory to Register 780--------------------------------------- 781 782This file promotes memory references to be register references. It promotes 783alloca instructions which only have loads and stores as uses. An ``alloca`` is 784transformed by using dominator frontiers to place phi nodes, then traversing 785the function in depth-first order to rewrite loads and stores as appropriate. 786This is just the standard SSA construction algorithm to construct "pruned" SSA 787form. 788 789``memcpyopt``: MemCpy Optimization 790---------------------------------- 791 792This pass performs various transformations related to eliminating ``memcpy`` 793calls, or transforming sets of stores into ``memset``\ s. 794 795``mergefunc``: Merge Functions 796------------------------------ 797 798This pass looks for equivalent functions that are mergeable and folds them. 799 800Total-ordering is introduced among the functions set: we define comparison 801that answers for every two functions which of them is greater. It allows to 802arrange functions into the binary tree. 803 804For every new function we check for equivalent in tree. 805 806If equivalent exists we fold such functions. If both functions are overridable, 807we move the functionality into a new internal function and leave two 808overridable thunks to it. 809 810If there is no equivalent, then we add this function to tree. 811 812Lookup routine has O(log(n)) complexity, while whole merging process has 813complexity of O(n*log(n)). 814 815Read 816:doc:`this <MergeFunctions>` 817article for more details. 818 819``mergereturn``: Unify function exit nodes 820------------------------------------------ 821 822Ensure that functions have at most one ``ret`` instruction in them. 823Additionally, it keeps track of which node is the new exit node of the CFG. 824 825``partial-inliner``: Partial Inliner 826------------------------------------ 827 828This pass performs partial inlining, typically by inlining an ``if`` statement 829that surrounds the body of the function. 830 831``reassociate``: Reassociate expressions 832---------------------------------------- 833 834This pass reassociates commutative expressions in an order that is designed to 835promote better constant propagation, GCSE, :ref:`LICM <passes-licm>`, PRE, etc. 836 837For example: 4 + (x + 5) ⇒ x + (4 + 5) 838 839In the implementation of this algorithm, constants are assigned rank = 0, 840function arguments are rank = 1, and other values are assigned ranks 841corresponding to the reverse post order traversal of current function (starting 842at 2), which effectively gives values in deep loops higher rank than values not 843in loops. 844 845``rel-lookup-table-converter``: Relative lookup table converter 846--------------------------------------------------------------- 847 848This pass converts lookup tables to PIC-friendly relative lookup tables. 849 850``reg2mem``: Demote all values to stack slots 851--------------------------------------------- 852 853This file demotes all registers to memory references. It is intended to be the 854inverse of :ref:`mem2reg <passes-mem2reg>`. By converting to ``load`` 855instructions, the only values live across basic blocks are ``alloca`` 856instructions and ``load`` instructions before ``phi`` nodes. It is intended 857that this should make CFG hacking much easier. To make later hacking easier, 858the entry block is split into two, such that all introduced ``alloca`` 859instructions (and nothing else) are in the entry block. 860 861``sroa``: Scalar Replacement of Aggregates 862------------------------------------------ 863 864The well-known scalar replacement of aggregates transformation. This transform 865breaks up ``alloca`` instructions of aggregate type (structure or array) into 866individual ``alloca`` instructions for each member if possible. Then, if 867possible, it transforms the individual ``alloca`` instructions into nice clean 868scalar SSA form. 869 870.. _passes-sccp: 871 872``sccp``: Sparse Conditional Constant Propagation 873------------------------------------------------- 874 875Sparse conditional constant propagation and merging, which can be summarized 876as: 877 878* Assumes values are constant unless proven otherwise 879* Assumes BasicBlocks are dead unless proven otherwise 880* Proves values to be constant, and replaces them with constants 881* Proves conditional branches to be unconditional 882 883Note that this pass has a habit of making definitions be dead. It is a good 884idea to run a :ref:`DCE <passes-dce>` pass sometime after running this pass. 885 886.. _passes-simplifycfg: 887 888``simplifycfg``: Simplify the CFG 889--------------------------------- 890 891Performs dead code elimination and basic block merging. Specifically: 892 893* Removes basic blocks with no predecessors. 894* Merges a basic block into its predecessor if there is only one and the 895 predecessor only has one successor. 896* Eliminates PHI nodes for basic blocks with a single predecessor. 897* Eliminates a basic block that only contains an unconditional branch. 898 899``sink``: Code sinking 900---------------------- 901 902This pass moves instructions into successor blocks, when possible, so that they 903aren't executed on paths where their results aren't needed. 904 905.. _passes-simple-loop-unswitch: 906 907``simple-loop-unswitch``: Unswitch loops 908---------------------------------------- 909 910This pass transforms loops that contain branches on loop-invariant conditions 911to have multiple loops. For example, it turns the left into the right code: 912 913.. code-block:: c++ 914 915 for (...) if (lic) 916 A for (...) 917 if (lic) A; B; C 918 B else 919 C for (...) 920 A; C 921 922This can increase the size of the code exponentially (doubling it every time a 923loop is unswitched) so we only unswitch if the resultant code will be smaller 924than a threshold. 925 926This pass expects :ref:`LICM <passes-licm>` to be run before it to hoist 927invariant conditions out of the loop, to make the unswitching opportunity 928obvious. 929 930``strip``: Strip all symbols from a module 931------------------------------------------ 932 933Performs code stripping. This transformation can delete: 934 935* names for virtual registers 936* symbols for internal globals and functions 937* debug information 938 939Note that this transformation makes code much less readable, so it should only 940be used in situations where the strip utility would be used, such as reducing 941code size or making it harder to reverse engineer code. 942 943``strip-dead-debug-info``: Strip debug info for unused symbols 944-------------------------------------------------------------- 945 946Performs code stripping. Similar to strip, but only strips debug info for 947unused symbols. 948 949``strip-dead-prototypes``: Strip Unused Function Prototypes 950----------------------------------------------------------- 951 952This pass loops over all of the functions in the input module, looking for dead 953declarations and removes them. Dead declarations are declarations of functions 954for which no implementation is available (i.e., declarations for unused library 955functions). 956 957``strip-debug-declare``: Strip all ``llvm.dbg.declare`` intrinsics and 958``#dbg_declare`` records. 959------------------------------------------------------------------- 960 961Performs code stripping. Similar to strip, but only strips 962``llvm.dbg.declare`` intrinsics. 963 964``strip-nondebug``: Strip all symbols, except dbg symbols, from a module 965------------------------------------------------------------------------ 966 967Performs code stripping. Similar to strip, but dbg info is preserved. 968 969``tailcallelim``: Tail Call Elimination 970--------------------------------------- 971 972This file transforms calls of the current function (self recursion) followed by 973a return instruction with a branch to the entry of the function, creating a 974loop. This pass also implements the following extensions to the basic 975algorithm: 976 977#. Trivial instructions between the call and return do not prevent the 978 transformation from taking place, though currently the analysis cannot 979 support moving any really useful instructions (only dead ones). 980#. This pass transforms functions that are prevented from being tail recursive 981 by an associative expression to use an accumulator variable, thus compiling 982 the typical naive factorial or fib implementation into efficient code. 983#. TRE is performed if the function returns void, if the return returns the 984 result returned by the call, or if the function returns a run-time constant 985 on all exits from the function. It is possible, though unlikely, that the 986 return returns something else (like constant 0), and can still be TRE'd. It 987 can be TRE'd if *all other* return instructions in the function return the 988 exact same value. 989#. If it can prove that callees do not access their caller stack frame, they 990 are marked as eligible for tail call elimination (by the code generator). 991 992Utility Passes 993============== 994 995This section describes the LLVM Utility Passes. 996 997``deadarghaX0r``: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE) 998----------------------------------------------------------------------- 999 1000Same as dead argument elimination, but deletes arguments to functions which are 1001external. This is only for use by :doc:`bugpoint <Bugpoint>`. 1002 1003``extract-blocks``: Extract Basic Blocks From Module (for bugpoint use) 1004----------------------------------------------------------------------- 1005 1006This pass is used by bugpoint to extract all blocks from the module into their 1007own functions. 1008 1009``instnamer``: Assign names to anonymous instructions 1010----------------------------------------------------- 1011 1012This is a little utility pass that gives instructions names, this is mostly 1013useful when diffing the effect of an optimization because deleting an unnamed 1014instruction can change all other instruction numbering, making the diff very 1015noisy. 1016 1017.. _passes-verify: 1018 1019``verify``: Module Verifier 1020--------------------------- 1021 1022Verifies an LLVM IR code. This is useful to run after an optimization which is 1023undergoing testing. Note that llvm-as verifies its input before emitting 1024bitcode, and also that malformed bitcode is likely to make LLVM crash. All 1025language front-ends are therefore encouraged to verify their output before 1026performing optimizing transformations. 1027 1028#. Both of a binary operator's parameters are of the same type. 1029#. Verify that the indices of mem access instructions match other operands. 1030#. Verify that arithmetic and other things are only performed on first-class 1031 types. Verify that shifts and logicals only happen on integrals f.e. 1032#. All of the constants in a switch statement are of the correct type. 1033#. The code is in valid SSA form. 1034#. It is illegal to put a label into any other type (like a structure) or to 1035 return one. 1036#. Only phi nodes can be self referential: ``%x = add i32 %x``, ``%x`` is 1037 invalid. 1038#. PHI nodes must have an entry for each predecessor, with no extras. 1039#. PHI nodes must be the first thing in a basic block, all grouped together. 1040#. PHI nodes must have at least one entry. 1041#. All basic blocks should only end with terminator insts, not contain them. 1042#. The entry node to a function must not have predecessors. 1043#. All Instructions must be embedded into a basic block. 1044#. Functions cannot take a void-typed parameter. 1045#. Verify that a function's argument list agrees with its declared type. 1046#. It is illegal to specify a name for a void value. 1047#. It is illegal to have an internal global value with no initializer. 1048#. It is illegal to have a ``ret`` instruction that returns a value that does 1049 not agree with the function return value type. 1050#. Function call argument types match the function prototype. 1051#. All other things that are tested by asserts spread about the code. 1052 1053Note that this does not provide full security verification (like Java), but 1054instead just tries to ensure that code is well-formed. 1055 1056.. _passes-view-cfg: 1057 1058``view-cfg``: View CFG of function 1059---------------------------------- 1060 1061Displays the control flow graph using the GraphViz tool. 1062Additionally the ``-cfg-func-name=<substring>`` option can be used to filter the 1063functions that are displayed. All functions that contain the specified substring 1064will be displayed. 1065 1066``view-cfg-only``: View CFG of function (with no function bodies) 1067----------------------------------------------------------------- 1068 1069Displays the control flow graph using the GraphViz tool, but omitting function 1070bodies. 1071Additionally the ``-cfg-func-name=<substring>`` option can be used to filter the 1072functions that are displayed. All functions that contain the specified substring 1073will be displayed. 1074 1075``view-dom``: View dominance tree of function 1076--------------------------------------------- 1077 1078Displays the dominator tree using the GraphViz tool. 1079 1080``view-dom-only``: View dominance tree of function (with no function bodies) 1081---------------------------------------------------------------------------- 1082 1083Displays the dominator tree using the GraphViz tool, but omitting function 1084bodies. 1085 1086``view-post-dom``: View postdominance tree of function 1087------------------------------------------------------ 1088 1089Displays the post dominator tree using the GraphViz tool. 1090 1091``view-post-dom-only``: View postdominance tree of function (with no function bodies) 1092------------------------------------------------------------------------------------- 1093 1094Displays the post dominator tree using the GraphViz tool, but omitting function 1095bodies. 1096 1097``transform-warning``: Report missed forced transformations 1098----------------------------------------------------------- 1099 1100Emits warnings about not yet applied forced transformations (e.g. from 1101``#pragma omp simd``). 1102