1.. 2 If Passes.html is up to date, the following "one-liner" should print 3 an empty diff. 4 5 egrep -e '^<tr><td><a href="#.*">-.*</a></td><td>.*</td></tr>$' \ 6 -e '^ <a name=".*">.*</a>$' < Passes.html >html; \ 7 perl >help <<'EOT' && diff -u help html; rm -f help html 8 open HTML, "<Passes.html" or die "open: Passes.html: $!\n"; 9 while (<HTML>) { 10 m:^<tr><td><a href="#(.*)">-.*</a></td><td>.*</td></tr>$: or next; 11 $order{$1} = sprintf("%03d", 1 + int %order); 12 } 13 open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n"; 14 while (<HELP>) { 15 m:^ -([^ ]+) +- (.*)$: or next; 16 my $o = $order{$1}; 17 $o = "000" unless defined $o; 18 push @x, "$o<tr><td><a href=\"#$1\">-$1</a></td><td>$2</td></tr>\n"; 19 push @y, "$o <a name=\"$1\">-$1: $2</a>\n"; 20 } 21 @x = map { s/^\d\d\d//; $_ } sort @x; 22 @y = map { s/^\d\d\d//; $_ } sort @y; 23 print @x, @y; 24 EOT 25 26 This (real) one-liner can also be helpful when converting comments to HTML: 27 28 perl -e '$/ = undef; for (split(/\n/, <>)) { s:^ *///? ?::; print " <p>\n" if !$on && $_ =~ /\S/; print " </p>\n" if $on && $_ =~ /^\s*$/; print " $_\n"; $on = ($_ =~ /\S/); } print " </p>\n" if $on' 29 30==================================== 31LLVM's Analysis and Transform Passes 32==================================== 33 34.. contents:: 35 :local: 36 37Introduction 38============ 39 40This document serves as a high level summary of the optimization features that 41LLVM provides. Optimizations are implemented as Passes that traverse some 42portion of a program to either collect information or transform the program. 43The table below divides the passes that LLVM provides into three categories. 44Analysis passes compute information that other passes can use or for debugging 45or program visualization purposes. Transform passes can use (or invalidate) 46the analysis passes. Transform passes all mutate the program in some way. 47Utility passes provides some utility but don't otherwise fit categorization. 48For example passes to extract functions to bitcode or write a module to bitcode 49are neither analysis nor transform passes. The table of contents above 50provides a quick summary of each pass and links to the more complete pass 51description later in the document. 52 53Analysis Passes 54=============== 55 56This section describes the LLVM Analysis Passes. 57 58``-aa-eval``: Exhaustive Alias Analysis Precision Evaluator 59----------------------------------------------------------- 60 61This is a simple N^2 alias analysis accuracy evaluator. Basically, for each 62function in the program, it simply queries to see how the alias analysis 63implementation answers alias queries between each pair of pointers in the 64function. 65 66This is inspired and adapted from code by: Naveen Neelakantam, Francesco 67Spadini, and Wojciech Stryjewski. 68 69``-basicaa``: Basic Alias Analysis (stateless AA impl) 70------------------------------------------------------ 71 72A basic alias analysis pass that implements identities (two different globals 73cannot alias, etc), but does no stateful analysis. 74 75``-basiccg``: Basic CallGraph Construction 76------------------------------------------ 77 78Yet to be written. 79 80``-count-aa``: Count Alias Analysis Query Responses 81--------------------------------------------------- 82 83A pass which can be used to count how many alias queries are being made and how 84the alias analysis implementation being used responds. 85 86``-da``: Dependence Analysis 87---------------------------- 88 89Dependence analysis framework, which is used to detect dependences in memory 90accesses. 91 92``-debug-aa``: AA use debugger 93------------------------------ 94 95This simple pass checks alias analysis users to ensure that if they create a 96new value, they do not query AA without informing it of the value. It acts as 97a shim over any other AA pass you want. 98 99Yes keeping track of every value in the program is expensive, but this is a 100debugging pass. 101 102``-domfrontier``: Dominance Frontier Construction 103------------------------------------------------- 104 105This pass is a simple dominator construction algorithm for finding forward 106dominator frontiers. 107 108``-domtree``: Dominator Tree Construction 109----------------------------------------- 110 111This pass is a simple dominator construction algorithm for finding forward 112dominators. 113 114 115``-dot-callgraph``: Print Call Graph to "dot" file 116-------------------------------------------------- 117 118This pass, only available in ``opt``, prints the call graph into a ``.dot`` 119graph. This graph can then be processed with the "dot" tool to convert it to 120postscript or some other suitable format. 121 122``-dot-cfg``: Print CFG of function to "dot" file 123------------------------------------------------- 124 125This pass, only available in ``opt``, prints the control flow graph 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-cfg-only``: Print CFG of function to "dot" file (with no function bodies) 130-------------------------------------------------------------------------------- 131 132This pass, only available in ``opt``, prints the control flow graph 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``-dot-dom``: Print dominance tree of function to "dot" file 138------------------------------------------------------------ 139 140This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` 141graph. This graph can then be processed with the :program:`dot` tool to 142convert it to postscript or some other suitable format. 143 144``-dot-dom-only``: Print dominance tree of function to "dot" file (with no function bodies) 145------------------------------------------------------------------------------------------- 146 147This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` 148graph, omitting the function bodies. This graph can then be processed with the 149:program:`dot` tool to convert it to postscript or some other suitable format. 150 151``-dot-postdom``: Print postdominance tree of function to "dot" file 152-------------------------------------------------------------------- 153 154This pass, only available in ``opt``, prints the post dominator tree into a 155``.dot`` graph. This graph can then be processed with the :program:`dot` tool 156to convert it to postscript or some other suitable format. 157 158``-dot-postdom-only``: Print postdominance tree of function to "dot" file (with no function bodies) 159--------------------------------------------------------------------------------------------------- 160 161This pass, only available in ``opt``, prints the post dominator tree into a 162``.dot`` graph, omitting the function bodies. This graph can then be processed 163with the :program:`dot` tool to convert it to postscript or some other suitable 164format. 165 166``-globalsmodref-aa``: Simple mod/ref analysis for globals 167---------------------------------------------------------- 168 169This simple pass provides alias and mod/ref information for global values that 170do not have their address taken, and keeps track of whether functions read or 171write memory (are "pure"). For this simple (but very common) case, we can 172provide pretty accurate and useful information. 173 174``-instcount``: Counts the various types of ``Instruction``\ s 175-------------------------------------------------------------- 176 177This pass collects the count of all instructions and reports them. 178 179``-intervals``: Interval Partition Construction 180----------------------------------------------- 181 182This analysis calculates and represents the interval partition of a function, 183or a preexisting interval partition. 184 185In this way, the interval partition may be used to reduce a flow graph down to 186its degenerate single node interval partition (unless it is irreducible). 187 188``-iv-users``: Induction Variable Users 189--------------------------------------- 190 191Bookkeeping for "interesting" users of expressions computed from induction 192variables. 193 194``-lazy-value-info``: Lazy Value Information Analysis 195----------------------------------------------------- 196 197Interface for lazy computation of value constraint information. 198 199``-libcall-aa``: LibCall Alias Analysis 200--------------------------------------- 201 202LibCall Alias Analysis. 203 204``-lint``: Statically lint-checks LLVM IR 205----------------------------------------- 206 207This pass statically checks for common and easily-identified constructs which 208produce undefined or likely unintended behavior in LLVM IR. 209 210It is not a guarantee of correctness, in two ways. First, it isn't 211comprehensive. There are checks which could be done statically which are not 212yet implemented. Some of these are indicated by TODO comments, but those 213aren't comprehensive either. Second, many conditions cannot be checked 214statically. This pass does no dynamic instrumentation, so it can't check for 215all possible problems. 216 217Another limitation is that it assumes all code will be executed. A store 218through a null pointer in a basic block which is never reached is harmless, but 219this pass will warn about it anyway. 220 221Optimization passes may make conditions that this pass checks for more or less 222obvious. If an optimization pass appears to be introducing a warning, it may 223be that the optimization pass is merely exposing an existing condition in the 224code. 225 226This code may be run before :ref:`instcombine <passes-instcombine>`. In many 227cases, instcombine checks for the same kinds of things and turns instructions 228with undefined behavior into unreachable (or equivalent). Because of this, 229this pass makes some effort to look through bitcasts and so on. 230 231``-loops``: Natural Loop Information 232------------------------------------ 233 234This analysis is used to identify natural loops and determine the loop depth of 235various nodes of the CFG. Note that the loops identified may actually be 236several natural loops that share the same header node... not just a single 237natural loop. 238 239``-memdep``: Memory Dependence Analysis 240--------------------------------------- 241 242An analysis that determines, for a given memory operation, what preceding 243memory operations it depends on. It builds on alias analysis information, and 244tries to provide a lazy, caching interface to a common kind of alias 245information query. 246 247``-module-debuginfo``: Decodes module-level debug info 248------------------------------------------------------ 249 250This pass decodes the debug info metadata in a module and prints in a 251(sufficiently-prepared-) human-readable form. 252 253For example, run this pass from ``opt`` along with the ``-analyze`` option, and 254it'll print to standard output. 255 256``-postdomfrontier``: Post-Dominance Frontier Construction 257---------------------------------------------------------- 258 259This pass is a simple post-dominator construction algorithm for finding 260post-dominator frontiers. 261 262``-postdomtree``: Post-Dominator Tree Construction 263-------------------------------------------------- 264 265This pass is a simple post-dominator construction algorithm for finding 266post-dominators. 267 268``-print-alias-sets``: Alias Set Printer 269---------------------------------------- 270 271Yet to be written. 272 273``-print-callgraph``: Print a call graph 274---------------------------------------- 275 276This pass, only available in ``opt``, prints the call graph to standard error 277in a human-readable form. 278 279``-print-callgraph-sccs``: Print SCCs of the Call Graph 280------------------------------------------------------- 281 282This pass, only available in ``opt``, prints the SCCs of the call graph to 283standard error in a human-readable form. 284 285``-print-cfg-sccs``: Print SCCs of each function CFG 286---------------------------------------------------- 287 288This pass, only available in ``opt``, printsthe SCCs of each function CFG to 289standard error in a human-readable fom. 290 291``-print-dom-info``: Dominator Info Printer 292------------------------------------------- 293 294Dominator Info Printer. 295 296``-print-externalfnconstants``: Print external fn callsites passed constants 297---------------------------------------------------------------------------- 298 299This pass, only available in ``opt``, prints out call sites to external 300functions that are called with constant arguments. This can be useful when 301looking for standard library functions we should constant fold or handle in 302alias analyses. 303 304``-print-function``: Print function to stderr 305--------------------------------------------- 306 307The ``PrintFunctionPass`` class is designed to be pipelined with other 308``FunctionPasses``, and prints out the functions of the module as they are 309processed. 310 311``-print-module``: Print module to stderr 312----------------------------------------- 313 314This pass simply prints out the entire module when it is executed. 315 316.. _passes-print-used-types: 317 318``-print-used-types``: Find Used Types 319-------------------------------------- 320 321This pass is used to seek out all of the types in use by the program. Note 322that this analysis explicitly does not include types only used by the symbol 323table. 324 325``-regions``: Detect single entry single exit regions 326----------------------------------------------------- 327 328The ``RegionInfo`` pass detects single entry single exit regions in a function, 329where a region is defined as any subgraph that is connected to the remaining 330graph at only two spots. Furthermore, an hierarchical region tree is built. 331 332``-scalar-evolution``: Scalar Evolution Analysis 333------------------------------------------------ 334 335The ``ScalarEvolution`` analysis can be used to analyze and catagorize scalar 336expressions in loops. It specializes in recognizing general induction 337variables, representing them with the abstract and opaque ``SCEV`` class. 338Given this analysis, trip counts of loops and other important properties can be 339obtained. 340 341This analysis is primarily useful for induction variable substitution and 342strength reduction. 343 344``-scev-aa``: ScalarEvolution-based Alias Analysis 345-------------------------------------------------- 346 347Simple alias analysis implemented in terms of ``ScalarEvolution`` queries. 348 349This differs from traditional loop dependence analysis in that it tests for 350dependencies within a single iteration of a loop, rather than dependencies 351between different iterations. 352 353``ScalarEvolution`` has a more complete understanding of pointer arithmetic 354than ``BasicAliasAnalysis``' collection of ad-hoc analyses. 355 356``-targetdata``: Target Data Layout 357----------------------------------- 358 359Provides other passes access to information on how the size and alignment 360required by the target ABI for various data types. 361 362Transform Passes 363================ 364 365This section describes the LLVM Transform Passes. 366 367``-adce``: Aggressive Dead Code Elimination 368------------------------------------------- 369 370ADCE aggressively tries to eliminate code. This pass is similar to :ref:`DCE 371<passes-dce>` but it assumes that values are dead until proven otherwise. This 372is similar to :ref:`SCCP <passes-sccp>`, except applied to the liveness of 373values. 374 375``-always-inline``: Inliner for ``always_inline`` functions 376----------------------------------------------------------- 377 378A custom inliner that handles only functions that are marked as "always 379inline". 380 381``-argpromotion``: Promote 'by reference' arguments to scalars 382-------------------------------------------------------------- 383 384This pass promotes "by reference" arguments to be "by value" arguments. In 385practice, this means looking for internal functions that have pointer 386arguments. If it can prove, through the use of alias analysis, that an 387argument is *only* loaded, then it can pass the value into the function instead 388of the address of the value. This can cause recursive simplification of code 389and lead to the elimination of allocas (especially in C++ template code like 390the STL). 391 392This pass also handles aggregate arguments that are passed into a function, 393scalarizing them if the elements of the aggregate are only loaded. Note that 394it refuses to scalarize aggregates which would require passing in more than 395three operands to the function, because passing thousands of operands for a 396large array or structure is unprofitable! 397 398Note that this transformation could also be done for arguments that are only 399stored to (returning the value instead), but does not currently. This case 400would be best handled when and if LLVM starts supporting multiple return values 401from functions. 402 403``-bb-vectorize``: Basic-Block Vectorization 404-------------------------------------------- 405 406This pass combines instructions inside basic blocks to form vector 407instructions. It iterates over each basic block, attempting to pair compatible 408instructions, repeating this process until no additional pairs are selected for 409vectorization. When the outputs of some pair of compatible instructions are 410used as inputs by some other pair of compatible instructions, those pairs are 411part of a potential vectorization chain. Instruction pairs are only fused into 412vector instructions when they are part of a chain longer than some threshold 413length. Moreover, the pass attempts to find the best possible chain for each 414pair of compatible instructions. These heuristics are intended to prevent 415vectorization in cases where it would not yield a performance increase of the 416resulting code. 417 418``-block-placement``: Profile Guided Basic Block Placement 419---------------------------------------------------------- 420 421This pass is a very simple profile guided basic block placement algorithm. The 422idea is to put frequently executed blocks together at the start of the function 423and hopefully increase the number of fall-through conditional branches. If 424there is no profile information for a particular function, this pass basically 425orders blocks in depth-first order. 426 427``-break-crit-edges``: Break critical edges in CFG 428-------------------------------------------------- 429 430Break all of the critical edges in the CFG by inserting a dummy basic block. 431It may be "required" by passes that cannot deal with critical edges. This 432transformation obviously invalidates the CFG, but can update forward dominator 433(set, immediate dominators, tree, and frontier) information. 434 435``-codegenprepare``: Optimize for code generation 436------------------------------------------------- 437 438This pass munges the code in the input function to better prepare it for 439SelectionDAG-based code generation. This works around limitations in its 440basic-block-at-a-time approach. It should eventually be removed. 441 442``-constmerge``: Merge Duplicate Global Constants 443------------------------------------------------- 444 445Merges duplicate global constants together into a single constant that is 446shared. This is useful because some passes (i.e., TraceValues) insert a lot of 447string constants into the program, regardless of whether or not an existing 448string is available. 449 450``-constprop``: Simple constant propagation 451------------------------------------------- 452 453This pass implements constant propagation and merging. It looks for 454instructions involving only constant operands and replaces them with a constant 455value instead of an instruction. For example: 456 457.. code-block:: llvm 458 459 add i32 1, 2 460 461becomes 462 463.. code-block:: llvm 464 465 i32 3 466 467NOTE: this pass has a habit of making definitions be dead. It is a good idea 468to run a :ref:`Dead Instruction Elimination <passes-die>` pass sometime after 469running this pass. 470 471.. _passes-dce: 472 473``-dce``: Dead Code Elimination 474------------------------------- 475 476Dead code elimination is similar to :ref:`dead instruction elimination 477<passes-die>`, but it rechecks instructions that were used by removed 478instructions to see if they are newly dead. 479 480``-deadargelim``: Dead Argument Elimination 481------------------------------------------- 482 483This pass deletes dead arguments from internal functions. Dead argument 484elimination removes arguments which are directly dead, as well as arguments 485only passed into function calls as dead arguments of other functions. This 486pass also deletes dead arguments in a similar way. 487 488This pass is often useful as a cleanup pass to run after aggressive 489interprocedural passes, which add possibly-dead arguments. 490 491``-deadtypeelim``: Dead Type Elimination 492---------------------------------------- 493 494This pass is used to cleanup the output of GCC. It eliminate names for types 495that are unused in the entire translation unit, using the :ref:`find used types 496<passes-print-used-types>` pass. 497 498.. _passes-die: 499 500``-die``: Dead Instruction Elimination 501-------------------------------------- 502 503Dead instruction elimination performs a single pass over the function, removing 504instructions that are obviously dead. 505 506``-dse``: Dead Store Elimination 507-------------------------------- 508 509A trivial dead store elimination that only considers basic-block local 510redundant stores. 511 512.. _passes-functionattrs: 513 514``-functionattrs``: Deduce function attributes 515---------------------------------------------- 516 517A simple interprocedural pass which walks the call-graph, looking for functions 518which do not access or only read non-local memory, and marking them 519``readnone``/``readonly``. In addition, it marks function arguments (of 520pointer type) "``nocapture``" if a call to the function does not create any 521copies of the pointer value that outlive the call. This more or less means 522that the pointer is only dereferenced, and not returned from the function or 523stored in a global. This pass is implemented as a bottom-up traversal of the 524call-graph. 525 526``-globaldce``: Dead Global Elimination 527--------------------------------------- 528 529This transform is designed to eliminate unreachable internal globals from the 530program. It uses an aggressive algorithm, searching out globals that are known 531to be alive. After it finds all of the globals which are needed, it deletes 532whatever is left over. This allows it to delete recursive chunks of the 533program which are unreachable. 534 535``-globalopt``: Global Variable Optimizer 536----------------------------------------- 537 538This pass transforms simple global variables that never have their address 539taken. If obviously true, it marks read/write globals as constant, deletes 540variables only stored to, etc. 541 542``-gvn``: Global Value Numbering 543-------------------------------- 544 545This pass performs global value numbering to eliminate fully and partially 546redundant instructions. It also performs redundant load elimination. 547 548.. _passes-indvars: 549 550``-indvars``: Canonicalize Induction Variables 551---------------------------------------------- 552 553This transformation analyzes and transforms the induction variables (and 554computations derived from them) into simpler forms suitable for subsequent 555analysis and transformation. 556 557This transformation makes the following changes to each loop with an 558identifiable induction variable: 559 560* All loops are transformed to have a *single* canonical induction variable 561 which starts at zero and steps by one. 562* The canonical induction variable is guaranteed to be the first PHI node in 563 the loop header block. 564* Any pointer arithmetic recurrences are raised to use array subscripts. 565 566If the trip count of a loop is computable, this pass also makes the following 567changes: 568 569* The exit condition for the loop is canonicalized to compare the induction 570 value against the exit value. This turns loops like: 571 572 .. code-block:: c++ 573 574 for (i = 7; i*i < 1000; ++i) 575 576 into 577 578 .. code-block:: c++ 579 580 for (i = 0; i != 25; ++i) 581 582* Any use outside of the loop of an expression derived from the indvar is 583 changed to compute the derived value outside of the loop, eliminating the 584 dependence on the exit value of the induction variable. If the only purpose 585 of the loop is to compute the exit value of some derived expression, this 586 transformation will make the loop dead. 587 588This transformation should be followed by strength reduction after all of the 589desired loop transformations have been performed. Additionally, on targets 590where it is profitable, the loop could be transformed to count down to zero 591(the "do loop" optimization). 592 593``-inline``: Function Integration/Inlining 594------------------------------------------ 595 596Bottom-up inlining of functions into callees. 597 598.. _passes-instcombine: 599 600``-instcombine``: Combine redundant instructions 601------------------------------------------------ 602 603Combine instructions to form fewer, simple instructions. This pass does not 604modify the CFG. This pass is where algebraic simplification happens. 605 606This pass combines things like: 607 608.. code-block:: llvm 609 610 %Y = add i32 %X, 1 611 %Z = add i32 %Y, 1 612 613into: 614 615.. code-block:: llvm 616 617 %Z = add i32 %X, 2 618 619This is a simple worklist driven algorithm. 620 621This pass guarantees that the following canonicalizations are performed on the 622program: 623 624#. If a binary operator has a constant operand, it is moved to the right-hand 625 side. 626#. Bitwise operators with constant operands are always grouped so that shifts 627 are performed first, then ``or``\ s, then ``and``\ s, then ``xor``\ s. 628#. Compare instructions are converted from ``<``, ``>``, ``≤``, or ``≥`` to 629 ``=`` or ``≠`` if possible. 630#. All ``cmp`` instructions on boolean values are replaced with logical 631 operations. 632#. ``add X, X`` is represented as ``mul X, 2`` ⇒ ``shl X, 1`` 633#. Multiplies with a constant power-of-two argument are transformed into 634 shifts. 635#. … etc. 636 637This pass can also simplify calls to specific well-known function calls (e.g. 638runtime library functions). For example, a call ``exit(3)`` that occurs within 639the ``main()`` function can be transformed into simply ``return 3``. Whether or 640not library calls are simplified is controlled by the 641:ref:`-functionattrs <passes-functionattrs>` pass and LLVM's knowledge of 642library calls on different targets. 643 644``-internalize``: Internalize Global Symbols 645-------------------------------------------- 646 647This pass loops over all of the functions in the input module, looking for a 648main function. If a main function is found, all other functions and all global 649variables with initializers are marked as internal. 650 651``-ipconstprop``: Interprocedural constant propagation 652------------------------------------------------------ 653 654This pass implements an *extremely* simple interprocedural constant propagation 655pass. It could certainly be improved in many different ways, like using a 656worklist. This pass makes arguments dead, but does not remove them. The 657existing dead argument elimination pass should be run after this to clean up 658the mess. 659 660``-ipsccp``: Interprocedural Sparse Conditional Constant Propagation 661-------------------------------------------------------------------- 662 663An interprocedural variant of :ref:`Sparse Conditional Constant Propagation 664<passes-sccp>`. 665 666``-jump-threading``: Jump Threading 667----------------------------------- 668 669Jump threading tries to find distinct threads of control flow running through a 670basic block. This pass looks at blocks that have multiple predecessors and 671multiple successors. If one or more of the predecessors of the block can be 672proven to always cause a jump to one of the successors, we forward the edge 673from the predecessor to the successor by duplicating the contents of this 674block. 675 676An example of when this can occur is code like this: 677 678.. code-block:: c++ 679 680 if () { ... 681 X = 4; 682 } 683 if (X < 3) { 684 685In this case, the unconditional branch at the end of the first if can be 686revectored to the false side of the second if. 687 688``-lcssa``: Loop-Closed SSA Form Pass 689------------------------------------- 690 691This pass transforms loops by placing phi nodes at the end of the loops for all 692values that are live across the loop boundary. For example, it turns the left 693into the right code: 694 695.. code-block:: c++ 696 697 for (...) for (...) 698 if (c) if (c) 699 X1 = ... X1 = ... 700 else else 701 X2 = ... X2 = ... 702 X3 = phi(X1, X2) X3 = phi(X1, X2) 703 ... = X3 + 4 X4 = phi(X3) 704 ... = X4 + 4 705 706This is still valid LLVM; the extra phi nodes are purely redundant, and will be 707trivially eliminated by ``InstCombine``. The major benefit of this 708transformation is that it makes many other loop optimizations, such as 709``LoopUnswitch``\ ing, simpler. 710 711.. _passes-licm: 712 713``-licm``: Loop Invariant Code Motion 714------------------------------------- 715 716This pass performs loop invariant code motion, attempting to remove as much 717code from the body of a loop as possible. It does this by either hoisting code 718into the preheader block, or by sinking code to the exit blocks if it is safe. 719This pass also promotes must-aliased memory locations in the loop to live in 720registers, thus hoisting and sinking "invariant" loads and stores. 721 722This pass uses alias analysis for two purposes: 723 724#. Moving loop invariant loads and calls out of loops. If we can determine 725 that a load or call inside of a loop never aliases anything stored to, we 726 can hoist it or sink it like any other instruction. 727 728#. Scalar Promotion of Memory. If there is a store instruction inside of the 729 loop, we try to move the store to happen AFTER the loop instead of inside of 730 the loop. This can only happen if a few conditions are true: 731 732 #. The pointer stored through is loop invariant. 733 #. There are no stores or loads in the loop which *may* alias the pointer. 734 There are no calls in the loop which mod/ref the pointer. 735 736 If these conditions are true, we can promote the loads and stores in the 737 loop of the pointer to use a temporary alloca'd variable. We then use the 738 :ref:`mem2reg <passes-mem2reg>` functionality to construct the appropriate 739 SSA form for the variable. 740 741``-loop-deletion``: Delete dead loops 742------------------------------------- 743 744This file implements the Dead Loop Deletion Pass. This pass is responsible for 745eliminating loops with non-infinite computable trip counts that have no side 746effects or volatile instructions, and do not contribute to the computation of 747the function's return value. 748 749.. _passes-loop-extract: 750 751``-loop-extract``: Extract loops into new functions 752--------------------------------------------------- 753 754A pass wrapper around the ``ExtractLoop()`` scalar transformation to extract 755each top-level loop into its own new function. If the loop is the *only* loop 756in a given function, it is not touched. This is a pass most useful for 757debugging via bugpoint. 758 759``-loop-extract-single``: Extract at most one loop into a new function 760---------------------------------------------------------------------- 761 762Similar to :ref:`Extract loops into new functions <passes-loop-extract>`, this 763pass extracts one natural loop from the program into a function if it can. 764This is used by :program:`bugpoint`. 765 766``-loop-reduce``: Loop Strength Reduction 767----------------------------------------- 768 769This pass performs a strength reduction on array references inside loops that 770have as one or more of their components the loop induction variable. This is 771accomplished by creating a new value to hold the initial value of the array 772access for the first iteration, and then creating a new GEP instruction in the 773loop to increment the value by the appropriate amount. 774 775``-loop-rotate``: Rotate Loops 776------------------------------ 777 778A simple loop rotation transformation. 779 780``-loop-simplify``: Canonicalize natural loops 781---------------------------------------------- 782 783This pass performs several transformations to transform natural loops into a 784simpler form, which makes subsequent analyses and transformations simpler and 785more effective. 786 787Loop pre-header insertion guarantees that there is a single, non-critical entry 788edge from outside of the loop to the loop header. This simplifies a number of 789analyses and transformations, such as :ref:`LICM <passes-licm>`. 790 791Loop exit-block insertion guarantees that all exit blocks from the loop (blocks 792which are outside of the loop that have predecessors inside of the loop) only 793have predecessors from inside of the loop (and are thus dominated by the loop 794header). This simplifies transformations such as store-sinking that are built 795into LICM. 796 797This pass also guarantees that loops will have exactly one backedge. 798 799Note that the :ref:`simplifycfg <passes-simplifycfg>` pass will clean up blocks 800which are split out but end up being unnecessary, so usage of this pass should 801not pessimize generated code. 802 803This pass obviously modifies the CFG, but updates loop information and 804dominator information. 805 806``-loop-unroll``: Unroll loops 807------------------------------ 808 809This pass implements a simple loop unroller. It works best when loops have 810been canonicalized by the :ref:`indvars <passes-indvars>` pass, allowing it to 811determine the trip counts of loops easily. 812 813``-loop-unswitch``: Unswitch loops 814---------------------------------- 815 816This pass transforms loops that contain branches on loop-invariant conditions 817to have multiple loops. For example, it turns the left into the right code: 818 819.. code-block:: c++ 820 821 for (...) if (lic) 822 A for (...) 823 if (lic) A; B; C 824 B else 825 C for (...) 826 A; C 827 828This can increase the size of the code exponentially (doubling it every time a 829loop is unswitched) so we only unswitch if the resultant code will be smaller 830than a threshold. 831 832This pass expects :ref:`LICM <passes-licm>` to be run before it to hoist 833invariant conditions out of the loop, to make the unswitching opportunity 834obvious. 835 836``-loweratomic``: Lower atomic intrinsics to non-atomic form 837------------------------------------------------------------ 838 839This pass lowers atomic intrinsics to non-atomic form for use in a known 840non-preemptible environment. 841 842The pass does not verify that the environment is non-preemptible (in general 843this would require knowledge of the entire call graph of the program including 844any libraries which may not be available in bitcode form); it simply lowers 845every atomic intrinsic. 846 847``-lowerinvoke``: Lower invokes to calls, for unwindless code generators 848------------------------------------------------------------------------ 849 850This transformation is designed for use by code generators which do not yet 851support stack unwinding. This pass converts ``invoke`` instructions to 852``call`` instructions, so that any exception-handling ``landingpad`` blocks 853become dead code (which can be removed by running the ``-simplifycfg`` pass 854afterwards). 855 856``-lowerswitch``: Lower ``SwitchInst``\ s to branches 857----------------------------------------------------- 858 859Rewrites switch instructions with a sequence of branches, which allows targets 860to get away with not implementing the switch instruction until it is 861convenient. 862 863.. _passes-mem2reg: 864 865``-mem2reg``: Promote Memory to Register 866---------------------------------------- 867 868This file promotes memory references to be register references. It promotes 869alloca instructions which only have loads and stores as uses. An ``alloca`` is 870transformed by using dominator frontiers to place phi nodes, then traversing 871the function in depth-first order to rewrite loads and stores as appropriate. 872This is just the standard SSA construction algorithm to construct "pruned" SSA 873form. 874 875``-memcpyopt``: MemCpy Optimization 876----------------------------------- 877 878This pass performs various transformations related to eliminating ``memcpy`` 879calls, or transforming sets of stores into ``memset``\ s. 880 881``-mergefunc``: Merge Functions 882------------------------------- 883 884This pass looks for equivalent functions that are mergable and folds them. 885 886Total-ordering is introduced among the functions set: we define comparison 887that answers for every two functions which of them is greater. It allows to 888arrange functions into the binary tree. 889 890For every new function we check for equivalent in tree. 891 892If equivalent exists we fold such functions. If both functions are overridable, 893we move the functionality into a new internal function and leave two 894overridable thunks to it. 895 896If there is no equivalent, then we add this function to tree. 897 898Lookup routine has O(log(n)) complexity, while whole merging process has 899complexity of O(n*log(n)). 900 901Read 902:doc:`this <MergeFunctions>` 903article for more details. 904 905``-mergereturn``: Unify function exit nodes 906------------------------------------------- 907 908Ensure that functions have at most one ``ret`` instruction in them. 909Additionally, it keeps track of which node is the new exit node of the CFG. 910 911``-partial-inliner``: Partial Inliner 912------------------------------------- 913 914This pass performs partial inlining, typically by inlining an ``if`` statement 915that surrounds the body of the function. 916 917``-prune-eh``: Remove unused exception handling info 918---------------------------------------------------- 919 920This file implements a simple interprocedural pass which walks the call-graph, 921turning invoke instructions into call instructions if and only if the callee 922cannot throw an exception. It implements this as a bottom-up traversal of the 923call-graph. 924 925``-reassociate``: Reassociate expressions 926----------------------------------------- 927 928This pass reassociates commutative expressions in an order that is designed to 929promote better constant propagation, GCSE, :ref:`LICM <passes-licm>`, PRE, etc. 930 931For example: 4 + (x + 5) ⇒ x + (4 + 5) 932 933In the implementation of this algorithm, constants are assigned rank = 0, 934function arguments are rank = 1, and other values are assigned ranks 935corresponding to the reverse post order traversal of current function (starting 936at 2), which effectively gives values in deep loops higher rank than values not 937in loops. 938 939``-reg2mem``: Demote all values to stack slots 940---------------------------------------------- 941 942This file demotes all registers to memory references. It is intended to be the 943inverse of :ref:`mem2reg <passes-mem2reg>`. By converting to ``load`` 944instructions, the only values live across basic blocks are ``alloca`` 945instructions and ``load`` instructions before ``phi`` nodes. It is intended 946that this should make CFG hacking much easier. To make later hacking easier, 947the entry block is split into two, such that all introduced ``alloca`` 948instructions (and nothing else) are in the entry block. 949 950``-sroa``: Scalar Replacement of Aggregates 951------------------------------------------------------ 952 953The well-known scalar replacement of aggregates transformation. This transform 954breaks up ``alloca`` instructions of aggregate type (structure or array) into 955individual ``alloca`` instructions for each member if possible. Then, if 956possible, it transforms the individual ``alloca`` instructions into nice clean 957scalar SSA form. 958 959.. _passes-sccp: 960 961``-sccp``: Sparse Conditional Constant Propagation 962-------------------------------------------------- 963 964Sparse conditional constant propagation and merging, which can be summarized 965as: 966 967* Assumes values are constant unless proven otherwise 968* Assumes BasicBlocks are dead unless proven otherwise 969* Proves values to be constant, and replaces them with constants 970* Proves conditional branches to be unconditional 971 972Note that this pass has a habit of making definitions be dead. It is a good 973idea to run a :ref:`DCE <passes-dce>` pass sometime after running this pass. 974 975.. _passes-simplifycfg: 976 977``-simplifycfg``: Simplify the CFG 978---------------------------------- 979 980Performs dead code elimination and basic block merging. Specifically: 981 982* Removes basic blocks with no predecessors. 983* Merges a basic block into its predecessor if there is only one and the 984 predecessor only has one successor. 985* Eliminates PHI nodes for basic blocks with a single predecessor. 986* Eliminates a basic block that only contains an unconditional branch. 987 988``-sink``: Code sinking 989----------------------- 990 991This pass moves instructions into successor blocks, when possible, so that they 992aren't executed on paths where their results aren't needed. 993 994``-strip``: Strip all symbols from a module 995------------------------------------------- 996 997Performs code stripping. This transformation can delete: 998 999* names for virtual registers 1000* symbols for internal globals and functions 1001* debug information 1002 1003Note that this transformation makes code much less readable, so it should only 1004be used in situations where the strip utility would be used, such as reducing 1005code size or making it harder to reverse engineer code. 1006 1007``-strip-dead-debug-info``: Strip debug info for unused symbols 1008--------------------------------------------------------------- 1009 1010.. FIXME: this description is the same as for -strip 1011 1012performs code stripping. this transformation can delete: 1013 1014* names for virtual registers 1015* symbols for internal globals and functions 1016* debug information 1017 1018note that this transformation makes code much less readable, so it should only 1019be used in situations where the strip utility would be used, such as reducing 1020code size or making it harder to reverse engineer code. 1021 1022``-strip-dead-prototypes``: Strip Unused Function Prototypes 1023------------------------------------------------------------ 1024 1025This pass loops over all of the functions in the input module, looking for dead 1026declarations and removes them. Dead declarations are declarations of functions 1027for which no implementation is available (i.e., declarations for unused library 1028functions). 1029 1030``-strip-debug-declare``: Strip all ``llvm.dbg.declare`` intrinsics 1031------------------------------------------------------------------- 1032 1033.. FIXME: this description is the same as for -strip 1034 1035This pass implements code stripping. Specifically, it can delete: 1036 1037#. names for virtual registers 1038#. symbols for internal globals and functions 1039#. debug information 1040 1041Note that this transformation makes code much less readable, so it should only 1042be used in situations where the 'strip' utility would be used, such as reducing 1043code size or making it harder to reverse engineer code. 1044 1045``-strip-nondebug``: Strip all symbols, except dbg symbols, from a module 1046------------------------------------------------------------------------- 1047 1048.. FIXME: this description is the same as for -strip 1049 1050This pass implements code stripping. Specifically, it can delete: 1051 1052#. names for virtual registers 1053#. symbols for internal globals and functions 1054#. debug information 1055 1056Note that this transformation makes code much less readable, so it should only 1057be used in situations where the 'strip' utility would be used, such as reducing 1058code size or making it harder to reverse engineer code. 1059 1060``-tailcallelim``: Tail Call Elimination 1061---------------------------------------- 1062 1063This file transforms calls of the current function (self recursion) followed by 1064a return instruction with a branch to the entry of the function, creating a 1065loop. This pass also implements the following extensions to the basic 1066algorithm: 1067 1068#. Trivial instructions between the call and return do not prevent the 1069 transformation from taking place, though currently the analysis cannot 1070 support moving any really useful instructions (only dead ones). 1071#. This pass transforms functions that are prevented from being tail recursive 1072 by an associative expression to use an accumulator variable, thus compiling 1073 the typical naive factorial or fib implementation into efficient code. 1074#. TRE is performed if the function returns void, if the return returns the 1075 result returned by the call, or if the function returns a run-time constant 1076 on all exits from the function. It is possible, though unlikely, that the 1077 return returns something else (like constant 0), and can still be TRE'd. It 1078 can be TRE'd if *all other* return instructions in the function return the 1079 exact same value. 1080#. If it can prove that callees do not access theier caller stack frame, they 1081 are marked as eligible for tail call elimination (by the code generator). 1082 1083Utility Passes 1084============== 1085 1086This section describes the LLVM Utility Passes. 1087 1088``-deadarghaX0r``: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE) 1089------------------------------------------------------------------------ 1090 1091Same as dead argument elimination, but deletes arguments to functions which are 1092external. This is only for use by :doc:`bugpoint <Bugpoint>`. 1093 1094``-extract-blocks``: Extract Basic Blocks From Module (for bugpoint use) 1095------------------------------------------------------------------------ 1096 1097This pass is used by bugpoint to extract all blocks from the module into their 1098own functions. 1099 1100``-instnamer``: Assign names to anonymous instructions 1101------------------------------------------------------ 1102 1103This is a little utility pass that gives instructions names, this is mostly 1104useful when diffing the effect of an optimization because deleting an unnamed 1105instruction can change all other instruction numbering, making the diff very 1106noisy. 1107 1108.. _passes-verify: 1109 1110``-verify``: Module Verifier 1111---------------------------- 1112 1113Verifies an LLVM IR code. This is useful to run after an optimization which is 1114undergoing testing. Note that llvm-as verifies its input before emitting 1115bitcode, and also that malformed bitcode is likely to make LLVM crash. All 1116language front-ends are therefore encouraged to verify their output before 1117performing optimizing transformations. 1118 1119#. Both of a binary operator's parameters are of the same type. 1120#. Verify that the indices of mem access instructions match other operands. 1121#. Verify that arithmetic and other things are only performed on first-class 1122 types. Verify that shifts and logicals only happen on integrals f.e. 1123#. All of the constants in a switch statement are of the correct type. 1124#. The code is in valid SSA form. 1125#. It is illegal to put a label into any other type (like a structure) or to 1126 return one. 1127#. Only phi nodes can be self referential: ``%x = add i32 %x``, ``%x`` is 1128 invalid. 1129#. PHI nodes must have an entry for each predecessor, with no extras. 1130#. PHI nodes must be the first thing in a basic block, all grouped together. 1131#. PHI nodes must have at least one entry. 1132#. All basic blocks should only end with terminator insts, not contain them. 1133#. The entry node to a function must not have predecessors. 1134#. All Instructions must be embedded into a basic block. 1135#. Functions cannot take a void-typed parameter. 1136#. Verify that a function's argument list agrees with its declared type. 1137#. It is illegal to specify a name for a void value. 1138#. It is illegal to have an internal global value with no initializer. 1139#. It is illegal to have a ``ret`` instruction that returns a value that does 1140 not agree with the function return value type. 1141#. Function call argument types match the function prototype. 1142#. All other things that are tested by asserts spread about the code. 1143 1144Note that this does not provide full security verification (like Java), but 1145instead just tries to ensure that code is well-formed. 1146 1147``-view-cfg``: View CFG of function 1148----------------------------------- 1149 1150Displays the control flow graph using the GraphViz tool. 1151 1152``-view-cfg-only``: View CFG of function (with no function bodies) 1153------------------------------------------------------------------ 1154 1155Displays the control flow graph using the GraphViz tool, but omitting function 1156bodies. 1157 1158``-view-dom``: View dominance tree of function 1159---------------------------------------------- 1160 1161Displays the dominator tree using the GraphViz tool. 1162 1163``-view-dom-only``: View dominance tree of function (with no function bodies) 1164----------------------------------------------------------------------------- 1165 1166Displays the dominator tree using the GraphViz tool, but omitting function 1167bodies. 1168 1169``-view-postdom``: View postdominance tree of function 1170------------------------------------------------------ 1171 1172Displays the post dominator tree using the GraphViz tool. 1173 1174``-view-postdom-only``: View postdominance tree of function (with no function bodies) 1175------------------------------------------------------------------------------------- 1176 1177Displays the post dominator tree using the GraphViz tool, but omitting function 1178bodies. 1179 1180