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``-basic-aa``: 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.. _passes-da: 87 88``-da``: Dependence Analysis 89---------------------------- 90 91Dependence analysis framework, which is used to detect dependences in memory 92accesses. 93 94``-debug-aa``: AA use debugger 95------------------------------ 96 97This simple pass checks alias analysis users to ensure that if they create a 98new value, they do not query AA without informing it of the value. It acts as 99a shim over any other AA pass you want. 100 101Yes keeping track of every value in the program is expensive, but this is a 102debugging pass. 103 104``-domfrontier``: Dominance Frontier Construction 105------------------------------------------------- 106 107This pass is a simple dominator construction algorithm for finding forward 108dominator frontiers. 109 110``-domtree``: Dominator Tree Construction 111----------------------------------------- 112 113This pass is a simple dominator construction algorithm for finding forward 114dominators. 115 116 117``-dot-callgraph``: Print Call Graph to "dot" file 118-------------------------------------------------- 119 120This pass, only available in ``opt``, prints the call graph into a ``.dot`` 121graph. This graph can then be processed with the "dot" tool to convert it to 122postscript or some other suitable format. 123 124``-dot-cfg``: Print CFG of function to "dot" file 125------------------------------------------------- 126 127This pass, only available in ``opt``, prints the control flow graph into a 128``.dot`` graph. This graph can then be processed with the :program:`dot` tool 129to convert it to postscript or some other suitable format. 130 131``-dot-cfg-only``: Print CFG of function to "dot" file (with no function bodies) 132-------------------------------------------------------------------------------- 133 134This pass, only available in ``opt``, prints the control flow graph into a 135``.dot`` graph, omitting the function bodies. This graph can then be processed 136with the :program:`dot` tool to convert it to postscript or some other suitable 137format. 138 139``-dot-dom``: Print dominance tree of function to "dot" file 140------------------------------------------------------------ 141 142This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` 143graph. This graph can then be processed with the :program:`dot` tool to 144convert it to postscript or some other suitable format. 145 146``-dot-dom-only``: Print dominance tree of function to "dot" file (with no function bodies) 147------------------------------------------------------------------------------------------- 148 149This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` 150graph, omitting the function bodies. This graph can then be processed with the 151:program:`dot` tool to convert it to postscript or some other suitable format. 152 153``-dot-postdom``: Print postdominance tree of function to "dot" file 154-------------------------------------------------------------------- 155 156This pass, only available in ``opt``, prints the post dominator tree into a 157``.dot`` graph. This graph can then be processed with the :program:`dot` tool 158to convert it to postscript or some other suitable format. 159 160``-dot-postdom-only``: Print postdominance tree of function to "dot" file (with no function bodies) 161--------------------------------------------------------------------------------------------------- 162 163This pass, only available in ``opt``, prints the post dominator tree into a 164``.dot`` graph, omitting the function bodies. This graph can then be processed 165with the :program:`dot` tool to convert it to postscript or some other suitable 166format. 167 168``-globalsmodref-aa``: Simple mod/ref analysis for globals 169---------------------------------------------------------- 170 171This simple pass provides alias and mod/ref information for global values that 172do not have their address taken, and keeps track of whether functions read or 173write memory (are "pure"). For this simple (but very common) case, we can 174provide pretty accurate and useful information. 175 176``-instcount``: Counts the various types of ``Instruction``\ s 177-------------------------------------------------------------- 178 179This pass collects the count of all instructions and reports them. 180 181``-intervals``: Interval Partition Construction 182----------------------------------------------- 183 184This analysis calculates and represents the interval partition of a function, 185or a preexisting interval partition. 186 187In this way, the interval partition may be used to reduce a flow graph down to 188its degenerate single node interval partition (unless it is irreducible). 189 190``-iv-users``: Induction Variable Users 191--------------------------------------- 192 193Bookkeeping for "interesting" users of expressions computed from induction 194variables. 195 196``-lazy-value-info``: Lazy Value Information Analysis 197----------------------------------------------------- 198 199Interface for lazy computation of value constraint information. 200 201``-libcall-aa``: LibCall Alias Analysis 202--------------------------------------- 203 204LibCall Alias Analysis. 205 206``-lint``: Statically lint-checks LLVM IR 207----------------------------------------- 208 209This pass statically checks for common and easily-identified constructs which 210produce undefined or likely unintended behavior in LLVM IR. 211 212It is not a guarantee of correctness, in two ways. First, it isn't 213comprehensive. There are checks which could be done statically which are not 214yet implemented. Some of these are indicated by TODO comments, but those 215aren't comprehensive either. Second, many conditions cannot be checked 216statically. This pass does no dynamic instrumentation, so it can't check for 217all possible problems. 218 219Another limitation is that it assumes all code will be executed. A store 220through a null pointer in a basic block which is never reached is harmless, but 221this pass will warn about it anyway. 222 223Optimization passes may make conditions that this pass checks for more or less 224obvious. If an optimization pass appears to be introducing a warning, it may 225be that the optimization pass is merely exposing an existing condition in the 226code. 227 228This code may be run before :ref:`instcombine <passes-instcombine>`. In many 229cases, instcombine checks for the same kinds of things and turns instructions 230with undefined behavior into unreachable (or equivalent). Because of this, 231this pass makes some effort to look through bitcasts and so on. 232 233``-loops``: Natural Loop Information 234------------------------------------ 235 236This analysis is used to identify natural loops and determine the loop depth of 237various nodes of the CFG. Note that the loops identified may actually be 238several natural loops that share the same header node... not just a single 239natural loop. 240 241``-memdep``: Memory Dependence Analysis 242--------------------------------------- 243 244An analysis that determines, for a given memory operation, what preceding 245memory operations it depends on. It builds on alias analysis information, and 246tries to provide a lazy, caching interface to a common kind of alias 247information query. 248 249``-module-debuginfo``: Decodes module-level debug info 250------------------------------------------------------ 251 252This pass decodes the debug info metadata in a module and prints in a 253(sufficiently-prepared-) human-readable form. 254 255For example, run this pass from ``opt`` along with the ``-analyze`` option, and 256it'll print to standard output. 257 258``-postdomfrontier``: Post-Dominance Frontier Construction 259---------------------------------------------------------- 260 261This pass is a simple post-dominator construction algorithm for finding 262post-dominator frontiers. 263 264``-postdomtree``: Post-Dominator Tree Construction 265-------------------------------------------------- 266 267This pass is a simple post-dominator construction algorithm for finding 268post-dominators. 269 270``-print-alias-sets``: Alias Set Printer 271---------------------------------------- 272 273Yet to be written. 274 275``-print-callgraph``: Print a call graph 276---------------------------------------- 277 278This pass, only available in ``opt``, prints the call graph to standard error 279in a human-readable form. 280 281``-print-callgraph-sccs``: Print SCCs of the Call Graph 282------------------------------------------------------- 283 284This pass, only available in ``opt``, prints the SCCs of the call graph to 285standard error in a human-readable form. 286 287``-print-cfg-sccs``: Print SCCs of each function CFG 288---------------------------------------------------- 289 290This pass, only available in ``opt``, printsthe SCCs of each function CFG to 291standard error in a human-readable fom. 292 293``-print-dom-info``: Dominator Info Printer 294------------------------------------------- 295 296Dominator Info Printer. 297 298``-print-externalfnconstants``: Print external fn callsites passed constants 299---------------------------------------------------------------------------- 300 301This pass, only available in ``opt``, prints out call sites to external 302functions that are called with constant arguments. This can be useful when 303looking for standard library functions we should constant fold or handle in 304alias analyses. 305 306``-print-function``: Print function to stderr 307--------------------------------------------- 308 309The ``PrintFunctionPass`` class is designed to be pipelined with other 310``FunctionPasses``, and prints out the functions of the module as they are 311processed. 312 313``-print-module``: Print module to stderr 314----------------------------------------- 315 316This pass simply prints out the entire module when it is executed. 317 318.. _passes-print-used-types: 319 320``-print-used-types``: Find Used Types 321-------------------------------------- 322 323This pass is used to seek out all of the types in use by the program. Note 324that this analysis explicitly does not include types only used by the symbol 325table. 326 327``-regions``: Detect single entry single exit regions 328----------------------------------------------------- 329 330The ``RegionInfo`` pass detects single entry single exit regions in a function, 331where a region is defined as any subgraph that is connected to the remaining 332graph at only two spots. Furthermore, a hierarchical region tree is built. 333 334.. _passes-scalar-evolution: 335 336``-scalar-evolution``: Scalar Evolution Analysis 337------------------------------------------------ 338 339The ``ScalarEvolution`` analysis can be used to analyze and categorize scalar 340expressions in loops. It specializes in recognizing general induction 341variables, representing them with the abstract and opaque ``SCEV`` class. 342Given this analysis, trip counts of loops and other important properties can be 343obtained. 344 345This analysis is primarily useful for induction variable substitution and 346strength reduction. 347 348``-scev-aa``: ScalarEvolution-based Alias Analysis 349-------------------------------------------------- 350 351Simple alias analysis implemented in terms of ``ScalarEvolution`` queries. 352 353This differs from traditional loop dependence analysis in that it tests for 354dependencies within a single iteration of a loop, rather than dependencies 355between different iterations. 356 357``ScalarEvolution`` has a more complete understanding of pointer arithmetic 358than ``BasicAliasAnalysis``' collection of ad-hoc analyses. 359 360``-stack-safety``: Stack Safety Analysis 361------------------------------------------------ 362 363The ``StackSafety`` analysis can be used to determine if stack allocated 364variables can be considered safe from memory access bugs. 365 366This analysis' primary purpose is to be used by sanitizers to avoid unnecessary 367instrumentation of safe variables. 368 369``-targetdata``: Target Data Layout 370----------------------------------- 371 372Provides other passes access to information on how the size and alignment 373required by the target ABI for various data types. 374 375Transform Passes 376================ 377 378This section describes the LLVM Transform Passes. 379 380``-adce``: Aggressive Dead Code Elimination 381------------------------------------------- 382 383ADCE aggressively tries to eliminate code. This pass is similar to :ref:`DCE 384<passes-dce>` but it assumes that values are dead until proven otherwise. This 385is similar to :ref:`SCCP <passes-sccp>`, except applied to the liveness of 386values. 387 388``-always-inline``: Inliner for ``always_inline`` functions 389----------------------------------------------------------- 390 391A custom inliner that handles only functions that are marked as "always 392inline". 393 394``-argpromotion``: Promote 'by reference' arguments to scalars 395-------------------------------------------------------------- 396 397This pass promotes "by reference" arguments to be "by value" arguments. In 398practice, this means looking for internal functions that have pointer 399arguments. If it can prove, through the use of alias analysis, that an 400argument is *only* loaded, then it can pass the value into the function instead 401of the address of the value. This can cause recursive simplification of code 402and lead to the elimination of allocas (especially in C++ template code like 403the STL). 404 405This pass also handles aggregate arguments that are passed into a function, 406scalarizing them if the elements of the aggregate are only loaded. Note that 407it refuses to scalarize aggregates which would require passing in more than 408three operands to the function, because passing thousands of operands for a 409large array or structure is unprofitable! 410 411Note that this transformation could also be done for arguments that are only 412stored to (returning the value instead), but does not currently. This case 413would be best handled when and if LLVM starts supporting multiple return values 414from functions. 415 416``-bb-vectorize``: Basic-Block Vectorization 417-------------------------------------------- 418 419This pass combines instructions inside basic blocks to form vector 420instructions. It iterates over each basic block, attempting to pair compatible 421instructions, repeating this process until no additional pairs are selected for 422vectorization. When the outputs of some pair of compatible instructions are 423used as inputs by some other pair of compatible instructions, those pairs are 424part of a potential vectorization chain. Instruction pairs are only fused into 425vector instructions when they are part of a chain longer than some threshold 426length. Moreover, the pass attempts to find the best possible chain for each 427pair of compatible instructions. These heuristics are intended to prevent 428vectorization in cases where it would not yield a performance increase of the 429resulting code. 430 431``-block-placement``: Profile Guided Basic Block Placement 432---------------------------------------------------------- 433 434This pass is a very simple profile guided basic block placement algorithm. The 435idea is to put frequently executed blocks together at the start of the function 436and hopefully increase the number of fall-through conditional branches. If 437there is no profile information for a particular function, this pass basically 438orders blocks in depth-first order. 439 440``-break-crit-edges``: Break critical edges in CFG 441-------------------------------------------------- 442 443Break all of the critical edges in the CFG by inserting a dummy basic block. 444It may be "required" by passes that cannot deal with critical edges. This 445transformation obviously invalidates the CFG, but can update forward dominator 446(set, immediate dominators, tree, and frontier) information. 447 448``-codegenprepare``: Optimize for code generation 449------------------------------------------------- 450 451This pass munges the code in the input function to better prepare it for 452SelectionDAG-based code generation. This works around limitations in its 453basic-block-at-a-time approach. It should eventually be removed. 454 455``-constmerge``: Merge Duplicate Global Constants 456------------------------------------------------- 457 458Merges duplicate global constants together into a single constant that is 459shared. This is useful because some passes (i.e., TraceValues) insert a lot of 460string constants into the program, regardless of whether or not an existing 461string is available. 462 463.. _passes-dce: 464 465``-dce``: Dead Code Elimination 466------------------------------- 467 468Dead code elimination is similar to :ref:`dead instruction elimination 469<passes-die>`, but it rechecks instructions that were used by removed 470instructions to see if they are newly dead. 471 472``-deadargelim``: Dead Argument Elimination 473------------------------------------------- 474 475This pass deletes dead arguments from internal functions. Dead argument 476elimination removes arguments which are directly dead, as well as arguments 477only passed into function calls as dead arguments of other functions. This 478pass also deletes dead arguments in a similar way. 479 480This pass is often useful as a cleanup pass to run after aggressive 481interprocedural passes, which add possibly-dead arguments. 482 483``-deadtypeelim``: Dead Type Elimination 484---------------------------------------- 485 486This pass is used to cleanup the output of GCC. It eliminate names for types 487that are unused in the entire translation unit, using the :ref:`find used types 488<passes-print-used-types>` pass. 489 490.. _passes-die: 491 492``-die``: Dead Instruction Elimination 493-------------------------------------- 494 495Dead instruction elimination performs a single pass over the function, removing 496instructions that are obviously dead. 497 498``-dse``: Dead Store Elimination 499-------------------------------- 500 501A trivial dead store elimination that only considers basic-block local 502redundant stores. 503 504.. _passes-function-attrs: 505 506``-function-attrs``: Deduce function attributes 507----------------------------------------------- 508 509A simple interprocedural pass which walks the call-graph, looking for functions 510which do not access or only read non-local memory, and marking them 511``readnone``/``readonly``. In addition, it marks function arguments (of 512pointer type) "``nocapture``" if a call to the function does not create any 513copies of the pointer value that outlive the call. This more or less means 514that the pointer is only dereferenced, and not returned from the function or 515stored in a global. This pass is implemented as a bottom-up traversal of the 516call-graph. 517 518``-globaldce``: Dead Global Elimination 519--------------------------------------- 520 521This transform is designed to eliminate unreachable internal globals from the 522program. It uses an aggressive algorithm, searching out globals that are known 523to be alive. After it finds all of the globals which are needed, it deletes 524whatever is left over. This allows it to delete recursive chunks of the 525program which are unreachable. 526 527``-globalopt``: Global Variable Optimizer 528----------------------------------------- 529 530This pass transforms simple global variables that never have their address 531taken. If obviously true, it marks read/write globals as constant, deletes 532variables only stored to, etc. 533 534``-gvn``: Global Value Numbering 535-------------------------------- 536 537This pass performs global value numbering to eliminate fully and partially 538redundant instructions. It also performs redundant load elimination. 539 540.. _passes-indvars: 541 542``-indvars``: Canonicalize Induction Variables 543---------------------------------------------- 544 545This transformation analyzes and transforms the induction variables (and 546computations derived from them) into simpler forms suitable for subsequent 547analysis and transformation. 548 549This transformation makes the following changes to each loop with an 550identifiable induction variable: 551 552* All loops are transformed to have a *single* canonical induction variable 553 which starts at zero and steps by one. 554* The canonical induction variable is guaranteed to be the first PHI node in 555 the loop header block. 556* Any pointer arithmetic recurrences are raised to use array subscripts. 557 558If the trip count of a loop is computable, this pass also makes the following 559changes: 560 561* The exit condition for the loop is canonicalized to compare the induction 562 value against the exit value. This turns loops like: 563 564 .. code-block:: c++ 565 566 for (i = 7; i*i < 1000; ++i) 567 568 into 569 570 .. code-block:: c++ 571 572 for (i = 0; i != 25; ++i) 573 574* Any use outside of the loop of an expression derived from the indvar is 575 changed to compute the derived value outside of the loop, eliminating the 576 dependence on the exit value of the induction variable. If the only purpose 577 of the loop is to compute the exit value of some derived expression, this 578 transformation will make the loop dead. 579 580This transformation should be followed by strength reduction after all of the 581desired loop transformations have been performed. Additionally, on targets 582where it is profitable, the loop could be transformed to count down to zero 583(the "do loop" optimization). 584 585``-inline``: Function Integration/Inlining 586------------------------------------------ 587 588Bottom-up inlining of functions into callees. 589 590.. _passes-instcombine: 591 592``-instcombine``: Combine redundant instructions 593------------------------------------------------ 594 595Combine instructions to form fewer, simple instructions. This pass does not 596modify the CFG. This pass is where algebraic simplification happens. 597 598This pass combines things like: 599 600.. code-block:: llvm 601 602 %Y = add i32 %X, 1 603 %Z = add i32 %Y, 1 604 605into: 606 607.. code-block:: llvm 608 609 %Z = add i32 %X, 2 610 611This is a simple worklist driven algorithm. 612 613This pass guarantees that the following canonicalizations are performed on the 614program: 615 616#. If a binary operator has a constant operand, it is moved to the right-hand 617 side. 618#. Bitwise operators with constant operands are always grouped so that shifts 619 are performed first, then ``or``\ s, then ``and``\ s, then ``xor``\ s. 620#. Compare instructions are converted from ``<``, ``>``, ``≤``, or ``≥`` to 621 ``=`` or ``≠`` if possible. 622#. All ``cmp`` instructions on boolean values are replaced with logical 623 operations. 624#. ``add X, X`` is represented as ``mul X, 2`` ⇒ ``shl X, 1`` 625#. Multiplies with a constant power-of-two argument are transformed into 626 shifts. 627#. … etc. 628 629This pass can also simplify calls to specific well-known function calls (e.g. 630runtime library functions). For example, a call ``exit(3)`` that occurs within 631the ``main()`` function can be transformed into simply ``return 3``. Whether or 632not library calls are simplified is controlled by the 633:ref:`-function-attrs <passes-function-attrs>` pass and LLVM's knowledge of 634library calls on different targets. 635 636.. _passes-aggressive-instcombine: 637 638``-aggressive-instcombine``: Combine expression patterns 639-------------------------------------------------------- 640 641Combine expression patterns to form expressions with fewer, simple instructions. 642This pass does not modify the CFG. 643 644For example, this pass reduce width of expressions post-dominated by TruncInst 645into smaller width when applicable. 646 647It differs from instcombine pass in that it contains pattern optimization that 648requires higher complexity than the O(1), thus, it should run fewer times than 649instcombine pass. 650 651``-internalize``: Internalize Global Symbols 652-------------------------------------------- 653 654This pass loops over all of the functions in the input module, looking for a 655main function. If a main function is found, all other functions and all global 656variables with initializers are marked as internal. 657 658``-ipsccp``: Interprocedural Sparse Conditional Constant Propagation 659-------------------------------------------------------------------- 660 661An interprocedural variant of :ref:`Sparse Conditional Constant Propagation 662<passes-sccp>`. 663 664``-jump-threading``: Jump Threading 665----------------------------------- 666 667Jump threading tries to find distinct threads of control flow running through a 668basic block. This pass looks at blocks that have multiple predecessors and 669multiple successors. If one or more of the predecessors of the block can be 670proven to always cause a jump to one of the successors, we forward the edge 671from the predecessor to the successor by duplicating the contents of this 672block. 673 674An example of when this can occur is code like this: 675 676.. code-block:: c++ 677 678 if () { ... 679 X = 4; 680 } 681 if (X < 3) { 682 683In this case, the unconditional branch at the end of the first if can be 684revectored to the false side of the second if. 685 686.. _passes-lcssa: 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. You can read more in the 710:ref:`loop terminology section for the LCSSA form <loop-terminology-lcssa>`. 711 712.. _passes-licm: 713 714``-licm``: Loop Invariant Code Motion 715------------------------------------- 716 717This pass performs loop invariant code motion, attempting to remove as much 718code from the body of a loop as possible. It does this by either hoisting code 719into the preheader block, or by sinking code to the exit blocks if it is safe. 720This pass also promotes must-aliased memory locations in the loop to live in 721registers, thus hoisting and sinking "invariant" loads and stores. 722 723Hoisting operations out of loops is a canonicalization transform. It enables 724and simplifies subsequent optimizations in the middle-end. Rematerialization 725of hoisted instructions to reduce register pressure is the responsibility of 726the back-end, which has more accurate information about register pressure and 727also handles other optimizations than LICM that increase live-ranges. 728 729This pass uses alias analysis for two purposes: 730 731#. Moving loop invariant loads and calls out of loops. If we can determine 732 that a load or call inside of a loop never aliases anything stored to, we 733 can hoist it or sink it like any other instruction. 734 735#. Scalar Promotion of Memory. If there is a store instruction inside of the 736 loop, we try to move the store to happen AFTER the loop instead of inside of 737 the loop. This can only happen if a few conditions are true: 738 739 #. The pointer stored through is loop invariant. 740 #. There are no stores or loads in the loop which *may* alias the pointer. 741 There are no calls in the loop which mod/ref the pointer. 742 743 If these conditions are true, we can promote the loads and stores in the 744 loop of the pointer to use a temporary alloca'd variable. We then use the 745 :ref:`mem2reg <passes-mem2reg>` functionality to construct the appropriate 746 SSA form for the variable. 747 748``-loop-deletion``: Delete dead loops 749------------------------------------- 750 751This file implements the Dead Loop Deletion Pass. This pass is responsible for 752eliminating loops with non-infinite computable trip counts that have no side 753effects or volatile instructions, and do not contribute to the computation of 754the function's return value. 755 756.. _passes-loop-extract: 757 758``-loop-extract``: Extract loops into new functions 759--------------------------------------------------- 760 761A pass wrapper around the ``ExtractLoop()`` scalar transformation to extract 762each top-level loop into its own new function. If the loop is the *only* loop 763in a given function, it is not touched. This is a pass most useful for 764debugging via bugpoint. 765 766``-loop-extract-single``: Extract at most one loop into a new function 767---------------------------------------------------------------------- 768 769Similar to :ref:`Extract loops into new functions <passes-loop-extract>`, this 770pass extracts one natural loop from the program into a function if it can. 771This is used by :program:`bugpoint`. 772 773``-loop-reduce``: Loop Strength Reduction 774----------------------------------------- 775 776This pass performs a strength reduction on array references inside loops that 777have as one or more of their components the loop induction variable. This is 778accomplished by creating a new value to hold the initial value of the array 779access for the first iteration, and then creating a new GEP instruction in the 780loop to increment the value by the appropriate amount. 781 782.. _passes-loop-rotate: 783 784``-loop-rotate``: Rotate Loops 785------------------------------ 786 787A simple loop rotation transformation. A summary of it can be found in 788:ref:`Loop Terminology for Rotated Loops <loop-terminology-loop-rotate>`. 789 790 791.. _passes-loop-simplify: 792 793``-loop-simplify``: Canonicalize natural loops 794---------------------------------------------- 795 796This pass performs several transformations to transform natural loops into a 797simpler form, which makes subsequent analyses and transformations simpler and 798more effective. A summary of it can be found in 799:ref:`Loop Terminology, Loop Simplify Form <loop-terminology-loop-simplify>`. 800 801Loop pre-header insertion guarantees that there is a single, non-critical entry 802edge from outside of the loop to the loop header. This simplifies a number of 803analyses and transformations, such as :ref:`LICM <passes-licm>`. 804 805Loop exit-block insertion guarantees that all exit blocks from the loop (blocks 806which are outside of the loop that have predecessors inside of the loop) only 807have predecessors from inside of the loop (and are thus dominated by the loop 808header). This simplifies transformations such as store-sinking that are built 809into LICM. 810 811This pass also guarantees that loops will have exactly one backedge. 812 813Note that the :ref:`simplifycfg <passes-simplifycfg>` pass will clean up blocks 814which are split out but end up being unnecessary, so usage of this pass should 815not pessimize generated code. 816 817This pass obviously modifies the CFG, but updates loop information and 818dominator information. 819 820``-loop-unroll``: Unroll loops 821------------------------------ 822 823This pass implements a simple loop unroller. It works best when loops have 824been canonicalized by the :ref:`indvars <passes-indvars>` pass, allowing it to 825determine the trip counts of loops easily. 826 827``-loop-unroll-and-jam``: Unroll and Jam loops 828---------------------------------------------- 829 830This pass implements a simple unroll and jam classical loop optimisation pass. 831It transforms loop from: 832 833.. code-block:: c++ 834 835 for i.. i+= 1 for i.. i+= 4 836 for j.. for j.. 837 code(i, j) code(i, j) 838 code(i+1, j) 839 code(i+2, j) 840 code(i+3, j) 841 remainder loop 842 843Which can be seen as unrolling the outer loop and "jamming" (fusing) the inner 844loops into one. When variables or loads can be shared in the new inner loop, this 845can lead to significant performance improvements. It uses 846:ref:`Dependence Analysis <passes-da>` for proving the transformations are safe. 847 848.. _passes-loop-unswitch: 849 850``-loop-unswitch``: Unswitch loops 851---------------------------------- 852 853This pass transforms loops that contain branches on loop-invariant conditions 854to have multiple loops. For example, it turns the left into the right code: 855 856.. code-block:: c++ 857 858 for (...) if (lic) 859 A for (...) 860 if (lic) A; B; C 861 B else 862 C for (...) 863 A; C 864 865This can increase the size of the code exponentially (doubling it every time a 866loop is unswitched) so we only unswitch if the resultant code will be smaller 867than a threshold. 868 869This pass expects :ref:`LICM <passes-licm>` to be run before it to hoist 870invariant conditions out of the loop, to make the unswitching opportunity 871obvious. 872 873``-loweratomic``: Lower atomic intrinsics to non-atomic form 874------------------------------------------------------------ 875 876This pass lowers atomic intrinsics to non-atomic form for use in a known 877non-preemptible environment. 878 879The pass does not verify that the environment is non-preemptible (in general 880this would require knowledge of the entire call graph of the program including 881any libraries which may not be available in bitcode form); it simply lowers 882every atomic intrinsic. 883 884``-lowerinvoke``: Lower invokes to calls, for unwindless code generators 885------------------------------------------------------------------------ 886 887This transformation is designed for use by code generators which do not yet 888support stack unwinding. This pass converts ``invoke`` instructions to 889``call`` instructions, so that any exception-handling ``landingpad`` blocks 890become dead code (which can be removed by running the ``-simplifycfg`` pass 891afterwards). 892 893``-lowerswitch``: Lower ``SwitchInst``\ s to branches 894----------------------------------------------------- 895 896Rewrites switch instructions with a sequence of branches, which allows targets 897to get away with not implementing the switch instruction until it is 898convenient. 899 900.. _passes-mem2reg: 901 902``-mem2reg``: Promote Memory to Register 903---------------------------------------- 904 905This file promotes memory references to be register references. It promotes 906alloca instructions which only have loads and stores as uses. An ``alloca`` is 907transformed by using dominator frontiers to place phi nodes, then traversing 908the function in depth-first order to rewrite loads and stores as appropriate. 909This is just the standard SSA construction algorithm to construct "pruned" SSA 910form. 911 912``-memcpyopt``: MemCpy Optimization 913----------------------------------- 914 915This pass performs various transformations related to eliminating ``memcpy`` 916calls, or transforming sets of stores into ``memset``\ s. 917 918``-mergefunc``: Merge Functions 919------------------------------- 920 921This pass looks for equivalent functions that are mergable and folds them. 922 923Total-ordering is introduced among the functions set: we define comparison 924that answers for every two functions which of them is greater. It allows to 925arrange functions into the binary tree. 926 927For every new function we check for equivalent in tree. 928 929If equivalent exists we fold such functions. If both functions are overridable, 930we move the functionality into a new internal function and leave two 931overridable thunks to it. 932 933If there is no equivalent, then we add this function to tree. 934 935Lookup routine has O(log(n)) complexity, while whole merging process has 936complexity of O(n*log(n)). 937 938Read 939:doc:`this <MergeFunctions>` 940article for more details. 941 942``-mergereturn``: Unify function exit nodes 943------------------------------------------- 944 945Ensure that functions have at most one ``ret`` instruction in them. 946Additionally, it keeps track of which node is the new exit node of the CFG. 947 948``-partial-inliner``: Partial Inliner 949------------------------------------- 950 951This pass performs partial inlining, typically by inlining an ``if`` statement 952that surrounds the body of the function. 953 954``-prune-eh``: Remove unused exception handling info 955---------------------------------------------------- 956 957This file implements a simple interprocedural pass which walks the call-graph, 958turning invoke instructions into call instructions if and only if the callee 959cannot throw an exception. It implements this as a bottom-up traversal of the 960call-graph. 961 962``-reassociate``: Reassociate expressions 963----------------------------------------- 964 965This pass reassociates commutative expressions in an order that is designed to 966promote better constant propagation, GCSE, :ref:`LICM <passes-licm>`, PRE, etc. 967 968For example: 4 + (x + 5) ⇒ x + (4 + 5) 969 970In the implementation of this algorithm, constants are assigned rank = 0, 971function arguments are rank = 1, and other values are assigned ranks 972corresponding to the reverse post order traversal of current function (starting 973at 2), which effectively gives values in deep loops higher rank than values not 974in loops. 975 976``-rel-lookup-table-converter``: Relative lookup table converter 977---------------------------------------------------------------- 978 979This pass converts lookup tables to PIC-friendly relative lookup tables. 980 981``-reg2mem``: Demote all values to stack slots 982---------------------------------------------- 983 984This file demotes all registers to memory references. It is intended to be the 985inverse of :ref:`mem2reg <passes-mem2reg>`. By converting to ``load`` 986instructions, the only values live across basic blocks are ``alloca`` 987instructions and ``load`` instructions before ``phi`` nodes. It is intended 988that this should make CFG hacking much easier. To make later hacking easier, 989the entry block is split into two, such that all introduced ``alloca`` 990instructions (and nothing else) are in the entry block. 991 992``-sroa``: Scalar Replacement of Aggregates 993------------------------------------------------------ 994 995The well-known scalar replacement of aggregates transformation. This transform 996breaks up ``alloca`` instructions of aggregate type (structure or array) into 997individual ``alloca`` instructions for each member if possible. Then, if 998possible, it transforms the individual ``alloca`` instructions into nice clean 999scalar SSA form. 1000 1001.. _passes-sccp: 1002 1003``-sccp``: Sparse Conditional Constant Propagation 1004-------------------------------------------------- 1005 1006Sparse conditional constant propagation and merging, which can be summarized 1007as: 1008 1009* Assumes values are constant unless proven otherwise 1010* Assumes BasicBlocks are dead unless proven otherwise 1011* Proves values to be constant, and replaces them with constants 1012* Proves conditional branches to be unconditional 1013 1014Note that this pass has a habit of making definitions be dead. It is a good 1015idea to run a :ref:`DCE <passes-dce>` pass sometime after running this pass. 1016 1017.. _passes-simplifycfg: 1018 1019``-simplifycfg``: Simplify the CFG 1020---------------------------------- 1021 1022Performs dead code elimination and basic block merging. Specifically: 1023 1024* Removes basic blocks with no predecessors. 1025* Merges a basic block into its predecessor if there is only one and the 1026 predecessor only has one successor. 1027* Eliminates PHI nodes for basic blocks with a single predecessor. 1028* Eliminates a basic block that only contains an unconditional branch. 1029 1030``-sink``: Code sinking 1031----------------------- 1032 1033This pass moves instructions into successor blocks, when possible, so that they 1034aren't executed on paths where their results aren't needed. 1035 1036``-strip``: Strip all symbols from a module 1037------------------------------------------- 1038 1039Performs code stripping. This transformation can delete: 1040 1041* names for virtual registers 1042* symbols for internal globals and functions 1043* debug information 1044 1045Note that this transformation makes code much less readable, so it should only 1046be used in situations where the strip utility would be used, such as reducing 1047code size or making it harder to reverse engineer code. 1048 1049``-strip-dead-debug-info``: Strip debug info for unused symbols 1050--------------------------------------------------------------- 1051 1052.. FIXME: this description is the same as for -strip 1053 1054performs code stripping. this transformation can delete: 1055 1056* names for virtual registers 1057* symbols for internal globals and functions 1058* debug information 1059 1060note that this transformation makes code much less readable, so it should only 1061be used in situations where the strip utility would be used, such as reducing 1062code size or making it harder to reverse engineer code. 1063 1064``-strip-dead-prototypes``: Strip Unused Function Prototypes 1065------------------------------------------------------------ 1066 1067This pass loops over all of the functions in the input module, looking for dead 1068declarations and removes them. Dead declarations are declarations of functions 1069for which no implementation is available (i.e., declarations for unused library 1070functions). 1071 1072``-strip-debug-declare``: Strip all ``llvm.dbg.declare`` intrinsics 1073------------------------------------------------------------------- 1074 1075.. FIXME: this description is the same as for -strip 1076 1077This pass implements code stripping. Specifically, it can delete: 1078 1079#. names for virtual registers 1080#. symbols for internal globals and functions 1081#. debug information 1082 1083Note that this transformation makes code much less readable, so it should only 1084be used in situations where the 'strip' utility would be used, such as reducing 1085code size or making it harder to reverse engineer code. 1086 1087``-strip-nondebug``: Strip all symbols, except dbg symbols, from a module 1088------------------------------------------------------------------------- 1089 1090.. FIXME: this description is the same as for -strip 1091 1092This pass implements code stripping. Specifically, it can delete: 1093 1094#. names for virtual registers 1095#. symbols for internal globals and functions 1096#. debug information 1097 1098Note that this transformation makes code much less readable, so it should only 1099be used in situations where the 'strip' utility would be used, such as reducing 1100code size or making it harder to reverse engineer code. 1101 1102``-tailcallelim``: Tail Call Elimination 1103---------------------------------------- 1104 1105This file transforms calls of the current function (self recursion) followed by 1106a return instruction with a branch to the entry of the function, creating a 1107loop. This pass also implements the following extensions to the basic 1108algorithm: 1109 1110#. Trivial instructions between the call and return do not prevent the 1111 transformation from taking place, though currently the analysis cannot 1112 support moving any really useful instructions (only dead ones). 1113#. This pass transforms functions that are prevented from being tail recursive 1114 by an associative expression to use an accumulator variable, thus compiling 1115 the typical naive factorial or fib implementation into efficient code. 1116#. TRE is performed if the function returns void, if the return returns the 1117 result returned by the call, or if the function returns a run-time constant 1118 on all exits from the function. It is possible, though unlikely, that the 1119 return returns something else (like constant 0), and can still be TRE'd. It 1120 can be TRE'd if *all other* return instructions in the function return the 1121 exact same value. 1122#. If it can prove that callees do not access their caller stack frame, they 1123 are marked as eligible for tail call elimination (by the code generator). 1124 1125Utility Passes 1126============== 1127 1128This section describes the LLVM Utility Passes. 1129 1130``-deadarghaX0r``: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE) 1131------------------------------------------------------------------------ 1132 1133Same as dead argument elimination, but deletes arguments to functions which are 1134external. This is only for use by :doc:`bugpoint <Bugpoint>`. 1135 1136``-extract-blocks``: Extract Basic Blocks From Module (for bugpoint use) 1137------------------------------------------------------------------------ 1138 1139This pass is used by bugpoint to extract all blocks from the module into their 1140own functions. 1141 1142``-instnamer``: Assign names to anonymous instructions 1143------------------------------------------------------ 1144 1145This is a little utility pass that gives instructions names, this is mostly 1146useful when diffing the effect of an optimization because deleting an unnamed 1147instruction can change all other instruction numbering, making the diff very 1148noisy. 1149 1150.. _passes-verify: 1151 1152``-verify``: Module Verifier 1153---------------------------- 1154 1155Verifies an LLVM IR code. This is useful to run after an optimization which is 1156undergoing testing. Note that llvm-as verifies its input before emitting 1157bitcode, and also that malformed bitcode is likely to make LLVM crash. All 1158language front-ends are therefore encouraged to verify their output before 1159performing optimizing transformations. 1160 1161#. Both of a binary operator's parameters are of the same type. 1162#. Verify that the indices of mem access instructions match other operands. 1163#. Verify that arithmetic and other things are only performed on first-class 1164 types. Verify that shifts and logicals only happen on integrals f.e. 1165#. All of the constants in a switch statement are of the correct type. 1166#. The code is in valid SSA form. 1167#. It is illegal to put a label into any other type (like a structure) or to 1168 return one. 1169#. Only phi nodes can be self referential: ``%x = add i32 %x``, ``%x`` is 1170 invalid. 1171#. PHI nodes must have an entry for each predecessor, with no extras. 1172#. PHI nodes must be the first thing in a basic block, all grouped together. 1173#. PHI nodes must have at least one entry. 1174#. All basic blocks should only end with terminator insts, not contain them. 1175#. The entry node to a function must not have predecessors. 1176#. All Instructions must be embedded into a basic block. 1177#. Functions cannot take a void-typed parameter. 1178#. Verify that a function's argument list agrees with its declared type. 1179#. It is illegal to specify a name for a void value. 1180#. It is illegal to have an internal global value with no initializer. 1181#. It is illegal to have a ``ret`` instruction that returns a value that does 1182 not agree with the function return value type. 1183#. Function call argument types match the function prototype. 1184#. All other things that are tested by asserts spread about the code. 1185 1186Note that this does not provide full security verification (like Java), but 1187instead just tries to ensure that code is well-formed. 1188 1189.. _passes-view-cfg: 1190 1191``-view-cfg``: View CFG of function 1192----------------------------------- 1193 1194Displays the control flow graph using the GraphViz tool. 1195 1196``-view-cfg-only``: View CFG of function (with no function bodies) 1197------------------------------------------------------------------ 1198 1199Displays the control flow graph using the GraphViz tool, but omitting function 1200bodies. 1201 1202``-view-dom``: View dominance tree of function 1203---------------------------------------------- 1204 1205Displays the dominator tree using the GraphViz tool. 1206 1207``-view-dom-only``: View dominance tree of function (with no function bodies) 1208----------------------------------------------------------------------------- 1209 1210Displays the dominator tree using the GraphViz tool, but omitting function 1211bodies. 1212 1213``-view-postdom``: View postdominance tree of function 1214------------------------------------------------------ 1215 1216Displays the post dominator tree using the GraphViz tool. 1217 1218``-view-postdom-only``: View postdominance tree of function (with no function bodies) 1219------------------------------------------------------------------------------------- 1220 1221Displays the post dominator tree using the GraphViz tool, but omitting function 1222bodies. 1223 1224``-transform-warning``: Report missed forced transformations 1225------------------------------------------------------------ 1226 1227Emits warnings about not yet applied forced transformations (e.g. from 1228``#pragma omp simd``). 1229