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 ==================================== 31 LLVM's Analysis and Transform Passes 32 ==================================== 33 34 .. contents:: 35 :local: 36 37 Introduction 38 ============ 39 40 This document serves as a high level summary of the optimization features that 41 LLVM provides. Optimizations are implemented as Passes that traverse some 42 portion of a program to either collect information or transform the program. 43 The table below divides the passes that LLVM provides into three categories. 44 Analysis passes compute information that other passes can use or for debugging 45 or program visualization purposes. Transform passes can use (or invalidate) 46 the analysis passes. Transform passes all mutate the program in some way. 47 Utility passes provides some utility but don't otherwise fit categorization. 48 For example passes to extract functions to bitcode or write a module to bitcode 49 are neither analysis nor transform passes. The table of contents above 50 provides a quick summary of each pass and links to the more complete pass 51 description later in the document. 52 53 Analysis Passes 54 =============== 55 56 This section describes the LLVM Analysis Passes. 57 58 ``-aa-eval``: Exhaustive Alias Analysis Precision Evaluator 59 ----------------------------------------------------------- 60 61 This is a simple N^2 alias analysis accuracy evaluator. Basically, for each 62 function in the program, it simply queries to see how the alias analysis 63 implementation answers alias queries between each pair of pointers in the 64 function. 65 66 This is inspired and adapted from code by: Naveen Neelakantam, Francesco 67 Spadini, and Wojciech Stryjewski. 68 69 ``-basicaa``: Basic Alias Analysis (stateless AA impl) 70 ------------------------------------------------------ 71 72 A basic alias analysis pass that implements identities (two different globals 73 cannot alias, etc), but does no stateful analysis. 74 75 ``-basiccg``: Basic CallGraph Construction 76 ------------------------------------------ 77 78 Yet to be written. 79 80 ``-count-aa``: Count Alias Analysis Query Responses 81 --------------------------------------------------- 82 83 A pass which can be used to count how many alias queries are being made and how 84 the alias analysis implementation being used responds. 85 86 ``-da``: Dependence Analysis 87 ---------------------------- 88 89 Dependence analysis framework, which is used to detect dependences in memory 90 accesses. 91 92 ``-debug-aa``: AA use debugger 93 ------------------------------ 94 95 This simple pass checks alias analysis users to ensure that if they create a 96 new value, they do not query AA without informing it of the value. It acts as 97 a shim over any other AA pass you want. 98 99 Yes keeping track of every value in the program is expensive, but this is a 100 debugging pass. 101 102 ``-domfrontier``: Dominance Frontier Construction 103 ------------------------------------------------- 104 105 This pass is a simple dominator construction algorithm for finding forward 106 dominator frontiers. 107 108 ``-domtree``: Dominator Tree Construction 109 ----------------------------------------- 110 111 This pass is a simple dominator construction algorithm for finding forward 112 dominators. 113 114 115 ``-dot-callgraph``: Print Call Graph to "dot" file 116 -------------------------------------------------- 117 118 This pass, only available in ``opt``, prints the call graph into a ``.dot`` 119 graph. This graph can then be processed with the "dot" tool to convert it to 120 postscript or some other suitable format. 121 122 ``-dot-cfg``: Print CFG of function to "dot" file 123 ------------------------------------------------- 124 125 This 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 127 to 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 -------------------------------------------------------------------------------- 131 132 This 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 134 with the :program:`dot` tool to convert it to postscript or some other suitable 135 format. 136 137 ``-dot-dom``: Print dominance tree of function to "dot" file 138 ------------------------------------------------------------ 139 140 This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` 141 graph. This graph can then be processed with the :program:`dot` tool to 142 convert 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 ------------------------------------------------------------------------------------------- 146 147 This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` 148 graph, 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 -------------------------------------------------------------------- 153 154 This 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 156 to 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 --------------------------------------------------------------------------------------------------- 160 161 This 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 163 with the :program:`dot` tool to convert it to postscript or some other suitable 164 format. 165 166 ``-globalsmodref-aa``: Simple mod/ref analysis for globals 167 ---------------------------------------------------------- 168 169 This simple pass provides alias and mod/ref information for global values that 170 do not have their address taken, and keeps track of whether functions read or 171 write memory (are "pure"). For this simple (but very common) case, we can 172 provide pretty accurate and useful information. 173 174 ``-instcount``: Counts the various types of ``Instruction``\ s 175 -------------------------------------------------------------- 176 177 This pass collects the count of all instructions and reports them. 178 179 ``-intervals``: Interval Partition Construction 180 ----------------------------------------------- 181 182 This analysis calculates and represents the interval partition of a function, 183 or a preexisting interval partition. 184 185 In this way, the interval partition may be used to reduce a flow graph down to 186 its degenerate single node interval partition (unless it is irreducible). 187 188 ``-iv-users``: Induction Variable Users 189 --------------------------------------- 190 191 Bookkeeping for "interesting" users of expressions computed from induction 192 variables. 193 194 ``-lazy-value-info``: Lazy Value Information Analysis 195 ----------------------------------------------------- 196 197 Interface for lazy computation of value constraint information. 198 199 ``-libcall-aa``: LibCall Alias Analysis 200 --------------------------------------- 201 202 LibCall Alias Analysis. 203 204 ``-lint``: Statically lint-checks LLVM IR 205 ----------------------------------------- 206 207 This pass statically checks for common and easily-identified constructs which 208 produce undefined or likely unintended behavior in LLVM IR. 209 210 It is not a guarantee of correctness, in two ways. First, it isn't 211 comprehensive. There are checks which could be done statically which are not 212 yet implemented. Some of these are indicated by TODO comments, but those 213 aren't comprehensive either. Second, many conditions cannot be checked 214 statically. This pass does no dynamic instrumentation, so it can't check for 215 all possible problems. 216 217 Another limitation is that it assumes all code will be executed. A store 218 through a null pointer in a basic block which is never reached is harmless, but 219 this pass will warn about it anyway. 220 221 Optimization passes may make conditions that this pass checks for more or less 222 obvious. If an optimization pass appears to be introducing a warning, it may 223 be that the optimization pass is merely exposing an existing condition in the 224 code. 225 226 This code may be run before :ref:`instcombine <passes-instcombine>`. In many 227 cases, instcombine checks for the same kinds of things and turns instructions 228 with undefined behavior into unreachable (or equivalent). Because of this, 229 this pass makes some effort to look through bitcasts and so on. 230 231 ``-loops``: Natural Loop Information 232 ------------------------------------ 233 234 This analysis is used to identify natural loops and determine the loop depth of 235 various nodes of the CFG. Note that the loops identified may actually be 236 several natural loops that share the same header node... not just a single 237 natural loop. 238 239 ``-memdep``: Memory Dependence Analysis 240 --------------------------------------- 241 242 An analysis that determines, for a given memory operation, what preceding 243 memory operations it depends on. It builds on alias analysis information, and 244 tries to provide a lazy, caching interface to a common kind of alias 245 information query. 246 247 ``-module-debuginfo``: Decodes module-level debug info 248 ------------------------------------------------------ 249 250 This pass decodes the debug info metadata in a module and prints in a 251 (sufficiently-prepared-) human-readable form. 252 253 For example, run this pass from ``opt`` along with the ``-analyze`` option, and 254 it'll print to standard output. 255 256 ``-no-aa``: No Alias Analysis (always returns 'may' alias) 257 ---------------------------------------------------------- 258 259 This is the default implementation of the Alias Analysis interface. It always 260 returns "I don't know" for alias queries. NoAA is unlike other alias analysis 261 implementations, in that it does not chain to a previous analysis. As such it 262 doesn't follow many of the rules that other alias analyses must. 263 264 ``-no-profile``: No Profile Information 265 --------------------------------------- 266 267 The default "no profile" implementation of the abstract ``ProfileInfo`` 268 interface. 269 270 ``-postdomfrontier``: Post-Dominance Frontier Construction 271 ---------------------------------------------------------- 272 273 This pass is a simple post-dominator construction algorithm for finding 274 post-dominator frontiers. 275 276 ``-postdomtree``: Post-Dominator Tree Construction 277 -------------------------------------------------- 278 279 This pass is a simple post-dominator construction algorithm for finding 280 post-dominators. 281 282 ``-print-alias-sets``: Alias Set Printer 283 ---------------------------------------- 284 285 Yet to be written. 286 287 ``-print-callgraph``: Print a call graph 288 ---------------------------------------- 289 290 This pass, only available in ``opt``, prints the call graph to standard error 291 in a human-readable form. 292 293 ``-print-callgraph-sccs``: Print SCCs of the Call Graph 294 ------------------------------------------------------- 295 296 This pass, only available in ``opt``, prints the SCCs of the call graph to 297 standard error in a human-readable form. 298 299 ``-print-cfg-sccs``: Print SCCs of each function CFG 300 ---------------------------------------------------- 301 302 This pass, only available in ``opt``, printsthe SCCs of each function CFG to 303 standard error in a human-readable fom. 304 305 ``-print-dbginfo``: Print debug info in human readable form 306 ----------------------------------------------------------- 307 308 Pass that prints instructions, and associated debug info: 309 310 #. source/line/col information 311 #. original variable name 312 #. original type name 313 314 ``-print-dom-info``: Dominator Info Printer 315 ------------------------------------------- 316 317 Dominator Info Printer. 318 319 ``-print-externalfnconstants``: Print external fn callsites passed constants 320 ---------------------------------------------------------------------------- 321 322 This pass, only available in ``opt``, prints out call sites to external 323 functions that are called with constant arguments. This can be useful when 324 looking for standard library functions we should constant fold or handle in 325 alias analyses. 326 327 ``-print-function``: Print function to stderr 328 --------------------------------------------- 329 330 The ``PrintFunctionPass`` class is designed to be pipelined with other 331 ``FunctionPasses``, and prints out the functions of the module as they are 332 processed. 333 334 ``-print-module``: Print module to stderr 335 ----------------------------------------- 336 337 This pass simply prints out the entire module when it is executed. 338 339 .. _passes-print-used-types: 340 341 ``-print-used-types``: Find Used Types 342 -------------------------------------- 343 344 This pass is used to seek out all of the types in use by the program. Note 345 that this analysis explicitly does not include types only used by the symbol 346 table. 347 348 ``-profile-estimator``: Estimate profiling information 349 ------------------------------------------------------ 350 351 Profiling information that estimates the profiling information in a very crude 352 and unimaginative way. 353 354 ``-profile-loader``: Load profile information from ``llvmprof.out`` 355 ------------------------------------------------------------------- 356 357 A concrete implementation of profiling information that loads the information 358 from a profile dump file. 359 360 ``-profile-verifier``: Verify profiling information 361 --------------------------------------------------- 362 363 Pass that checks profiling information for plausibility. 364 365 ``-regions``: Detect single entry single exit regions 366 ----------------------------------------------------- 367 368 The ``RegionInfo`` pass detects single entry single exit regions in a function, 369 where a region is defined as any subgraph that is connected to the remaining 370 graph at only two spots. Furthermore, an hierarchical region tree is built. 371 372 ``-scalar-evolution``: Scalar Evolution Analysis 373 ------------------------------------------------ 374 375 The ``ScalarEvolution`` analysis can be used to analyze and catagorize scalar 376 expressions in loops. It specializes in recognizing general induction 377 variables, representing them with the abstract and opaque ``SCEV`` class. 378 Given this analysis, trip counts of loops and other important properties can be 379 obtained. 380 381 This analysis is primarily useful for induction variable substitution and 382 strength reduction. 383 384 ``-scev-aa``: ScalarEvolution-based Alias Analysis 385 -------------------------------------------------- 386 387 Simple alias analysis implemented in terms of ``ScalarEvolution`` queries. 388 389 This differs from traditional loop dependence analysis in that it tests for 390 dependencies within a single iteration of a loop, rather than dependencies 391 between different iterations. 392 393 ``ScalarEvolution`` has a more complete understanding of pointer arithmetic 394 than ``BasicAliasAnalysis``' collection of ad-hoc analyses. 395 396 ``-targetdata``: Target Data Layout 397 ----------------------------------- 398 399 Provides other passes access to information on how the size and alignment 400 required by the target ABI for various data types. 401 402 Transform Passes 403 ================ 404 405 This section describes the LLVM Transform Passes. 406 407 ``-adce``: Aggressive Dead Code Elimination 408 ------------------------------------------- 409 410 ADCE aggressively tries to eliminate code. This pass is similar to :ref:`DCE 411 <passes-dce>` but it assumes that values are dead until proven otherwise. This 412 is similar to :ref:`SCCP <passes-sccp>`, except applied to the liveness of 413 values. 414 415 ``-always-inline``: Inliner for ``always_inline`` functions 416 ----------------------------------------------------------- 417 418 A custom inliner that handles only functions that are marked as "always 419 inline". 420 421 ``-argpromotion``: Promote 'by reference' arguments to scalars 422 -------------------------------------------------------------- 423 424 This pass promotes "by reference" arguments to be "by value" arguments. In 425 practice, this means looking for internal functions that have pointer 426 arguments. If it can prove, through the use of alias analysis, that an 427 argument is *only* loaded, then it can pass the value into the function instead 428 of the address of the value. This can cause recursive simplification of code 429 and lead to the elimination of allocas (especially in C++ template code like 430 the STL). 431 432 This pass also handles aggregate arguments that are passed into a function, 433 scalarizing them if the elements of the aggregate are only loaded. Note that 434 it refuses to scalarize aggregates which would require passing in more than 435 three operands to the function, because passing thousands of operands for a 436 large array or structure is unprofitable! 437 438 Note that this transformation could also be done for arguments that are only 439 stored to (returning the value instead), but does not currently. This case 440 would be best handled when and if LLVM starts supporting multiple return values 441 from functions. 442 443 ``-bb-vectorize``: Basic-Block Vectorization 444 -------------------------------------------- 445 446 This pass combines instructions inside basic blocks to form vector 447 instructions. It iterates over each basic block, attempting to pair compatible 448 instructions, repeating this process until no additional pairs are selected for 449 vectorization. When the outputs of some pair of compatible instructions are 450 used as inputs by some other pair of compatible instructions, those pairs are 451 part of a potential vectorization chain. Instruction pairs are only fused into 452 vector instructions when they are part of a chain longer than some threshold 453 length. Moreover, the pass attempts to find the best possible chain for each 454 pair of compatible instructions. These heuristics are intended to prevent 455 vectorization in cases where it would not yield a performance increase of the 456 resulting code. 457 458 ``-block-placement``: Profile Guided Basic Block Placement 459 ---------------------------------------------------------- 460 461 This pass is a very simple profile guided basic block placement algorithm. The 462 idea is to put frequently executed blocks together at the start of the function 463 and hopefully increase the number of fall-through conditional branches. If 464 there is no profile information for a particular function, this pass basically 465 orders blocks in depth-first order. 466 467 ``-break-crit-edges``: Break critical edges in CFG 468 -------------------------------------------------- 469 470 Break all of the critical edges in the CFG by inserting a dummy basic block. 471 It may be "required" by passes that cannot deal with critical edges. This 472 transformation obviously invalidates the CFG, but can update forward dominator 473 (set, immediate dominators, tree, and frontier) information. 474 475 ``-codegenprepare``: Optimize for code generation 476 ------------------------------------------------- 477 478 This pass munges the code in the input function to better prepare it for 479 SelectionDAG-based code generation. This works around limitations in it's 480 basic-block-at-a-time approach. It should eventually be removed. 481 482 ``-constmerge``: Merge Duplicate Global Constants 483 ------------------------------------------------- 484 485 Merges duplicate global constants together into a single constant that is 486 shared. This is useful because some passes (i.e., TraceValues) insert a lot of 487 string constants into the program, regardless of whether or not an existing 488 string is available. 489 490 ``-constprop``: Simple constant propagation 491 ------------------------------------------- 492 493 This file implements constant propagation and merging. It looks for 494 instructions involving only constant operands and replaces them with a constant 495 value instead of an instruction. For example: 496 497 .. code-block:: llvm 498 499 add i32 1, 2 500 501 becomes 502 503 .. code-block:: llvm 504 505 i32 3 506 507 NOTE: this pass has a habit of making definitions be dead. It is a good idea 508 to to run a :ref:`Dead Instruction Elimination <passes-die>` pass sometime 509 after running this pass. 510 511 .. _passes-dce: 512 513 ``-dce``: Dead Code Elimination 514 ------------------------------- 515 516 Dead code elimination is similar to :ref:`dead instruction elimination 517 <passes-die>`, but it rechecks instructions that were used by removed 518 instructions to see if they are newly dead. 519 520 ``-deadargelim``: Dead Argument Elimination 521 ------------------------------------------- 522 523 This pass deletes dead arguments from internal functions. Dead argument 524 elimination removes arguments which are directly dead, as well as arguments 525 only passed into function calls as dead arguments of other functions. This 526 pass also deletes dead arguments in a similar way. 527 528 This pass is often useful as a cleanup pass to run after aggressive 529 interprocedural passes, which add possibly-dead arguments. 530 531 ``-deadtypeelim``: Dead Type Elimination 532 ---------------------------------------- 533 534 This pass is used to cleanup the output of GCC. It eliminate names for types 535 that are unused in the entire translation unit, using the :ref:`find used types 536 <passes-print-used-types>` pass. 537 538 .. _passes-die: 539 540 ``-die``: Dead Instruction Elimination 541 -------------------------------------- 542 543 Dead instruction elimination performs a single pass over the function, removing 544 instructions that are obviously dead. 545 546 ``-dse``: Dead Store Elimination 547 -------------------------------- 548 549 A trivial dead store elimination that only considers basic-block local 550 redundant stores. 551 552 ``-functionattrs``: Deduce function attributes 553 ---------------------------------------------- 554 555 A simple interprocedural pass which walks the call-graph, looking for functions 556 which do not access or only read non-local memory, and marking them 557 ``readnone``/``readonly``. In addition, it marks function arguments (of 558 pointer type) "``nocapture``" if a call to the function does not create any 559 copies of the pointer value that outlive the call. This more or less means 560 that the pointer is only dereferenced, and not returned from the function or 561 stored in a global. This pass is implemented as a bottom-up traversal of the 562 call-graph. 563 564 ``-globaldce``: Dead Global Elimination 565 --------------------------------------- 566 567 This transform is designed to eliminate unreachable internal globals from the 568 program. It uses an aggressive algorithm, searching out globals that are known 569 to be alive. After it finds all of the globals which are needed, it deletes 570 whatever is left over. This allows it to delete recursive chunks of the 571 program which are unreachable. 572 573 ``-globalopt``: Global Variable Optimizer 574 ----------------------------------------- 575 576 This pass transforms simple global variables that never have their address 577 taken. If obviously true, it marks read/write globals as constant, deletes 578 variables only stored to, etc. 579 580 ``-gvn``: Global Value Numbering 581 -------------------------------- 582 583 This pass performs global value numbering to eliminate fully and partially 584 redundant instructions. It also performs redundant load elimination. 585 586 .. _passes-indvars: 587 588 ``-indvars``: Canonicalize Induction Variables 589 ---------------------------------------------- 590 591 This transformation analyzes and transforms the induction variables (and 592 computations derived from them) into simpler forms suitable for subsequent 593 analysis and transformation. 594 595 This transformation makes the following changes to each loop with an 596 identifiable induction variable: 597 598 * All loops are transformed to have a *single* canonical induction variable 599 which starts at zero and steps by one. 600 * The canonical induction variable is guaranteed to be the first PHI node in 601 the loop header block. 602 * Any pointer arithmetic recurrences are raised to use array subscripts. 603 604 If the trip count of a loop is computable, this pass also makes the following 605 changes: 606 607 * The exit condition for the loop is canonicalized to compare the induction 608 value against the exit value. This turns loops like: 609 610 .. code-block:: c++ 611 612 for (i = 7; i*i < 1000; ++i) 613 614 into 615 616 .. code-block:: c++ 617 618 for (i = 0; i != 25; ++i) 619 620 * Any use outside of the loop of an expression derived from the indvar is 621 changed to compute the derived value outside of the loop, eliminating the 622 dependence on the exit value of the induction variable. If the only purpose 623 of the loop is to compute the exit value of some derived expression, this 624 transformation will make the loop dead. 625 626 This transformation should be followed by strength reduction after all of the 627 desired loop transformations have been performed. Additionally, on targets 628 where it is profitable, the loop could be transformed to count down to zero 629 (the "do loop" optimization). 630 631 ``-inline``: Function Integration/Inlining 632 ------------------------------------------ 633 634 Bottom-up inlining of functions into callees. 635 636 ``-insert-edge-profiling``: Insert instrumentation for edge profiling 637 --------------------------------------------------------------------- 638 639 This pass instruments the specified program with counters for edge profiling. 640 Edge profiling can give a reasonable approximation of the hot paths through a 641 program, and is used for a wide variety of program transformations. 642 643 Note that this implementation is very nave. It inserts a counter for *every* 644 edge in the program, instead of using control flow information to prune the 645 number of counters inserted. 646 647 ``-insert-optimal-edge-profiling``: Insert optimal instrumentation for edge profiling 648 ------------------------------------------------------------------------------------- 649 650 This pass instruments the specified program with counters for edge profiling. 651 Edge profiling can give a reasonable approximation of the hot paths through a 652 program, and is used for a wide variety of program transformations. 653 654 .. _passes-instcombine: 655 656 ``-instcombine``: Combine redundant instructions 657 ------------------------------------------------ 658 659 Combine instructions to form fewer, simple instructions. This pass does not 660 modify the CFG This pass is where algebraic simplification happens. 661 662 This pass combines things like: 663 664 .. code-block:: llvm 665 666 %Y = add i32 %X, 1 667 %Z = add i32 %Y, 1 668 669 into: 670 671 .. code-block:: llvm 672 673 %Z = add i32 %X, 2 674 675 This is a simple worklist driven algorithm. 676 677 This pass guarantees that the following canonicalizations are performed on the 678 program: 679 680 #. If a binary operator has a constant operand, it is moved to the right-hand 681 side. 682 #. Bitwise operators with constant operands are always grouped so that shifts 683 are performed first, then ``or``\ s, then ``and``\ s, then ``xor``\ s. 684 #. Compare instructions are converted from ``<``, ``>``, ````, or ```` to 685 ``=`` or ```` if possible. 686 #. All ``cmp`` instructions on boolean values are replaced with logical 687 operations. 688 #. ``add X, X`` is represented as ``mul X, 2`` ``shl X, 1`` 689 #. Multiplies with a constant power-of-two argument are transformed into 690 shifts. 691 #. etc. 692 693 ``-internalize``: Internalize Global Symbols 694 -------------------------------------------- 695 696 This pass loops over all of the functions in the input module, looking for a 697 main function. If a main function is found, all other functions and all global 698 variables with initializers are marked as internal. 699 700 ``-ipconstprop``: Interprocedural constant propagation 701 ------------------------------------------------------ 702 703 This pass implements an *extremely* simple interprocedural constant propagation 704 pass. It could certainly be improved in many different ways, like using a 705 worklist. This pass makes arguments dead, but does not remove them. The 706 existing dead argument elimination pass should be run after this to clean up 707 the mess. 708 709 ``-ipsccp``: Interprocedural Sparse Conditional Constant Propagation 710 -------------------------------------------------------------------- 711 712 An interprocedural variant of :ref:`Sparse Conditional Constant Propagation 713 <passes-sccp>`. 714 715 ``-jump-threading``: Jump Threading 716 ----------------------------------- 717 718 Jump threading tries to find distinct threads of control flow running through a 719 basic block. This pass looks at blocks that have multiple predecessors and 720 multiple successors. If one or more of the predecessors of the block can be 721 proven to always cause a jump to one of the successors, we forward the edge 722 from the predecessor to the successor by duplicating the contents of this 723 block. 724 725 An example of when this can occur is code like this: 726 727 .. code-block:: c++ 728 729 if () { ... 730 X = 4; 731 } 732 if (X < 3) { 733 734 In this case, the unconditional branch at the end of the first if can be 735 revectored to the false side of the second if. 736 737 ``-lcssa``: Loop-Closed SSA Form Pass 738 ------------------------------------- 739 740 This pass transforms loops by placing phi nodes at the end of the loops for all 741 values that are live across the loop boundary. For example, it turns the left 742 into the right code: 743 744 .. code-block:: c++ 745 746 for (...) for (...) 747 if (c) if (c) 748 X1 = ... X1 = ... 749 else else 750 X2 = ... X2 = ... 751 X3 = phi(X1, X2) X3 = phi(X1, X2) 752 ... = X3 + 4 X4 = phi(X3) 753 ... = X4 + 4 754 755 This is still valid LLVM; the extra phi nodes are purely redundant, and will be 756 trivially eliminated by ``InstCombine``. The major benefit of this 757 transformation is that it makes many other loop optimizations, such as 758 ``LoopUnswitch``\ ing, simpler. 759 760 .. _passes-licm: 761 762 ``-licm``: Loop Invariant Code Motion 763 ------------------------------------- 764 765 This pass performs loop invariant code motion, attempting to remove as much 766 code from the body of a loop as possible. It does this by either hoisting code 767 into the preheader block, or by sinking code to the exit blocks if it is safe. 768 This pass also promotes must-aliased memory locations in the loop to live in 769 registers, thus hoisting and sinking "invariant" loads and stores. 770 771 This pass uses alias analysis for two purposes: 772 773 #. Moving loop invariant loads and calls out of loops. If we can determine 774 that a load or call inside of a loop never aliases anything stored to, we 775 can hoist it or sink it like any other instruction. 776 777 #. Scalar Promotion of Memory. If there is a store instruction inside of the 778 loop, we try to move the store to happen AFTER the loop instead of inside of 779 the loop. This can only happen if a few conditions are true: 780 781 #. The pointer stored through is loop invariant. 782 #. There are no stores or loads in the loop which *may* alias the pointer. 783 There are no calls in the loop which mod/ref the pointer. 784 785 If these conditions are true, we can promote the loads and stores in the 786 loop of the pointer to use a temporary alloca'd variable. We then use the 787 :ref:`mem2reg <passes-mem2reg>` functionality to construct the appropriate 788 SSA form for the variable. 789 790 ``-loop-deletion``: Delete dead loops 791 ------------------------------------- 792 793 This file implements the Dead Loop Deletion Pass. This pass is responsible for 794 eliminating loops with non-infinite computable trip counts that have no side 795 effects or volatile instructions, and do not contribute to the computation of 796 the function's return value. 797 798 .. _passes-loop-extract: 799 800 ``-loop-extract``: Extract loops into new functions 801 --------------------------------------------------- 802 803 A pass wrapper around the ``ExtractLoop()`` scalar transformation to extract 804 each top-level loop into its own new function. If the loop is the *only* loop 805 in a given function, it is not touched. This is a pass most useful for 806 debugging via bugpoint. 807 808 ``-loop-extract-single``: Extract at most one loop into a new function 809 ---------------------------------------------------------------------- 810 811 Similar to :ref:`Extract loops into new functions <passes-loop-extract>`, this 812 pass extracts one natural loop from the program into a function if it can. 813 This is used by :program:`bugpoint`. 814 815 ``-loop-reduce``: Loop Strength Reduction 816 ----------------------------------------- 817 818 This pass performs a strength reduction on array references inside loops that 819 have as one or more of their components the loop induction variable. This is 820 accomplished by creating a new value to hold the initial value of the array 821 access for the first iteration, and then creating a new GEP instruction in the 822 loop to increment the value by the appropriate amount. 823 824 ``-loop-rotate``: Rotate Loops 825 ------------------------------ 826 827 A simple loop rotation transformation. 828 829 ``-loop-simplify``: Canonicalize natural loops 830 ---------------------------------------------- 831 832 This pass performs several transformations to transform natural loops into a 833 simpler form, which makes subsequent analyses and transformations simpler and 834 more effective. 835 836 Loop pre-header insertion guarantees that there is a single, non-critical entry 837 edge from outside of the loop to the loop header. This simplifies a number of 838 analyses and transformations, such as :ref:`LICM <passes-licm>`. 839 840 Loop exit-block insertion guarantees that all exit blocks from the loop (blocks 841 which are outside of the loop that have predecessors inside of the loop) only 842 have predecessors from inside of the loop (and are thus dominated by the loop 843 header). This simplifies transformations such as store-sinking that are built 844 into LICM. 845 846 This pass also guarantees that loops will have exactly one backedge. 847 848 Note that the :ref:`simplifycfg <passes-simplifycfg>` pass will clean up blocks 849 which are split out but end up being unnecessary, so usage of this pass should 850 not pessimize generated code. 851 852 This pass obviously modifies the CFG, but updates loop information and 853 dominator information. 854 855 ``-loop-unroll``: Unroll loops 856 ------------------------------ 857 858 This pass implements a simple loop unroller. It works best when loops have 859 been canonicalized by the :ref:`indvars <passes-indvars>` pass, allowing it to 860 determine the trip counts of loops easily. 861 862 ``-loop-unswitch``: Unswitch loops 863 ---------------------------------- 864 865 This pass transforms loops that contain branches on loop-invariant conditions 866 to have multiple loops. For example, it turns the left into the right code: 867 868 .. code-block:: c++ 869 870 for (...) if (lic) 871 A for (...) 872 if (lic) A; B; C 873 B else 874 C for (...) 875 A; C 876 877 This can increase the size of the code exponentially (doubling it every time a 878 loop is unswitched) so we only unswitch if the resultant code will be smaller 879 than a threshold. 880 881 This pass expects :ref:`LICM <passes-licm>` to be run before it to hoist 882 invariant conditions out of the loop, to make the unswitching opportunity 883 obvious. 884 885 ``-loweratomic``: Lower atomic intrinsics to non-atomic form 886 ------------------------------------------------------------ 887 888 This pass lowers atomic intrinsics to non-atomic form for use in a known 889 non-preemptible environment. 890 891 The pass does not verify that the environment is non-preemptible (in general 892 this would require knowledge of the entire call graph of the program including 893 any libraries which may not be available in bitcode form); it simply lowers 894 every atomic intrinsic. 895 896 ``-lowerinvoke``: Lower invoke and unwind, for unwindless code generators 897 ------------------------------------------------------------------------- 898 899 This transformation is designed for use by code generators which do not yet 900 support stack unwinding. This pass supports two models of exception handling 901 lowering, the "cheap" support and the "expensive" support. 902 903 "Cheap" exception handling support gives the program the ability to execute any 904 program which does not "throw an exception", by turning "``invoke``" 905 instructions into calls and by turning "``unwind``" instructions into calls to 906 ``abort()``. If the program does dynamically use the "``unwind``" instruction, 907 the program will print a message then abort. 908 909 "Expensive" exception handling support gives the full exception handling 910 support to the program at the cost of making the "``invoke``" instruction 911 really expensive. It basically inserts ``setjmp``/``longjmp`` calls to emulate 912 the exception handling as necessary. 913 914 Because the "expensive" support slows down programs a lot, and EH is only used 915 for a subset of the programs, it must be specifically enabled by the 916 ``-enable-correct-eh-support`` option. 917 918 Note that after this pass runs the CFG is not entirely accurate (exceptional 919 control flow edges are not correct anymore) so only very simple things should 920 be done after the ``lowerinvoke`` pass has run (like generation of native 921 code). This should not be used as a general purpose "my LLVM-to-LLVM pass 922 doesn't support the ``invoke`` instruction yet" lowering pass. 923 924 ``-lowerswitch``: Lower ``SwitchInst``\ s to branches 925 ----------------------------------------------------- 926 927 Rewrites switch instructions with a sequence of branches, which allows targets 928 to get away with not implementing the switch instruction until it is 929 convenient. 930 931 .. _passes-mem2reg: 932 933 ``-mem2reg``: Promote Memory to Register 934 ---------------------------------------- 935 936 This file promotes memory references to be register references. It promotes 937 alloca instructions which only have loads and stores as uses. An ``alloca`` is 938 transformed by using dominator frontiers to place phi nodes, then traversing 939 the function in depth-first order to rewrite loads and stores as appropriate. 940 This is just the standard SSA construction algorithm to construct "pruned" SSA 941 form. 942 943 ``-memcpyopt``: MemCpy Optimization 944 ----------------------------------- 945 946 This pass performs various transformations related to eliminating ``memcpy`` 947 calls, or transforming sets of stores into ``memset``\ s. 948 949 ``-mergefunc``: Merge Functions 950 ------------------------------- 951 952 This pass looks for equivalent functions that are mergable and folds them. 953 954 A hash is computed from the function, based on its type and number of basic 955 blocks. 956 957 Once all hashes are computed, we perform an expensive equality comparison on 958 each function pair. This takes n^2/2 comparisons per bucket, so it's important 959 that the hash function be high quality. The equality comparison iterates 960 through each instruction in each basic block. 961 962 When a match is found the functions are folded. If both functions are 963 overridable, we move the functionality into a new internal function and leave 964 two overridable thunks to it. 965 966 ``-mergereturn``: Unify function exit nodes 967 ------------------------------------------- 968 969 Ensure that functions have at most one ``ret`` instruction in them. 970 Additionally, it keeps track of which node is the new exit node of the CFG. 971 972 ``-partial-inliner``: Partial Inliner 973 ------------------------------------- 974 975 This pass performs partial inlining, typically by inlining an ``if`` statement 976 that surrounds the body of the function. 977 978 ``-prune-eh``: Remove unused exception handling info 979 ---------------------------------------------------- 980 981 This file implements a simple interprocedural pass which walks the call-graph, 982 turning invoke instructions into call instructions if and only if the callee 983 cannot throw an exception. It implements this as a bottom-up traversal of the 984 call-graph. 985 986 ``-reassociate``: Reassociate expressions 987 ----------------------------------------- 988 989 This pass reassociates commutative expressions in an order that is designed to 990 promote better constant propagation, GCSE, :ref:`LICM <passes-licm>`, PRE, etc. 991 992 For example: 4 + (x + 5) x + (4 + 5) 993 994 In the implementation of this algorithm, constants are assigned rank = 0, 995 function arguments are rank = 1, and other values are assigned ranks 996 corresponding to the reverse post order traversal of current function (starting 997 at 2), which effectively gives values in deep loops higher rank than values not 998 in loops. 999 1000 ``-reg2mem``: Demote all values to stack slots 1001 ---------------------------------------------- 1002 1003 This file demotes all registers to memory references. It is intended to be the 1004 inverse of :ref:`mem2reg <passes-mem2reg>`. By converting to ``load`` 1005 instructions, the only values live across basic blocks are ``alloca`` 1006 instructions and ``load`` instructions before ``phi`` nodes. It is intended 1007 that this should make CFG hacking much easier. To make later hacking easier, 1008 the entry block is split into two, such that all introduced ``alloca`` 1009 instructions (and nothing else) are in the entry block. 1010 1011 ``-scalarrepl``: Scalar Replacement of Aggregates (DT) 1012 ------------------------------------------------------ 1013 1014 The well-known scalar replacement of aggregates transformation. This transform 1015 breaks up ``alloca`` instructions of aggregate type (structure or array) into 1016 individual ``alloca`` instructions for each member if possible. Then, if 1017 possible, it transforms the individual ``alloca`` instructions into nice clean 1018 scalar SSA form. 1019 1020 This combines a simple scalar replacement of aggregates algorithm with the 1021 :ref:`mem2reg <passes-mem2reg>` algorithm because often interact, especially 1022 for C++ programs. As such, iterating between ``scalarrepl``, then 1023 :ref:`mem2reg <passes-mem2reg>` until we run out of things to promote works 1024 well. 1025 1026 .. _passes-sccp: 1027 1028 ``-sccp``: Sparse Conditional Constant Propagation 1029 -------------------------------------------------- 1030 1031 Sparse conditional constant propagation and merging, which can be summarized 1032 as: 1033 1034 * Assumes values are constant unless proven otherwise 1035 * Assumes BasicBlocks are dead unless proven otherwise 1036 * Proves values to be constant, and replaces them with constants 1037 * Proves conditional branches to be unconditional 1038 1039 Note that this pass has a habit of making definitions be dead. It is a good 1040 idea to to run a :ref:`DCE <passes-dce>` pass sometime after running this pass. 1041 1042 ``-simplify-libcalls``: Simplify well-known library calls 1043 --------------------------------------------------------- 1044 1045 Applies a variety of small optimizations for calls to specific well-known 1046 function calls (e.g. runtime library functions). For example, a call 1047 ``exit(3)`` that occurs within the ``main()`` function can be transformed into 1048 simply ``return 3``. 1049 1050 .. _passes-simplifycfg: 1051 1052 ``-simplifycfg``: Simplify the CFG 1053 ---------------------------------- 1054 1055 Performs dead code elimination and basic block merging. Specifically: 1056 1057 * Removes basic blocks with no predecessors. 1058 * Merges a basic block into its predecessor if there is only one and the 1059 predecessor only has one successor. 1060 * Eliminates PHI nodes for basic blocks with a single predecessor. 1061 * Eliminates a basic block that only contains an unconditional branch. 1062 1063 ``-sink``: Code sinking 1064 ----------------------- 1065 1066 This pass moves instructions into successor blocks, when possible, so that they 1067 aren't executed on paths where their results aren't needed. 1068 1069 ``-strip``: Strip all symbols from a module 1070 ------------------------------------------- 1071 1072 Performs code stripping. This transformation can delete: 1073 1074 * names for virtual registers 1075 * symbols for internal globals and functions 1076 * debug information 1077 1078 Note that this transformation makes code much less readable, so it should only 1079 be used in situations where the strip utility would be used, such as reducing 1080 code size or making it harder to reverse engineer code. 1081 1082 ``-strip-dead-debug-info``: Strip debug info for unused symbols 1083 --------------------------------------------------------------- 1084 1085 .. FIXME: this description is the same as for -strip 1086 1087 performs code stripping. this transformation can delete: 1088 1089 * names for virtual registers 1090 * symbols for internal globals and functions 1091 * debug information 1092 1093 note that this transformation makes code much less readable, so it should only 1094 be used in situations where the strip utility would be used, such as reducing 1095 code size or making it harder to reverse engineer code. 1096 1097 ``-strip-dead-prototypes``: Strip Unused Function Prototypes 1098 ------------------------------------------------------------ 1099 1100 This pass loops over all of the functions in the input module, looking for dead 1101 declarations and removes them. Dead declarations are declarations of functions 1102 for which no implementation is available (i.e., declarations for unused library 1103 functions). 1104 1105 ``-strip-debug-declare``: Strip all ``llvm.dbg.declare`` intrinsics 1106 ------------------------------------------------------------------- 1107 1108 .. FIXME: this description is the same as for -strip 1109 1110 This pass implements code stripping. Specifically, it can delete: 1111 1112 #. names for virtual registers 1113 #. symbols for internal globals and functions 1114 #. debug information 1115 1116 Note that this transformation makes code much less readable, so it should only 1117 be used in situations where the 'strip' utility would be used, such as reducing 1118 code size or making it harder to reverse engineer code. 1119 1120 ``-strip-nondebug``: Strip all symbols, except dbg symbols, from a module 1121 ------------------------------------------------------------------------- 1122 1123 .. FIXME: this description is the same as for -strip 1124 1125 This pass implements code stripping. Specifically, it can delete: 1126 1127 #. names for virtual registers 1128 #. symbols for internal globals and functions 1129 #. debug information 1130 1131 Note that this transformation makes code much less readable, so it should only 1132 be used in situations where the 'strip' utility would be used, such as reducing 1133 code size or making it harder to reverse engineer code. 1134 1135 ``-tailcallelim``: Tail Call Elimination 1136 ---------------------------------------- 1137 1138 This file transforms calls of the current function (self recursion) followed by 1139 a return instruction with a branch to the entry of the function, creating a 1140 loop. This pass also implements the following extensions to the basic 1141 algorithm: 1142 1143 #. Trivial instructions between the call and return do not prevent the 1144 transformation from taking place, though currently the analysis cannot 1145 support moving any really useful instructions (only dead ones). 1146 #. This pass transforms functions that are prevented from being tail recursive 1147 by an associative expression to use an accumulator variable, thus compiling 1148 the typical naive factorial or fib implementation into efficient code. 1149 #. TRE is performed if the function returns void, if the return returns the 1150 result returned by the call, or if the function returns a run-time constant 1151 on all exits from the function. It is possible, though unlikely, that the 1152 return returns something else (like constant 0), and can still be TRE'd. It 1153 can be TRE'd if *all other* return instructions in the function return the 1154 exact same value. 1155 #. If it can prove that callees do not access theier caller stack frame, they 1156 are marked as eligible for tail call elimination (by the code generator). 1157 1158 Utility Passes 1159 ============== 1160 1161 This section describes the LLVM Utility Passes. 1162 1163 ``-deadarghaX0r``: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE) 1164 ------------------------------------------------------------------------ 1165 1166 Same as dead argument elimination, but deletes arguments to functions which are 1167 external. This is only for use by :doc:`bugpoint <Bugpoint>`. 1168 1169 ``-extract-blocks``: Extract Basic Blocks From Module (for bugpoint use) 1170 ------------------------------------------------------------------------ 1171 1172 This pass is used by bugpoint to extract all blocks from the module into their 1173 own functions. 1174 1175 ``-instnamer``: Assign names to anonymous instructions 1176 ------------------------------------------------------ 1177 1178 This is a little utility pass that gives instructions names, this is mostly 1179 useful when diffing the effect of an optimization because deleting an unnamed 1180 instruction can change all other instruction numbering, making the diff very 1181 noisy. 1182 1183 ``-preverify``: Preliminary module verification 1184 ----------------------------------------------- 1185 1186 Ensures that the module is in the form required by the :ref:`Module Verifier 1187 <passes-verify>` pass. Running the verifier runs this pass automatically, so 1188 there should be no need to use it directly. 1189 1190 .. _passes-verify: 1191 1192 ``-verify``: Module Verifier 1193 ---------------------------- 1194 1195 Verifies an LLVM IR code. This is useful to run after an optimization which is 1196 undergoing testing. Note that llvm-as verifies its input before emitting 1197 bitcode, and also that malformed bitcode is likely to make LLVM crash. All 1198 language front-ends are therefore encouraged to verify their output before 1199 performing optimizing transformations. 1200 1201 #. Both of a binary operator's parameters are of the same type. 1202 #. Verify that the indices of mem access instructions match other operands. 1203 #. Verify that arithmetic and other things are only performed on first-class 1204 types. Verify that shifts and logicals only happen on integrals f.e. 1205 #. All of the constants in a switch statement are of the correct type. 1206 #. The code is in valid SSA form. 1207 #. It is illegal to put a label into any other type (like a structure) or to 1208 return one. 1209 #. Only phi nodes can be self referential: ``%x = add i32 %x``, ``%x`` is 1210 invalid. 1211 #. PHI nodes must have an entry for each predecessor, with no extras. 1212 #. PHI nodes must be the first thing in a basic block, all grouped together. 1213 #. PHI nodes must have at least one entry. 1214 #. All basic blocks should only end with terminator insts, not contain them. 1215 #. The entry node to a function must not have predecessors. 1216 #. All Instructions must be embedded into a basic block. 1217 #. Functions cannot take a void-typed parameter. 1218 #. Verify that a function's argument list agrees with its declared type. 1219 #. It is illegal to specify a name for a void value. 1220 #. It is illegal to have an internal global value with no initializer. 1221 #. It is illegal to have a ``ret`` instruction that returns a value that does 1222 not agree with the function return value type. 1223 #. Function call argument types match the function prototype. 1224 #. All other things that are tested by asserts spread about the code. 1225 1226 Note that this does not provide full security verification (like Java), but 1227 instead just tries to ensure that code is well-formed. 1228 1229 ``-view-cfg``: View CFG of function 1230 ----------------------------------- 1231 1232 Displays the control flow graph using the GraphViz tool. 1233 1234 ``-view-cfg-only``: View CFG of function (with no function bodies) 1235 ------------------------------------------------------------------ 1236 1237 Displays the control flow graph using the GraphViz tool, but omitting function 1238 bodies. 1239 1240 ``-view-dom``: View dominance tree of function 1241 ---------------------------------------------- 1242 1243 Displays the dominator tree using the GraphViz tool. 1244 1245 ``-view-dom-only``: View dominance tree of function (with no function bodies) 1246 ----------------------------------------------------------------------------- 1247 1248 Displays the dominator tree using the GraphViz tool, but omitting function 1249 bodies. 1250 1251 ``-view-postdom``: View postdominance tree of function 1252 ------------------------------------------------------ 1253 1254 Displays the post dominator tree using the GraphViz tool. 1255 1256 ``-view-postdom-only``: View postdominance tree of function (with no function bodies) 1257 ------------------------------------------------------------------------------------- 1258 1259 Displays the post dominator tree using the GraphViz tool, but omitting function 1260 bodies. 1261 1262
