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diff --git a/docs/Passes.rst b/docs/Passes.rst new file mode 100644 index 0000000000000..d279eca3afb65 --- /dev/null +++ b/docs/Passes.rst @@ -0,0 +1,1261 @@ +.. + If Passes.html is up to date, the following "one-liner" should print + an empty diff. + + egrep -e '^<tr><td><a href="#.*">-.*</a></td><td>.*</td></tr>$' \ + -e '^ <a name=".*">.*</a>$' < Passes.html >html; \ + perl >help <<'EOT' && diff -u help html; rm -f help html + open HTML, "<Passes.html" or die "open: Passes.html: $!\n"; + while (<HTML>) { + m:^<tr><td><a href="#(.*)">-.*</a></td><td>.*</td></tr>$: or next; + $order{$1} = sprintf("%03d", 1 + int %order); + } + open HELP, "../Release/bin/opt -help|" or die "open: opt -help: $!\n"; + while (<HELP>) { + m:^ -([^ ]+) +- (.*)$: or next; + my $o = $order{$1}; + $o = "000" unless defined $o; + push @x, "$o<tr><td><a href=\"#$1\">-$1</a></td><td>$2</td></tr>\n"; + push @y, "$o <a name=\"$1\">-$1: $2</a>\n"; + } + @x = map { s/^\d\d\d//; $_ } sort @x; + @y = map { s/^\d\d\d//; $_ } sort @y; + print @x, @y; + EOT + + This (real) one-liner can also be helpful when converting comments to HTML: + + perl -e '$/ = 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' + +==================================== +LLVM's Analysis and Transform Passes +==================================== + +.. contents:: + :local: + +Introduction +============ + +This document serves as a high level summary of the optimization features that +LLVM provides. Optimizations are implemented as Passes that traverse some +portion of a program to either collect information or transform the program. +The table below divides the passes that LLVM provides into three categories. +Analysis passes compute information that other passes can use or for debugging +or program visualization purposes. Transform passes can use (or invalidate) +the analysis passes. Transform passes all mutate the program in some way. +Utility passes provides some utility but don't otherwise fit categorization. +For example passes to extract functions to bitcode or write a module to bitcode +are neither analysis nor transform passes. The table of contents above +provides a quick summary of each pass and links to the more complete pass +description later in the document. + +Analysis Passes +=============== + +This section describes the LLVM Analysis Passes. + +``-aa-eval``: Exhaustive Alias Analysis Precision Evaluator +----------------------------------------------------------- + +This is a simple N^2 alias analysis accuracy evaluator. Basically, for each +function in the program, it simply queries to see how the alias analysis +implementation answers alias queries between each pair of pointers in the +function. + +This is inspired and adapted from code by: Naveen Neelakantam, Francesco +Spadini, and Wojciech Stryjewski. + +``-basicaa``: Basic Alias Analysis (stateless AA impl) +------------------------------------------------------ + +A basic alias analysis pass that implements identities (two different globals +cannot alias, etc), but does no stateful analysis. + +``-basiccg``: Basic CallGraph Construction +------------------------------------------ + +Yet to be written. + +``-count-aa``: Count Alias Analysis Query Responses +--------------------------------------------------- + +A pass which can be used to count how many alias queries are being made and how +the alias analysis implementation being used responds. + +``-da``: Dependence Analysis +---------------------------- + +Dependence analysis framework, which is used to detect dependences in memory +accesses. + +``-debug-aa``: AA use debugger +------------------------------ + +This simple pass checks alias analysis users to ensure that if they create a +new value, they do not query AA without informing it of the value. It acts as +a shim over any other AA pass you want. + +Yes keeping track of every value in the program is expensive, but this is a +debugging pass. + +``-domfrontier``: Dominance Frontier Construction +------------------------------------------------- + +This pass is a simple dominator construction algorithm for finding forward +dominator frontiers. + +``-domtree``: Dominator Tree Construction +----------------------------------------- + +This pass is a simple dominator construction algorithm for finding forward +dominators. + + +``-dot-callgraph``: Print Call Graph to "dot" file +-------------------------------------------------- + +This pass, only available in ``opt``, prints the call graph into a ``.dot`` +graph. This graph can then be processed with the "dot" tool to convert it to +postscript or some other suitable format. + +``-dot-cfg``: Print CFG of function to "dot" file +------------------------------------------------- + +This pass, only available in ``opt``, prints the control flow graph into a +``.dot`` graph. This graph can then be processed with the :program:`dot` tool +to convert it to postscript or some other suitable format. + +``-dot-cfg-only``: Print CFG of function to "dot" file (with no function bodies) +-------------------------------------------------------------------------------- + +This pass, only available in ``opt``, prints the control flow graph into a +``.dot`` graph, omitting the function bodies. This graph can then be processed +with the :program:`dot` tool to convert it to postscript or some other suitable +format. + +``-dot-dom``: Print dominance tree of function to "dot" file +------------------------------------------------------------ + +This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` +graph. This graph can then be processed with the :program:`dot` tool to +convert it to postscript or some other suitable format. + +``-dot-dom-only``: Print dominance tree of function to "dot" file (with no function bodies) +------------------------------------------------------------------------------------------- + +This pass, only available in ``opt``, prints the dominator tree into a ``.dot`` +graph, omitting the function bodies. This graph can then be processed with the +:program:`dot` tool to convert it to postscript or some other suitable format. + +``-dot-postdom``: Print postdominance tree of function to "dot" file +-------------------------------------------------------------------- + +This pass, only available in ``opt``, prints the post dominator tree into a +``.dot`` graph. This graph can then be processed with the :program:`dot` tool +to convert it to postscript or some other suitable format. + +``-dot-postdom-only``: Print postdominance tree of function to "dot" file (with no function bodies) +--------------------------------------------------------------------------------------------------- + +This pass, only available in ``opt``, prints the post dominator tree into a +``.dot`` graph, omitting the function bodies. This graph can then be processed +with the :program:`dot` tool to convert it to postscript or some other suitable +format. + +``-globalsmodref-aa``: Simple mod/ref analysis for globals +---------------------------------------------------------- + +This simple pass provides alias and mod/ref information for global values that +do not have their address taken, and keeps track of whether functions read or +write memory (are "pure"). For this simple (but very common) case, we can +provide pretty accurate and useful information. + +``-instcount``: Counts the various types of ``Instruction``\ s +-------------------------------------------------------------- + +This pass collects the count of all instructions and reports them. + +``-intervals``: Interval Partition Construction +----------------------------------------------- + +This analysis calculates and represents the interval partition of a function, +or a preexisting interval partition. + +In this way, the interval partition may be used to reduce a flow graph down to +its degenerate single node interval partition (unless it is irreducible). + +``-iv-users``: Induction Variable Users +--------------------------------------- + +Bookkeeping for "interesting" users of expressions computed from induction +variables. + +``-lazy-value-info``: Lazy Value Information Analysis +----------------------------------------------------- + +Interface for lazy computation of value constraint information. + +``-libcall-aa``: LibCall Alias Analysis +--------------------------------------- + +LibCall Alias Analysis. + +``-lint``: Statically lint-checks LLVM IR +----------------------------------------- + +This pass statically checks for common and easily-identified constructs which +produce undefined or likely unintended behavior in LLVM IR. + +It is not a guarantee of correctness, in two ways. First, it isn't +comprehensive. There are checks which could be done statically which are not +yet implemented. Some of these are indicated by TODO comments, but those +aren't comprehensive either. Second, many conditions cannot be checked +statically. This pass does no dynamic instrumentation, so it can't check for +all possible problems. + +Another limitation is that it assumes all code will be executed. A store +through a null pointer in a basic block which is never reached is harmless, but +this pass will warn about it anyway. + +Optimization passes may make conditions that this pass checks for more or less +obvious. If an optimization pass appears to be introducing a warning, it may +be that the optimization pass is merely exposing an existing condition in the +code. + +This code may be run before :ref:`instcombine <passes-instcombine>`. In many +cases, instcombine checks for the same kinds of things and turns instructions +with undefined behavior into unreachable (or equivalent). Because of this, +this pass makes some effort to look through bitcasts and so on. + +``-loops``: Natural Loop Information +------------------------------------ + +This analysis is used to identify natural loops and determine the loop depth of +various nodes of the CFG. Note that the loops identified may actually be +several natural loops that share the same header node... not just a single +natural loop. + +``-memdep``: Memory Dependence Analysis +--------------------------------------- + +An analysis that determines, for a given memory operation, what preceding +memory operations it depends on. It builds on alias analysis information, and +tries to provide a lazy, caching interface to a common kind of alias +information query. + +``-module-debuginfo``: Decodes module-level debug info +------------------------------------------------------ + +This pass decodes the debug info metadata in a module and prints in a +(sufficiently-prepared-) human-readable form. + +For example, run this pass from ``opt`` along with the ``-analyze`` option, and +it'll print to standard output. + +``-no-aa``: No Alias Analysis (always returns 'may' alias) +---------------------------------------------------------- + +This is the default implementation of the Alias Analysis interface. It always +returns "I don't know" for alias queries. NoAA is unlike other alias analysis +implementations, in that it does not chain to a previous analysis. As such it +doesn't follow many of the rules that other alias analyses must. + +``-no-profile``: No Profile Information +--------------------------------------- + +The default "no profile" implementation of the abstract ``ProfileInfo`` +interface. + +``-postdomfrontier``: Post-Dominance Frontier Construction +---------------------------------------------------------- + +This pass is a simple post-dominator construction algorithm for finding +post-dominator frontiers. + +``-postdomtree``: Post-Dominator Tree Construction +-------------------------------------------------- + +This pass is a simple post-dominator construction algorithm for finding +post-dominators. + +``-print-alias-sets``: Alias Set Printer +---------------------------------------- + +Yet to be written. + +``-print-callgraph``: Print a call graph +---------------------------------------- + +This pass, only available in ``opt``, prints the call graph to standard error +in a human-readable form. + +``-print-callgraph-sccs``: Print SCCs of the Call Graph +------------------------------------------------------- + +This pass, only available in ``opt``, prints the SCCs of the call graph to +standard error in a human-readable form. + +``-print-cfg-sccs``: Print SCCs of each function CFG +---------------------------------------------------- + +This pass, only available in ``opt``, printsthe SCCs of each function CFG to +standard error in a human-readable fom. + +``-print-dbginfo``: Print debug info in human readable form +----------------------------------------------------------- + +Pass that prints instructions, and associated debug info: + +#. source/line/col information +#. original variable name +#. original type name + +``-print-dom-info``: Dominator Info Printer +------------------------------------------- + +Dominator Info Printer. + +``-print-externalfnconstants``: Print external fn callsites passed constants +---------------------------------------------------------------------------- + +This pass, only available in ``opt``, prints out call sites to external +functions that are called with constant arguments. This can be useful when +looking for standard library functions we should constant fold or handle in +alias analyses. + +``-print-function``: Print function to stderr +--------------------------------------------- + +The ``PrintFunctionPass`` class is designed to be pipelined with other +``FunctionPasses``, and prints out the functions of the module as they are +processed. + +``-print-module``: Print module to stderr +----------------------------------------- + +This pass simply prints out the entire module when it is executed. + +.. _passes-print-used-types: + +``-print-used-types``: Find Used Types +-------------------------------------- + +This pass is used to seek out all of the types in use by the program. Note +that this analysis explicitly does not include types only used by the symbol +table. + +``-profile-estimator``: Estimate profiling information +------------------------------------------------------ + +Profiling information that estimates the profiling information in a very crude +and unimaginative way. + +``-profile-loader``: Load profile information from ``llvmprof.out`` +------------------------------------------------------------------- + +A concrete implementation of profiling information that loads the information +from a profile dump file. + +``-profile-verifier``: Verify profiling information +--------------------------------------------------- + +Pass that checks profiling information for plausibility. + +``-regions``: Detect single entry single exit regions +----------------------------------------------------- + +The ``RegionInfo`` pass detects single entry single exit regions in a function, +where a region is defined as any subgraph that is connected to the remaining +graph at only two spots. Furthermore, an hierarchical region tree is built. + +``-scalar-evolution``: Scalar Evolution Analysis +------------------------------------------------ + +The ``ScalarEvolution`` analysis can be used to analyze and catagorize scalar +expressions in loops. It specializes in recognizing general induction +variables, representing them with the abstract and opaque ``SCEV`` class. +Given this analysis, trip counts of loops and other important properties can be +obtained. + +This analysis is primarily useful for induction variable substitution and +strength reduction. + +``-scev-aa``: ScalarEvolution-based Alias Analysis +-------------------------------------------------- + +Simple alias analysis implemented in terms of ``ScalarEvolution`` queries. + +This differs from traditional loop dependence analysis in that it tests for +dependencies within a single iteration of a loop, rather than dependencies +between different iterations. + +``ScalarEvolution`` has a more complete understanding of pointer arithmetic +than ``BasicAliasAnalysis``' collection of ad-hoc analyses. + +``-targetdata``: Target Data Layout +----------------------------------- + +Provides other passes access to information on how the size and alignment +required by the target ABI for various data types. + +Transform Passes +================ + +This section describes the LLVM Transform Passes. + +``-adce``: Aggressive Dead Code Elimination +------------------------------------------- + +ADCE aggressively tries to eliminate code. This pass is similar to :ref:`DCE +<passes-dce>` but it assumes that values are dead until proven otherwise. This +is similar to :ref:`SCCP <passes-sccp>`, except applied to the liveness of +values. + +``-always-inline``: Inliner for ``always_inline`` functions +----------------------------------------------------------- + +A custom inliner that handles only functions that are marked as "always +inline". + +``-argpromotion``: Promote 'by reference' arguments to scalars +-------------------------------------------------------------- + +This pass promotes "by reference" arguments to be "by value" arguments. In +practice, this means looking for internal functions that have pointer +arguments. If it can prove, through the use of alias analysis, that an +argument is *only* loaded, then it can pass the value into the function instead +of the address of the value. This can cause recursive simplification of code +and lead to the elimination of allocas (especially in C++ template code like +the STL). + +This pass also handles aggregate arguments that are passed into a function, +scalarizing them if the elements of the aggregate are only loaded. Note that +it refuses to scalarize aggregates which would require passing in more than +three operands to the function, because passing thousands of operands for a +large array or structure is unprofitable! + +Note that this transformation could also be done for arguments that are only +stored to (returning the value instead), but does not currently. This case +would be best handled when and if LLVM starts supporting multiple return values +from functions. + +``-bb-vectorize``: Basic-Block Vectorization +-------------------------------------------- + +This pass combines instructions inside basic blocks to form vector +instructions. It iterates over each basic block, attempting to pair compatible +instructions, repeating this process until no additional pairs are selected for +vectorization. When the outputs of some pair of compatible instructions are +used as inputs by some other pair of compatible instructions, those pairs are +part of a potential vectorization chain. Instruction pairs are only fused into +vector instructions when they are part of a chain longer than some threshold +length. Moreover, the pass attempts to find the best possible chain for each +pair of compatible instructions. These heuristics are intended to prevent +vectorization in cases where it would not yield a performance increase of the +resulting code. + +``-block-placement``: Profile Guided Basic Block Placement +---------------------------------------------------------- + +This pass is a very simple profile guided basic block placement algorithm. The +idea is to put frequently executed blocks together at the start of the function +and hopefully increase the number of fall-through conditional branches. If +there is no profile information for a particular function, this pass basically +orders blocks in depth-first order. + +``-break-crit-edges``: Break critical edges in CFG +-------------------------------------------------- + +Break all of the critical edges in the CFG by inserting a dummy basic block. +It may be "required" by passes that cannot deal with critical edges. This +transformation obviously invalidates the CFG, but can update forward dominator +(set, immediate dominators, tree, and frontier) information. + +``-codegenprepare``: Optimize for code generation +------------------------------------------------- + +This pass munges the code in the input function to better prepare it for +SelectionDAG-based code generation. This works around limitations in it's +basic-block-at-a-time approach. It should eventually be removed. + +``-constmerge``: Merge Duplicate Global Constants +------------------------------------------------- + +Merges duplicate global constants together into a single constant that is +shared. This is useful because some passes (i.e., TraceValues) insert a lot of +string constants into the program, regardless of whether or not an existing +string is available. + +``-constprop``: Simple constant propagation +------------------------------------------- + +This file implements constant propagation and merging. It looks for +instructions involving only constant operands and replaces them with a constant +value instead of an instruction. For example: + +.. code-block:: llvm + + add i32 1, 2 + +becomes + +.. code-block:: llvm + + i32 3 + +NOTE: this pass has a habit of making definitions be dead. It is a good idea +to to run a :ref:`Dead Instruction Elimination <passes-die>` pass sometime +after running this pass. + +.. _passes-dce: + +``-dce``: Dead Code Elimination +------------------------------- + +Dead code elimination is similar to :ref:`dead instruction elimination +<passes-die>`, but it rechecks instructions that were used by removed +instructions to see if they are newly dead. + +``-deadargelim``: Dead Argument Elimination +------------------------------------------- + +This pass deletes dead arguments from internal functions. Dead argument +elimination removes arguments which are directly dead, as well as arguments +only passed into function calls as dead arguments of other functions. This +pass also deletes dead arguments in a similar way. + +This pass is often useful as a cleanup pass to run after aggressive +interprocedural passes, which add possibly-dead arguments. + +``-deadtypeelim``: Dead Type Elimination +---------------------------------------- + +This pass is used to cleanup the output of GCC. It eliminate names for types +that are unused in the entire translation unit, using the :ref:`find used types +<passes-print-used-types>` pass. + +.. _passes-die: + +``-die``: Dead Instruction Elimination +-------------------------------------- + +Dead instruction elimination performs a single pass over the function, removing +instructions that are obviously dead. + +``-dse``: Dead Store Elimination +-------------------------------- + +A trivial dead store elimination that only considers basic-block local +redundant stores. + +``-functionattrs``: Deduce function attributes +---------------------------------------------- + +A simple interprocedural pass which walks the call-graph, looking for functions +which do not access or only read non-local memory, and marking them +``readnone``/``readonly``. In addition, it marks function arguments (of +pointer type) "``nocapture``" if a call to the function does not create any +copies of the pointer value that outlive the call. This more or less means +that the pointer is only dereferenced, and not returned from the function or +stored in a global. This pass is implemented as a bottom-up traversal of the +call-graph. + +``-globaldce``: Dead Global Elimination +--------------------------------------- + +This transform is designed to eliminate unreachable internal globals from the +program. It uses an aggressive algorithm, searching out globals that are known +to be alive. After it finds all of the globals which are needed, it deletes +whatever is left over. This allows it to delete recursive chunks of the +program which are unreachable. + +``-globalopt``: Global Variable Optimizer +----------------------------------------- + +This pass transforms simple global variables that never have their address +taken. If obviously true, it marks read/write globals as constant, deletes +variables only stored to, etc. + +``-gvn``: Global Value Numbering +-------------------------------- + +This pass performs global value numbering to eliminate fully and partially +redundant instructions. It also performs redundant load elimination. + +.. _passes-indvars: + +``-indvars``: Canonicalize Induction Variables +---------------------------------------------- + +This transformation analyzes and transforms the induction variables (and +computations derived from them) into simpler forms suitable for subsequent +analysis and transformation. + +This transformation makes the following changes to each loop with an +identifiable induction variable: + +* All loops are transformed to have a *single* canonical induction variable + which starts at zero and steps by one. +* The canonical induction variable is guaranteed to be the first PHI node in + the loop header block. +* Any pointer arithmetic recurrences are raised to use array subscripts. + +If the trip count of a loop is computable, this pass also makes the following +changes: + +* The exit condition for the loop is canonicalized to compare the induction + value against the exit value. This turns loops like: + + .. code-block:: c++ + + for (i = 7; i*i < 1000; ++i) + + into + + .. code-block:: c++ + + for (i = 0; i != 25; ++i) + +* Any use outside of the loop of an expression derived from the indvar is + changed to compute the derived value outside of the loop, eliminating the + dependence on the exit value of the induction variable. If the only purpose + of the loop is to compute the exit value of some derived expression, this + transformation will make the loop dead. + +This transformation should be followed by strength reduction after all of the +desired loop transformations have been performed. Additionally, on targets +where it is profitable, the loop could be transformed to count down to zero +(the "do loop" optimization). + +``-inline``: Function Integration/Inlining +------------------------------------------ + +Bottom-up inlining of functions into callees. + +``-insert-edge-profiling``: Insert instrumentation for edge profiling +--------------------------------------------------------------------- + +This pass instruments the specified program with counters for edge profiling. +Edge profiling can give a reasonable approximation of the hot paths through a +program, and is used for a wide variety of program transformations. + +Note that this implementation is very naïve. It inserts a counter for *every* +edge in the program, instead of using control flow information to prune the +number of counters inserted. + +``-insert-optimal-edge-profiling``: Insert optimal instrumentation for edge profiling +------------------------------------------------------------------------------------- + +This pass instruments the specified program with counters for edge profiling. +Edge profiling can give a reasonable approximation of the hot paths through a +program, and is used for a wide variety of program transformations. + +.. _passes-instcombine: + +``-instcombine``: Combine redundant instructions +------------------------------------------------ + +Combine instructions to form fewer, simple instructions. This pass does not +modify the CFG This pass is where algebraic simplification happens. + +This pass combines things like: + +.. code-block:: llvm + + %Y = add i32 %X, 1 + %Z = add i32 %Y, 1 + +into: + +.. code-block:: llvm + + %Z = add i32 %X, 2 + +This is a simple worklist driven algorithm. + +This pass guarantees that the following canonicalizations are performed on the +program: + +#. If a binary operator has a constant operand, it is moved to the right-hand + side. +#. Bitwise operators with constant operands are always grouped so that shifts + are performed first, then ``or``\ s, then ``and``\ s, then ``xor``\ s. +#. Compare instructions are converted from ``<``, ``>``, ``≤``, or ``≥`` to + ``=`` or ``≠`` if possible. +#. All ``cmp`` instructions on boolean values are replaced with logical + operations. +#. ``add X, X`` is represented as ``mul X, 2`` ⇒ ``shl X, 1`` +#. Multiplies with a constant power-of-two argument are transformed into + shifts. +#. … etc. + +``-internalize``: Internalize Global Symbols +-------------------------------------------- + +This pass loops over all of the functions in the input module, looking for a +main function. If a main function is found, all other functions and all global +variables with initializers are marked as internal. + +``-ipconstprop``: Interprocedural constant propagation +------------------------------------------------------ + +This pass implements an *extremely* simple interprocedural constant propagation +pass. It could certainly be improved in many different ways, like using a +worklist. This pass makes arguments dead, but does not remove them. The +existing dead argument elimination pass should be run after this to clean up +the mess. + +``-ipsccp``: Interprocedural Sparse Conditional Constant Propagation +-------------------------------------------------------------------- + +An interprocedural variant of :ref:`Sparse Conditional Constant Propagation +<passes-sccp>`. + +``-jump-threading``: Jump Threading +----------------------------------- + +Jump threading tries to find distinct threads of control flow running through a +basic block. This pass looks at blocks that have multiple predecessors and +multiple successors. If one or more of the predecessors of the block can be +proven to always cause a jump to one of the successors, we forward the edge +from the predecessor to the successor by duplicating the contents of this +block. + +An example of when this can occur is code like this: + +.. code-block:: c++ + + if () { ... + X = 4; + } + if (X < 3) { + +In this case, the unconditional branch at the end of the first if can be +revectored to the false side of the second if. + +``-lcssa``: Loop-Closed SSA Form Pass +------------------------------------- + +This pass transforms loops by placing phi nodes at the end of the loops for all +values that are live across the loop boundary. For example, it turns the left +into the right code: + +.. code-block:: c++ + + for (...) for (...) + if (c) if (c) + X1 = ... X1 = ... + else else + X2 = ... X2 = ... + X3 = phi(X1, X2) X3 = phi(X1, X2) + ... = X3 + 4 X4 = phi(X3) + ... = X4 + 4 + +This is still valid LLVM; the extra phi nodes are purely redundant, and will be +trivially eliminated by ``InstCombine``. The major benefit of this +transformation is that it makes many other loop optimizations, such as +``LoopUnswitch``\ ing, simpler. + +.. _passes-licm: + +``-licm``: Loop Invariant Code Motion +------------------------------------- + +This pass performs loop invariant code motion, attempting to remove as much +code from the body of a loop as possible. It does this by either hoisting code +into the preheader block, or by sinking code to the exit blocks if it is safe. +This pass also promotes must-aliased memory locations in the loop to live in +registers, thus hoisting and sinking "invariant" loads and stores. + +This pass uses alias analysis for two purposes: + +#. Moving loop invariant loads and calls out of loops. If we can determine + that a load or call inside of a loop never aliases anything stored to, we + can hoist it or sink it like any other instruction. + +#. Scalar Promotion of Memory. If there is a store instruction inside of the + loop, we try to move the store to happen AFTER the loop instead of inside of + the loop. This can only happen if a few conditions are true: + + #. The pointer stored through is loop invariant. + #. There are no stores or loads in the loop which *may* alias the pointer. + There are no calls in the loop which mod/ref the pointer. + + If these conditions are true, we can promote the loads and stores in the + loop of the pointer to use a temporary alloca'd variable. We then use the + :ref:`mem2reg <passes-mem2reg>` functionality to construct the appropriate + SSA form for the variable. + +``-loop-deletion``: Delete dead loops +------------------------------------- + +This file implements the Dead Loop Deletion Pass. This pass is responsible for +eliminating loops with non-infinite computable trip counts that have no side +effects or volatile instructions, and do not contribute to the computation of +the function's return value. + +.. _passes-loop-extract: + +``-loop-extract``: Extract loops into new functions +--------------------------------------------------- + +A pass wrapper around the ``ExtractLoop()`` scalar transformation to extract +each top-level loop into its own new function. If the loop is the *only* loop +in a given function, it is not touched. This is a pass most useful for +debugging via bugpoint. + +``-loop-extract-single``: Extract at most one loop into a new function +---------------------------------------------------------------------- + +Similar to :ref:`Extract loops into new functions <passes-loop-extract>`, this +pass extracts one natural loop from the program into a function if it can. +This is used by :program:`bugpoint`. + +``-loop-reduce``: Loop Strength Reduction +----------------------------------------- + +This pass performs a strength reduction on array references inside loops that +have as one or more of their components the loop induction variable. This is +accomplished by creating a new value to hold the initial value of the array +access for the first iteration, and then creating a new GEP instruction in the +loop to increment the value by the appropriate amount. + +``-loop-rotate``: Rotate Loops +------------------------------ + +A simple loop rotation transformation. + +``-loop-simplify``: Canonicalize natural loops +---------------------------------------------- + +This pass performs several transformations to transform natural loops into a +simpler form, which makes subsequent analyses and transformations simpler and +more effective. + +Loop pre-header insertion guarantees that there is a single, non-critical entry +edge from outside of the loop to the loop header. This simplifies a number of +analyses and transformations, such as :ref:`LICM <passes-licm>`. + +Loop exit-block insertion guarantees that all exit blocks from the loop (blocks +which are outside of the loop that have predecessors inside of the loop) only +have predecessors from inside of the loop (and are thus dominated by the loop +header). This simplifies transformations such as store-sinking that are built +into LICM. + +This pass also guarantees that loops will have exactly one backedge. + +Note that the :ref:`simplifycfg <passes-simplifycfg>` pass will clean up blocks +which are split out but end up being unnecessary, so usage of this pass should +not pessimize generated code. + +This pass obviously modifies the CFG, but updates loop information and +dominator information. + +``-loop-unroll``: Unroll loops +------------------------------ + +This pass implements a simple loop unroller. It works best when loops have +been canonicalized by the :ref:`indvars <passes-indvars>` pass, allowing it to +determine the trip counts of loops easily. + +``-loop-unswitch``: Unswitch loops +---------------------------------- + +This pass transforms loops that contain branches on loop-invariant conditions +to have multiple loops. For example, it turns the left into the right code: + +.. code-block:: c++ + + for (...) if (lic) + A for (...) + if (lic) A; B; C + B else + C for (...) + A; C + +This can increase the size of the code exponentially (doubling it every time a +loop is unswitched) so we only unswitch if the resultant code will be smaller +than a threshold. + +This pass expects :ref:`LICM <passes-licm>` to be run before it to hoist +invariant conditions out of the loop, to make the unswitching opportunity +obvious. + +``-loweratomic``: Lower atomic intrinsics to non-atomic form +------------------------------------------------------------ + +This pass lowers atomic intrinsics to non-atomic form for use in a known +non-preemptible environment. + +The pass does not verify that the environment is non-preemptible (in general +this would require knowledge of the entire call graph of the program including +any libraries which may not be available in bitcode form); it simply lowers +every atomic intrinsic. + +``-lowerinvoke``: Lower invoke and unwind, for unwindless code generators +------------------------------------------------------------------------- + +This transformation is designed for use by code generators which do not yet +support stack unwinding. This pass supports two models of exception handling +lowering, the "cheap" support and the "expensive" support. + +"Cheap" exception handling support gives the program the ability to execute any +program which does not "throw an exception", by turning "``invoke``" +instructions into calls and by turning "``unwind``" instructions into calls to +``abort()``. If the program does dynamically use the "``unwind``" instruction, +the program will print a message then abort. + +"Expensive" exception handling support gives the full exception handling +support to the program at the cost of making the "``invoke``" instruction +really expensive. It basically inserts ``setjmp``/``longjmp`` calls to emulate +the exception handling as necessary. + +Because the "expensive" support slows down programs a lot, and EH is only used +for a subset of the programs, it must be specifically enabled by the +``-enable-correct-eh-support`` option. + +Note that after this pass runs the CFG is not entirely accurate (exceptional +control flow edges are not correct anymore) so only very simple things should +be done after the ``lowerinvoke`` pass has run (like generation of native +code). This should not be used as a general purpose "my LLVM-to-LLVM pass +doesn't support the ``invoke`` instruction yet" lowering pass. + +``-lowerswitch``: Lower ``SwitchInst``\ s to branches +----------------------------------------------------- + +Rewrites switch instructions with a sequence of branches, which allows targets +to get away with not implementing the switch instruction until it is +convenient. + +.. _passes-mem2reg: + +``-mem2reg``: Promote Memory to Register +---------------------------------------- + +This file promotes memory references to be register references. It promotes +alloca instructions which only have loads and stores as uses. An ``alloca`` is +transformed by using dominator frontiers to place phi nodes, then traversing +the function in depth-first order to rewrite loads and stores as appropriate. +This is just the standard SSA construction algorithm to construct "pruned" SSA +form. + +``-memcpyopt``: MemCpy Optimization +----------------------------------- + +This pass performs various transformations related to eliminating ``memcpy`` +calls, or transforming sets of stores into ``memset``\ s. + +``-mergefunc``: Merge Functions +------------------------------- + +This pass looks for equivalent functions that are mergable and folds them. + +A hash is computed from the function, based on its type and number of basic +blocks. + +Once all hashes are computed, we perform an expensive equality comparison on +each function pair. This takes n^2/2 comparisons per bucket, so it's important +that the hash function be high quality. The equality comparison iterates +through each instruction in each basic block. + +When a match is found the functions are folded. If both functions are +overridable, we move the functionality into a new internal function and leave +two overridable thunks to it. + +``-mergereturn``: Unify function exit nodes +------------------------------------------- + +Ensure that functions have at most one ``ret`` instruction in them. +Additionally, it keeps track of which node is the new exit node of the CFG. + +``-partial-inliner``: Partial Inliner +------------------------------------- + +This pass performs partial inlining, typically by inlining an ``if`` statement +that surrounds the body of the function. + +``-prune-eh``: Remove unused exception handling info +---------------------------------------------------- + +This file implements a simple interprocedural pass which walks the call-graph, +turning invoke instructions into call instructions if and only if the callee +cannot throw an exception. It implements this as a bottom-up traversal of the +call-graph. + +``-reassociate``: Reassociate expressions +----------------------------------------- + +This pass reassociates commutative expressions in an order that is designed to +promote better constant propagation, GCSE, :ref:`LICM <passes-licm>`, PRE, etc. + +For example: 4 + (x + 5) ⇒ x + (4 + 5) + +In the implementation of this algorithm, constants are assigned rank = 0, +function arguments are rank = 1, and other values are assigned ranks +corresponding to the reverse post order traversal of current function (starting +at 2), which effectively gives values in deep loops higher rank than values not +in loops. + +``-reg2mem``: Demote all values to stack slots +---------------------------------------------- + +This file demotes all registers to memory references. It is intended to be the +inverse of :ref:`mem2reg <passes-mem2reg>`. By converting to ``load`` +instructions, the only values live across basic blocks are ``alloca`` +instructions and ``load`` instructions before ``phi`` nodes. It is intended +that this should make CFG hacking much easier. To make later hacking easier, +the entry block is split into two, such that all introduced ``alloca`` +instructions (and nothing else) are in the entry block. + +``-scalarrepl``: Scalar Replacement of Aggregates (DT) +------------------------------------------------------ + +The well-known scalar replacement of aggregates transformation. This transform +breaks up ``alloca`` instructions of aggregate type (structure or array) into +individual ``alloca`` instructions for each member if possible. Then, if +possible, it transforms the individual ``alloca`` instructions into nice clean +scalar SSA form. + +This combines a simple scalar replacement of aggregates algorithm with the +:ref:`mem2reg <passes-mem2reg>` algorithm because they often interact, +especially for C++ programs. As such, iterating between ``scalarrepl``, then +:ref:`mem2reg <passes-mem2reg>` until we run out of things to promote works +well. + +.. _passes-sccp: + +``-sccp``: Sparse Conditional Constant Propagation +-------------------------------------------------- + +Sparse conditional constant propagation and merging, which can be summarized +as: + +* Assumes values are constant unless proven otherwise +* Assumes BasicBlocks are dead unless proven otherwise +* Proves values to be constant, and replaces them with constants +* Proves conditional branches to be unconditional + +Note that this pass has a habit of making definitions be dead. It is a good +idea to to run a :ref:`DCE <passes-dce>` pass sometime after running this pass. + +``-simplify-libcalls``: Simplify well-known library calls +--------------------------------------------------------- + +Applies a variety of small optimizations for calls to specific well-known +function calls (e.g. runtime library functions). For example, a call +``exit(3)`` that occurs within the ``main()`` function can be transformed into +simply ``return 3``. + +.. _passes-simplifycfg: + +``-simplifycfg``: Simplify the CFG +---------------------------------- + +Performs dead code elimination and basic block merging. Specifically: + +* Removes basic blocks with no predecessors. +* Merges a basic block into its predecessor if there is only one and the + predecessor only has one successor. +* Eliminates PHI nodes for basic blocks with a single predecessor. +* Eliminates a basic block that only contains an unconditional branch. + +``-sink``: Code sinking +----------------------- + +This pass moves instructions into successor blocks, when possible, so that they +aren't executed on paths where their results aren't needed. + +``-strip``: Strip all symbols from a module +------------------------------------------- + +Performs code stripping. This transformation can delete: + +* names for virtual registers +* symbols for internal globals and functions +* debug information + +Note that this transformation makes code much less readable, so it should only +be used in situations where the strip utility would be used, such as reducing +code size or making it harder to reverse engineer code. + +``-strip-dead-debug-info``: Strip debug info for unused symbols +--------------------------------------------------------------- + +.. FIXME: this description is the same as for -strip + +performs code stripping. this transformation can delete: + +* names for virtual registers +* symbols for internal globals and functions +* debug information + +note that this transformation makes code much less readable, so it should only +be used in situations where the strip utility would be used, such as reducing +code size or making it harder to reverse engineer code. + +``-strip-dead-prototypes``: Strip Unused Function Prototypes +------------------------------------------------------------ + +This pass loops over all of the functions in the input module, looking for dead +declarations and removes them. Dead declarations are declarations of functions +for which no implementation is available (i.e., declarations for unused library +functions). + +``-strip-debug-declare``: Strip all ``llvm.dbg.declare`` intrinsics +------------------------------------------------------------------- + +.. FIXME: this description is the same as for -strip + +This pass implements code stripping. Specifically, it can delete: + +#. names for virtual registers +#. symbols for internal globals and functions +#. debug information + +Note that this transformation makes code much less readable, so it should only +be used in situations where the 'strip' utility would be used, such as reducing +code size or making it harder to reverse engineer code. + +``-strip-nondebug``: Strip all symbols, except dbg symbols, from a module +------------------------------------------------------------------------- + +.. FIXME: this description is the same as for -strip + +This pass implements code stripping. Specifically, it can delete: + +#. names for virtual registers +#. symbols for internal globals and functions +#. debug information + +Note that this transformation makes code much less readable, so it should only +be used in situations where the 'strip' utility would be used, such as reducing +code size or making it harder to reverse engineer code. + +``-tailcallelim``: Tail Call Elimination +---------------------------------------- + +This file transforms calls of the current function (self recursion) followed by +a return instruction with a branch to the entry of the function, creating a +loop. This pass also implements the following extensions to the basic +algorithm: + +#. Trivial instructions between the call and return do not prevent the + transformation from taking place, though currently the analysis cannot + support moving any really useful instructions (only dead ones). +#. This pass transforms functions that are prevented from being tail recursive + by an associative expression to use an accumulator variable, thus compiling + the typical naive factorial or fib implementation into efficient code. +#. TRE is performed if the function returns void, if the return returns the + result returned by the call, or if the function returns a run-time constant + on all exits from the function. It is possible, though unlikely, that the + return returns something else (like constant 0), and can still be TRE'd. It + can be TRE'd if *all other* return instructions in the function return the + exact same value. +#. If it can prove that callees do not access theier caller stack frame, they + are marked as eligible for tail call elimination (by the code generator). + +Utility Passes +============== + +This section describes the LLVM Utility Passes. + +``-deadarghaX0r``: Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE) +------------------------------------------------------------------------ + +Same as dead argument elimination, but deletes arguments to functions which are +external. This is only for use by :doc:`bugpoint <Bugpoint>`. + +``-extract-blocks``: Extract Basic Blocks From Module (for bugpoint use) +------------------------------------------------------------------------ + +This pass is used by bugpoint to extract all blocks from the module into their +own functions. + +``-instnamer``: Assign names to anonymous instructions +------------------------------------------------------ + +This is a little utility pass that gives instructions names, this is mostly +useful when diffing the effect of an optimization because deleting an unnamed +instruction can change all other instruction numbering, making the diff very +noisy. + +``-preverify``: Preliminary module verification +----------------------------------------------- + +Ensures that the module is in the form required by the :ref:`Module Verifier +<passes-verify>` pass. Running the verifier runs this pass automatically, so +there should be no need to use it directly. + +.. _passes-verify: + +``-verify``: Module Verifier +---------------------------- + +Verifies an LLVM IR code. This is useful to run after an optimization which is +undergoing testing. Note that llvm-as verifies its input before emitting +bitcode, and also that malformed bitcode is likely to make LLVM crash. All +language front-ends are therefore encouraged to verify their output before +performing optimizing transformations. + +#. Both of a binary operator's parameters are of the same type. +#. Verify that the indices of mem access instructions match other operands. +#. Verify that arithmetic and other things are only performed on first-class + types. Verify that shifts and logicals only happen on integrals f.e. +#. All of the constants in a switch statement are of the correct type. +#. The code is in valid SSA form. +#. It is illegal to put a label into any other type (like a structure) or to + return one. +#. Only phi nodes can be self referential: ``%x = add i32 %x``, ``%x`` is + invalid. +#. PHI nodes must have an entry for each predecessor, with no extras. +#. PHI nodes must be the first thing in a basic block, all grouped together. +#. PHI nodes must have at least one entry. +#. All basic blocks should only end with terminator insts, not contain them. +#. The entry node to a function must not have predecessors. +#. All Instructions must be embedded into a basic block. +#. Functions cannot take a void-typed parameter. +#. Verify that a function's argument list agrees with its declared type. +#. It is illegal to specify a name for a void value. +#. It is illegal to have an internal global value with no initializer. +#. It is illegal to have a ``ret`` instruction that returns a value that does + not agree with the function return value type. +#. Function call argument types match the function prototype. +#. All other things that are tested by asserts spread about the code. + +Note that this does not provide full security verification (like Java), but +instead just tries to ensure that code is well-formed. + +``-view-cfg``: View CFG of function +----------------------------------- + +Displays the control flow graph using the GraphViz tool. + +``-view-cfg-only``: View CFG of function (with no function bodies) +------------------------------------------------------------------ + +Displays the control flow graph using the GraphViz tool, but omitting function +bodies. + +``-view-dom``: View dominance tree of function +---------------------------------------------- + +Displays the dominator tree using the GraphViz tool. + +``-view-dom-only``: View dominance tree of function (with no function bodies) +----------------------------------------------------------------------------- + +Displays the dominator tree using the GraphViz tool, but omitting function +bodies. + +``-view-postdom``: View postdominance tree of function +------------------------------------------------------ + +Displays the post dominator tree using the GraphViz tool. + +``-view-postdom-only``: View postdominance tree of function (with no function bodies) +------------------------------------------------------------------------------------- + +Displays the post dominator tree using the GraphViz tool, but omitting function +bodies. + |