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====================================
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.
.. _passes-da:
``-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.
``-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-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.
``-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 its
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 pass 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 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.
.. _passes-functionattrs:
``-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.
.. _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.
This pass can also simplify 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``. Whether or
not library calls are simplified is controlled by the
:ref:`-functionattrs <passes-functionattrs>` pass and LLVM's knowledge of
library calls on different targets.
.. _passes-aggressive-instcombine:
``-aggressive-instcombine``: Combine expression patterns
--------------------------------------------------------
Combine expression patterns to form expressions with fewer, simple instructions.
This pass does not modify the CFG.
For example, this pass reduce width of expressions post-dominated by TruncInst
into smaller width when applicable.
It differs from instcombine pass in that it contains pattern optimization that
requires higher complexity than the O(1), thus, it should run fewer times than
instcombine pass.
``-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-unroll-and-jam``: Unroll and Jam loops
----------------------------------------------
This pass implements a simple unroll and jam classical loop optimisation pass.
It transforms loop from:
.. code-block:: c++
for i.. i+= 1 for i.. i+= 4
for j.. for j..
code(i, j) code(i, j)
code(i+1, j)
code(i+2, j)
code(i+3, j)
remainder loop
Which can be seen as unrolling the outer loop and "jamming" (fusing) the inner
loops into one. When variables or loads can be shared in the new inner loop, this
can lead to significant performance improvements. It uses
:ref:`Dependence Analysis <passes-da>` for proving the transformations are safe.
``-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 invokes to calls, for unwindless code generators
------------------------------------------------------------------------
This transformation is designed for use by code generators which do not yet
support stack unwinding. This pass converts ``invoke`` instructions to
``call`` instructions, so that any exception-handling ``landingpad`` blocks
become dead code (which can be removed by running the ``-simplifycfg`` pass
afterwards).
``-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.
Total-ordering is introduced among the functions set: we define comparison
that answers for every two functions which of them is greater. It allows to
arrange functions into the binary tree.
For every new function we check for equivalent in tree.
If equivalent exists we fold such functions. If both functions are overridable,
we move the functionality into a new internal function and leave two
overridable thunks to it.
If there is no equivalent, then we add this function to tree.
Lookup routine has O(log(n)) complexity, while whole merging process has
complexity of O(n*log(n)).
Read
:doc:`this <MergeFunctions>`
article for more details.
``-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.
``-sroa``: Scalar Replacement of Aggregates
------------------------------------------------------
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.
.. _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 run a :ref:`DCE <passes-dce>` pass sometime after running this pass.
.. _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.
.. _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.