llvm/docs/Passes.md

# LLVM's Analysis and Transform Passes


```{toctree}
:hidden:

KernelInfo
LoopFusion

```

## Introduction

:::{warning}
This document is not updated frequently, and the list of passes
is most likely incomplete. It is possible to list passes known by the opt
tool using `opt -print-passes`.
:::

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 provide 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.

### `basic-aa`: 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.

(passes-da)=

### `da`: Dependence Analysis

Dependence analysis framework, which is used to detect dependencies in memory
accesses.

### `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.
Additionally, the `-cfg-func-name=<substring>` option can be used to filter the
functions that are printed. All functions that contain the specified substring
will be printed.

### `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.
Additionally, the `-cfg-func-name=<substring>` option can be used to filter the
functions that are printed. All functions that contain the specified substring
will be printed.

### `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-post-dom`: 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-post-dom-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.

### `globals-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.

### `iv-users`: Induction Variable Users

Bookkeeping for "interesting" users of expressions computed from induction
variables.

### `kernel-info`: GPU Kernel Info

Reports various statistics for codes compiled for GPUs.  This pass is
{doc}`documented separately <KernelInfo>`.

### `lazy-value-info`: Lazy Value Information Analysis

Interface for lazy computation of value constraint information.

### `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.

### `print<module-debuginfo>`: Decodes module-level debug info

This pass decodes the debug info metadata in a module and prints it to standard output in a
(sufficiently-prepared-) human-readable form.

### `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`, prints the SCCs of each function CFG to
standard error in a human-readable form.

### `function(print)`: 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.

### `module(print)`: Print module to stderr

This pass simply prints out the entire module when it is executed.

### `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, a hierarchical region tree is built.

(passes-scalar-evolution)=

### `scalar-evolution`: Scalar Evolution Analysis

The `ScalarEvolution` analysis can be used to analyze and categorize 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.

### `stack-safety`: Stack Safety Analysis

The `StackSafety` analysis can be used to determine if stack allocated
variables can be considered safe from memory access bugs.

This analysis' primary purpose is to be used by sanitizers to avoid unnecessary
instrumentation of safe variables.

## 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.

### `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.

(passes-dce)=

### `dce`: Dead Code Elimination

Dead code elimination is similar to dead instruction elimination, 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.

### `dse`: Dead Store Elimination

A trivial dead store elimination that only considers basic-block local
redundant stores.

(passes-function-attrs)=

### `function-attrs`: 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:

  ```c++
  for (i = 7; i*i < 1000; ++i)

  into
  ```
  ```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, simpler instructions.  This pass does not
modify the CFG. This pass is where algebraic simplification happens.

This pass combines things like:

```llvm
%Y = add i32 %X, 1
%Z = add i32 %Y, 1
```
into:

```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:

1. If a binary operator has a constant operand, it is moved to the right-hand
   side.
1. Bitwise operators with constant operands are always grouped so that shifts
   are performed first, then `or`s, then `and`s, then `xor`s.
1. Compare instructions are converted from `<`, `>`, `≤`, or `≥` to
   `=` or `≠` if possible.
1. All `cmp` instructions on boolean values are replaced with logical
   operations.
1. `add X, X` is represented as `mul X, 2` ⇒ `shl X, 1`
1. Multiplies with a constant power-of-two argument are transformed into
   shifts.
1. … 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}`-function-attrs <passes-function-attrs>` 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, simpler instructions.

For example, this pass reduces the width of expressions post-dominated by `TruncInst`
into smaller width when applicable.

It differs from instcombine pass in that it can modify CFG and 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.

### `ipsccp`: Interprocedural Sparse Conditional Constant Propagation

An interprocedural variant of
{ref}`Sparse Conditional Constant Propagation <passes-sccp>`.

### `normalize`: Transforms IR into a normal form that's easier to diff

This pass aims to transform LLVM Modules into a normal form by reordering and
renaming instructions while preserving the same semantics. The normalizer makes
it easier to spot semantic differences while diffing two modules which have
undergone two different passes.

### `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:

```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.

(passes-lcssa)=

### `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:

```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. You can read more in the
{ref}`loop terminology section for the LCSSA form <loop-terminology-lcssa>`.

(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.

Hoisting operations out of loops is a canonicalization transform. It enables
and simplifies subsequent optimizations in the middle-end. Rematerialization
of hoisted instructions to reduce register pressure is the responsibility of
the back-end, which has more accurate information about register pressure and
also handles other optimizations than LICM that increase live-ranges.

This pass uses alias analysis for two purposes:

1. 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.

1. 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:

   1. The pointer stored through is loop invariant.
   1. 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.

### `loop-fusion`: Loop Fusion

Merges adjacent loops when it can prove the transformation preserves the
program's semantics.  This pass is {doc}`documented separately <LoopFusion>`.

### `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.

(passes-loop-rotate)=

### `loop-rotate`: Rotate Loops

A simple loop rotation transformation.  A summary of it can be found in
{ref}`Loop Terminology for Rotated Loops <loop-terminology-loop-rotate>`.


(passes-loop-simplify)=

### `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. A summary of it can be found in
{ref}`Loop Terminology, Loop Simplify Form <loop-terminology-loop-simplify>`.

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 a loop from:

```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.

### `lower-global-dtors`: Lower global destructors

This pass lowers global module destructors (`llvm.global_dtors`) by creating
wrapper functions that are registered as global constructors in
`llvm.global_ctors` and which contain a call to `__cxa_atexit` to register
their destructor functions.

### `lower-atomic`: 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.

### `lower-invoke`: 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).

### `lower-switch`: 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 mergeable 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 a binary tree.

For every new function we check for equivalent in the 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.

### `partial-inliner`: Partial Inliner

This pass performs partial inlining, typically by inlining an `if` statement
that surrounds the body of the function.

### `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 the current function (starting
at 2), which effectively gives values in deep loops higher rank than values not
in loops.

### `rel-lookup-table-converter`: Relative lookup table converter

This pass converts lookup tables to PIC-friendly relative lookup tables.

### `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.

(passes-simple-loop-unswitch)=

### `simple-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:

```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.

### `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

Performs code stripping. Similar to strip, but only strips debug info for
unused symbols.

### `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 and `#dbg_declare` records.

Performs code stripping. Similar to strip, but only strips
`llvm.dbg.declare` intrinsics.

### `strip-nondebug`: Strip all symbols, except dbg symbols, from a module

Performs code stripping. Similar to strip, but dbg info is preserved.

### `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:

1. 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).
1. 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.
1. 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.
1. If it can prove that callees do not access their 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.

### `extract-blocks`: Extract Basic Blocks From Module

This pass extracts 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 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.

1. Both of a binary operator's parameters are of the same type.
1. Verify that the indices of mem access instructions match other operands.
1. Verify that arithmetic and other things are only performed on first-class
   types.  Verify that shifts and logicals only happen on integrals f.e.
1. All of the constants in a switch statement are of the correct type.
1. The code is in valid SSA form.
1. It is illegal to put a label into any other type (like a structure) or to
   return one.
1. Only phi nodes can be self referential: `%x = add i32 %x`, `%x` is
   invalid.
1. PHI nodes must have an entry for each predecessor, with no extras.
1. PHI nodes must be the first thing in a basic block, all grouped together.
1. PHI nodes must have at least one entry.
1. All basic blocks should only end with terminator insts, not contain them.
1. The entry node to a function must not have predecessors.
1. All Instructions must be embedded into a basic block.
1. Functions cannot take a void-typed parameter.
1. Verify that a function's argument list agrees with its declared type.
1. It is illegal to specify a name for a void value.
1. It is illegal to have an internal global value with no initializer.
1. It is illegal to have a `ret` instruction that returns a value that does
   not agree with the function return value type.
1. Function call argument types match the function prototype.
1. 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.

(passes-view-cfg)=

### `view-cfg`: View CFG of function

Displays the control flow graph using the GraphViz tool.
Additionally, the `-cfg-func-name=<substring>` option can be used to filter the
functions that are displayed. All functions that contain the specified substring
will be displayed.

### `view-cfg-only`: View CFG of function (with no function bodies)

Displays the control flow graph using the GraphViz tool, but omitting function
bodies.
Additionally, the `-cfg-func-name=<substring>` option can be used to filter the
functions that are displayed. All functions that contain the specified substring
will be displayed.

### `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-post-dom`: View postdominance tree of function

Displays the post dominator tree using the GraphViz tool.

### `view-post-dom-only`: View postdominance tree of function (with no function bodies)

Displays the post dominator tree using the GraphViz tool, but omitting function
bodies.

### `transform-warning`: Report missed forced transformations

Emits warnings about not yet applied forced transformations (e.g. from
`#pragma omp simd`).