path: root/docs/Lexicon.rst

================
The LLVM Lexicon
================

.. note::

    This document is a work in progress!

Definitions
===========

A
-

**ADCE**
    Aggressive Dead Code Elimination

**AST**
    Abstract Syntax Tree.

    Due to Clang's influence (mostly the fact that parsing and semantic
    analysis are so intertwined for C and especially C++), the typical
    working definition of AST in the LLVM community is roughly "the
    compiler's first complete symbolic (as opposed to textual)
    representation of an input program".
    As such, an "AST" might be a more general graph instead of a "tree"
    (consider the symbolic representation for the type of a typical "linked
    list node"). This working definition is closer to what some authors
    call an "annotated abstract syntax tree".

    Consult your favorite compiler book or search engine for more details.

B
-

.. _lexicon-bb-vectorization:

**BB Vectorization**
    Basic-Block Vectorization

**BURS**
    Bottom Up Rewriting System --- A method of instruction selection for code
    generation.  An example is the `BURG
    <http://www.program-transformation.org/Transform/BURG>`_ tool.

C
-

**CSE**
    Common Subexpression Elimination. An optimization that removes common
    subexpression compuation. For example ``(a+b)*(a+b)`` has two subexpressions
    that are the same: ``(a+b)``. This optimization would perform the addition
    only once and then perform the multiply (but only if it's computationally
    correct/safe).

D
-

**DAG**
    Directed Acyclic Graph

.. _derived pointer:
.. _derived pointers:

**Derived Pointer**
    A pointer to the interior of an object, such that a garbage collector is
    unable to use the pointer for reachability analysis. While a derived pointer
    is live, the corresponding object pointer must be kept in a root, otherwise
    the collector might free the referenced object. With copying collectors,
    derived pointers pose an additional hazard that they may be invalidated at
    any `safe point`_. This term is used in opposition to `object pointer`_.

**DSA**
    Data Structure Analysis

**DSE**
    Dead Store Elimination

F
-

**FCA**
    First Class Aggregate

G
-

**GC**
    Garbage Collection. The practice of using reachability analysis instead of
    explicit memory management to reclaim unused memory.

H
-

.. _heap:

**Heap**
    In garbage collection, the region of memory which is managed using
    reachability analysis.

I
-

**IPA**
    Inter-Procedural Analysis. Refers to any variety of code analysis that
    occurs between procedures, functions or compilation units (modules).

**IPO**
    Inter-Procedural Optimization. Refers to any variety of code optimization
    that occurs between procedures, functions or compilation units (modules).

**ISel**
    Instruction Selection

L
-

**LCSSA**
    Loop-Closed Static Single Assignment Form

**LICM**
    Loop Invariant Code Motion

**Load-VN**
    Load Value Numbering

**LTO**
    Link-Time Optimization

M
-

**MC**
    Machine Code

N
-

**NFC**
  "No functional change". Used in a commit message to indicate that a patch
  is a pure refactoring/cleanup.
  Usually used in the first line, so it is visible without opening the
  actual commit email.

O
-
.. _object pointer:
.. _object pointers:

**Object Pointer**
    A pointer to an object such that the garbage collector is able to trace
    references contained within the object. This term is used in opposition to
    `derived pointer`_.

P
-

**PRE**
    Partial Redundancy Elimination

R
-

**RAUW**

    Replace All Uses With. The functions ``User::replaceUsesOfWith()``,
    ``Value::replaceAllUsesWith()``, and
    ``Constant::replaceUsesOfWithOnConstant()`` implement the replacement of one
    Value with another by iterating over its def/use chain and fixing up all of
    the pointers to point to the new value.  See
    also `def/use chains <ProgrammersManual.html#iterating-over-def-use-use-def-chains>`_.

**Reassociation**
    Rearranging associative expressions to promote better redundancy elimination
    and other optimization.  For example, changing ``(A+B-A)`` into ``(B+A-A)``,
    permitting it to be optimized into ``(B+0)`` then ``(B)``.

.. _roots:
.. _stack roots:

**Root**
    In garbage collection, a pointer variable lying outside of the `heap`_ from
    which the collector begins its reachability analysis. In the context of code
    generation, "root" almost always refers to a "stack root" --- a local or
    temporary variable within an executing function.

**RPO**
    Reverse postorder

S
-

.. _safe point:

**Safe Point**
    In garbage collection, it is necessary to identify `stack roots`_ so that
    reachability analysis may proceed. It may be infeasible to provide this
    information for every instruction, so instead the information may is
    calculated only at designated safe points. With a copying collector,
    `derived pointers`_ must not be retained across safe points and `object
    pointers`_ must be reloaded from stack roots.

**SDISel**
    Selection DAG Instruction Selection.

**SCC**
    Strongly Connected Component

**SCCP**
    Sparse Conditional Constant Propagation

**SLP**
    Superword-Level Parallelism, same as :ref:`Basic-Block Vectorization
    <lexicon-bb-vectorization>`.

**SRoA**
    Scalar Replacement of Aggregates

**SSA**
    Static Single Assignment

**Stack Map**
    In garbage collection, metadata emitted by the code generator which
    identifies `roots`_ within the stack frame of an executing function.

T
-

**TBAA**
    Type-Based Alias Analysis