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.. _lexicon:
================
The LLVM Lexicon
================
.. note::
This document is a work in progress!
Definitions
===========
A
-
**ADCE**
Aggressive Dead Code Elimination
B
-
**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 compulationally
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
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#iterate_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.</dd>
**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
**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.
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