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195 lines
4.3 KiB
ReStructuredText
195 lines
4.3 KiB
ReStructuredText
.. _lexicon:
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================
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The LLVM Lexicon
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================
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.. note::
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This document is a work in progress!
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Definitions
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===========
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A
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-
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**ADCE**
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Aggressive Dead Code Elimination
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B
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-
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**BURS**
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Bottom Up Rewriting System --- A method of instruction selection for code
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generation. An example is the `BURG
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<http://www.program-transformation.org/Transform/BURG>`_ tool.
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C
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-
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**CSE**
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Common Subexpression Elimination. An optimization that removes common
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subexpression compuation. For example ``(a+b)*(a+b)`` has two subexpressions
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that are the same: ``(a+b)``. This optimization would perform the addition
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only once and then perform the multiply (but only if it's compulationally
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correct/safe).
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D
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-
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**DAG**
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Directed Acyclic Graph
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.. _derived pointer:
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.. _derived pointers:
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**Derived Pointer**
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A pointer to the interior of an object, such that a garbage collector is
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unable to use the pointer for reachability analysis. While a derived pointer
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is live, the corresponding object pointer must be kept in a root, otherwise
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the collector might free the referenced object. With copying collectors,
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derived pointers pose an additional hazard that they may be invalidated at
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any `safe point`_. This term is used in opposition to `object pointer`_.
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**DSA**
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Data Structure Analysis
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**DSE**
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Dead Store Elimination
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F
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-
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**FCA**
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First Class Aggregate
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G
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-
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**GC**
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Garbage Collection. The practice of using reachability analysis instead of
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explicit memory management to reclaim unused memory.
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H
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-
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.. _heap:
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**Heap**
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In garbage collection, the region of memory which is managed using
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reachability analysis.
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I
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-
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**IPA**
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Inter-Procedural Analysis. Refers to any variety of code analysis that
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occurs between procedures, functions or compilation units (modules).
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**IPO**
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Inter-Procedural Optimization. Refers to any variety of code optimization
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that occurs between procedures, functions or compilation units (modules).
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**ISel**
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Instruction Selection
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L
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-
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**LCSSA**
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Loop-Closed Static Single Assignment Form
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**LICM**
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Loop Invariant Code Motion
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**Load-VN**
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Load Value Numbering
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**LTO**
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Link-Time Optimization
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M
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-
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**MC**
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Machine Code
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O
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-
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.. _object pointer:
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.. _object pointers:
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**Object Pointer**
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A pointer to an object such that the garbage collector is able to trace
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references contained within the object. This term is used in opposition to
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`derived pointer`_.
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P
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-
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**PRE**
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Partial Redundancy Elimination
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R
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-
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**RAUW**
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Replace All Uses With. The functions ``User::replaceUsesOfWith()``,
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``Value::replaceAllUsesWith()``, and
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``Constant::replaceUsesOfWithOnConstant()`` implement the replacement of one
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Value with another by iterating over its def/use chain and fixing up all of
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the pointers to point to the new value. See
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also `def/use chains <ProgrammersManual.html#iterate_chains>`_.
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**Reassociation**
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Rearranging associative expressions to promote better redundancy elimination
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and other optimization. For example, changing ``(A+B-A)`` into ``(B+A-A)``,
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permitting it to be optimized into ``(B+0)`` then ``(B)``.
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.. _roots:
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.. _stack roots:
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**Root**
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In garbage collection, a pointer variable lying outside of the `heap`_ from
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which the collector begins its reachability analysis. In the context of code
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generation, "root" almost always refers to a "stack root" --- a local or
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temporary variable within an executing function.</dd>
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**RPO**
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Reverse postorder
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S
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-
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.. _safe point:
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**Safe Point**
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In garbage collection, it is necessary to identify `stack roots`_ so that
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reachability analysis may proceed. It may be infeasible to provide this
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information for every instruction, so instead the information may is
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calculated only at designated safe points. With a copying collector,
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`derived pointers`_ must not be retained across safe points and `object
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pointers`_ must be reloaded from stack roots.
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**SDISel**
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Selection DAG Instruction Selection.
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**SCC**
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Strongly Connected Component
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**SCCP**
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Sparse Conditional Constant Propagation
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**SRoA**
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Scalar Replacement of Aggregates
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**SSA**
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Static Single Assignment
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**Stack Map**
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In garbage collection, metadata emitted by the code generator which
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identifies `roots`_ within the stack frame of an executing function.
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