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295 lines
8.1 KiB
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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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**AST**
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Abstract Syntax Tree.
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Due to Clang's influence (mostly the fact that parsing and semantic
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analysis are so intertwined for C and especially C++), the typical
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working definition of AST in the LLVM community is roughly "the
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compiler's first complete symbolic (as opposed to textual)
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representation of an input program".
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As such, an "AST" might be a more general graph instead of a "tree"
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(consider the symbolic representation for the type of a typical "linked
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list node"). This working definition is closer to what some authors
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call an "annotated abstract syntax tree".
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Consult your favorite compiler book or search engine for more details.
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B
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-
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.. _lexicon-bb-vectorization:
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**BB Vectorization**
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Basic-Block Vectorization
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**BDCE**
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Bit-tracking dead code elimination. Some bit-wise instructions (shifts,
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ands, ors, etc.) "kill" some of their input bits -- that is, they make it
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such that those bits can be either zero or one without affecting control or
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data flow of a program. The BDCE pass removes instructions that only
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compute these dead bits.
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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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**CFI**
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Call Frame Information. Used in DWARF debug info and in C++ unwind info
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to show how the function prolog lays out the stack frame.
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**CIE**
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Common Information Entry. A kind of CFI used to reduce the size of FDEs.
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The compiler creates a CIE which contains the information common across all
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the FDEs. Each FDE then points to its CIE.
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**CSE**
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Common Subexpression Elimination. An optimization that removes common
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subexpression computation. For example ``(a+b)*(a+b)`` has two
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subexpressions that are the same: ``(a+b)``. This optimization would
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perform the addition only once and then perform the multiply (but only if
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it's computationally 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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**FDE**
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Frame Description Entry. A kind of CFI used to describe the stack frame of
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one function.
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G
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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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**GEP**
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``GetElementPtr``. An LLVM IR instruction that is used to get the address
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of a subelement of an aggregate data structure. It is documented in detail
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`here <http://llvm.org/docs/GetElementPtr.html>`_.
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**GVN**
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Global Value Numbering. GVN is a pass that partitions values computed by a
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function into congruence classes. Values ending up in the same congruence
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class are guaranteed to be the same for every execution of the program.
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In that respect, congruency is a compile-time approximation of equivalence
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of values at runtime.
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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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**ICE**
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Internal Compiler Error. This abbreviation is used to describe errors
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that occur in LLVM or Clang as they are compiling source code. For example,
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if a valid C++ source program were to trigger an assert in Clang when
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compiled, that could be referred to as an "ICE".
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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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**LCSSA**
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Loop-Closed Static Single Assignment Form
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**LGTM**
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"Looks Good To Me". In a review thread, this indicates that the
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reviewer thinks that the patch is okay to commit.
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**LICM**
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Loop Invariant Code Motion
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**LSDA**
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Language Specific Data Area. C++ "zero cost" unwinding is built on top a
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generic unwinding mechanism. As the unwinder walks each frame, it calls
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a "personality" function to do language specific analysis. Each function's
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FDE points to an optional LSDA which is passed to the personality function.
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For C++, the LSDA contain info about the type and location of catch
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statements in that function.
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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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N
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-
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.. _nfc:
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**NFC**
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"No functional change". Used in a commit message to indicate that a patch
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is a pure refactoring/cleanup.
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Usually used in the first line, so it is visible without opening the
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actual commit email.
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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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**PR**
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Problem report. A bug filed on `the LLVM Bug Tracking System
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<https://bugs.llvm.org/enter_bug.cgi>`_.
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**PRE**
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Partial Redundancy Elimination
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R
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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#iterating-over-def-use-use-def-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.
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**RPO**
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Reverse postorder
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S
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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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**SLP**
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Superword-Level Parallelism, same as :ref:`Basic-Block Vectorization
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<lexicon-bb-vectorization>`.
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**Splat**
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Splat refers to a vector of identical scalar elements.
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The term is based on the PowerPC Altivec instructions that provided
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this functionality in hardware. For example, "vsplth" and the corresponding
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software intrinsic "vec_splat()". Examples of other hardware names for this
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action include "duplicate" (ARM) and "broadcast" (x86).
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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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T
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-
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**TBAA**
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Type-Based Alias Analysis
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