1================ 2The LLVM Lexicon 3================ 4 5.. note:: 6 7 This document is a work in progress! 8 9Definitions 10=========== 11 12A 13- 14 15**ADCE** 16 Aggressive Dead Code Elimination 17 18**AST** 19 Abstract Syntax Tree. 20 21 Due to Clang's influence (mostly the fact that parsing and semantic 22 analysis are so intertwined for C and especially C++), the typical 23 working definition of AST in the LLVM community is roughly "the 24 compiler's first complete symbolic (as opposed to textual) 25 representation of an input program". 26 As such, an "AST" might be a more general graph instead of a "tree" 27 (consider the symbolic representation for the type of a typical "linked 28 list node"). This working definition is closer to what some authors 29 call an "annotated abstract syntax tree". 30 31 Consult your favorite compiler book or search engine for more details. 32 33B 34- 35 36.. _lexicon-bb-vectorization: 37 38**BB Vectorization** 39 Basic-Block Vectorization 40 41**BURS** 42 Bottom Up Rewriting System --- A method of instruction selection for code 43 generation. An example is the `BURG 44 <http://www.program-transformation.org/Transform/BURG>`_ tool. 45 46C 47- 48 49**CFI** 50 Call Frame Information. Used in DWARF debug info and in C++ unwind info 51 to show how the function prolog lays out the stack frame. 52 53**CIE** 54 Common Information Entry. A kind of CFI used to reduce the size of FDEs. 55 The compiler creates a CIE which contains the information common across all 56 the FDEs. Each FDE then points to its CIE. 57 58**CSE** 59 Common Subexpression Elimination. An optimization that removes common 60 subexpression compuation. For example ``(a+b)*(a+b)`` has two subexpressions 61 that are the same: ``(a+b)``. This optimization would perform the addition 62 only once and then perform the multiply (but only if it's computationally 63 correct/safe). 64 65D 66- 67 68**DAG** 69 Directed Acyclic Graph 70 71.. _derived pointer: 72.. _derived pointers: 73 74**Derived Pointer** 75 A pointer to the interior of an object, such that a garbage collector is 76 unable to use the pointer for reachability analysis. While a derived pointer 77 is live, the corresponding object pointer must be kept in a root, otherwise 78 the collector might free the referenced object. With copying collectors, 79 derived pointers pose an additional hazard that they may be invalidated at 80 any `safe point`_. This term is used in opposition to `object pointer`_. 81 82**DSA** 83 Data Structure Analysis 84 85**DSE** 86 Dead Store Elimination 87 88F 89- 90 91**FCA** 92 First Class Aggregate 93 94**FDE** 95 Frame Description Entry. A kind of CFI used to describe the stack frame of 96 one function. 97 98G 99- 100 101**GC** 102 Garbage Collection. The practice of using reachability analysis instead of 103 explicit memory management to reclaim unused memory. 104 105H 106- 107 108.. _heap: 109 110**Heap** 111 In garbage collection, the region of memory which is managed using 112 reachability analysis. 113 114I 115- 116 117**IPA** 118 Inter-Procedural Analysis. Refers to any variety of code analysis that 119 occurs between procedures, functions or compilation units (modules). 120 121**IPO** 122 Inter-Procedural Optimization. Refers to any variety of code optimization 123 that occurs between procedures, functions or compilation units (modules). 124 125**ISel** 126 Instruction Selection 127 128L 129- 130 131**LCSSA** 132 Loop-Closed Static Single Assignment Form 133 134**LGTM** 135 "Looks Good To Me". In a review thread, this indicates that the 136 reviewer thinks that the patch is okay to commit. 137 138**LICM** 139 Loop Invariant Code Motion 140 141**LSDA** 142 Language Specific Data Area. C++ "zero cost" unwinding is built on top a 143 generic unwinding mechanism. As the unwinder walks each frame, it calls 144 a "personality" function to do language specific analysis. Each function's 145 FDE points to an optional LSDA which is passed to the personality function. 146 For C++, the LSDA contain info about the type and location of catch 147 statements in that function. 148 149**Load-VN** 150 Load Value Numbering 151 152**LTO** 153 Link-Time Optimization 154 155M 156- 157 158**MC** 159 Machine Code 160 161N 162- 163 164**NFC** 165 "No functional change". Used in a commit message to indicate that a patch 166 is a pure refactoring/cleanup. 167 Usually used in the first line, so it is visible without opening the 168 actual commit email. 169 170O 171- 172.. _object pointer: 173.. _object pointers: 174 175**Object Pointer** 176 A pointer to an object such that the garbage collector is able to trace 177 references contained within the object. This term is used in opposition to 178 `derived pointer`_. 179 180P 181- 182 183**PR** 184 Problem report. A bug filed on `the LLVM Bug Tracking System 185 <http://llvm.org/bugs/enter_bug.cgi>`_. 186 187**PRE** 188 Partial Redundancy Elimination 189 190R 191- 192 193**RAUW** 194 195 Replace All Uses With. The functions ``User::replaceUsesOfWith()``, 196 ``Value::replaceAllUsesWith()``, and 197 ``Constant::replaceUsesOfWithOnConstant()`` implement the replacement of one 198 Value with another by iterating over its def/use chain and fixing up all of 199 the pointers to point to the new value. See 200 also `def/use chains <ProgrammersManual.html#iterating-over-def-use-use-def-chains>`_. 201 202**Reassociation** 203 Rearranging associative expressions to promote better redundancy elimination 204 and other optimization. For example, changing ``(A+B-A)`` into ``(B+A-A)``, 205 permitting it to be optimized into ``(B+0)`` then ``(B)``. 206 207.. _roots: 208.. _stack roots: 209 210**Root** 211 In garbage collection, a pointer variable lying outside of the `heap`_ from 212 which the collector begins its reachability analysis. In the context of code 213 generation, "root" almost always refers to a "stack root" --- a local or 214 temporary variable within an executing function. 215 216**RPO** 217 Reverse postorder 218 219S 220- 221 222.. _safe point: 223 224**Safe Point** 225 In garbage collection, it is necessary to identify `stack roots`_ so that 226 reachability analysis may proceed. It may be infeasible to provide this 227 information for every instruction, so instead the information may is 228 calculated only at designated safe points. With a copying collector, 229 `derived pointers`_ must not be retained across safe points and `object 230 pointers`_ must be reloaded from stack roots. 231 232**SDISel** 233 Selection DAG Instruction Selection. 234 235**SCC** 236 Strongly Connected Component 237 238**SCCP** 239 Sparse Conditional Constant Propagation 240 241**SLP** 242 Superword-Level Parallelism, same as :ref:`Basic-Block Vectorization 243 <lexicon-bb-vectorization>`. 244 245**SRoA** 246 Scalar Replacement of Aggregates 247 248**SSA** 249 Static Single Assignment 250 251**Stack Map** 252 In garbage collection, metadata emitted by the code generator which 253 identifies `roots`_ within the stack frame of an executing function. 254 255T 256- 257 258**TBAA** 259 Type-Based Alias Analysis 260 261