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**BDCE** 42 Bit-tracking dead code elimination. Some bit-wise instructions (shifts, 43 ands, ors, etc.) "kill" some of their input bits -- that is, they make it 44 such that those bits can be either zero or one without affecting control or 45 data flow of a program. The BDCE pass removes instructions that only 46 compute these dead bits. 47 48**BURS** 49 Bottom Up Rewriting System --- A method of instruction selection for code 50 generation. An example is the `BURG 51 <http://www.program-transformation.org/Transform/BURG>`_ tool. 52 53C 54- 55 56**CFI** 57 Call Frame Information. Used in DWARF debug info and in C++ unwind info 58 to show how the function prolog lays out the stack frame. 59 60**CIE** 61 Common Information Entry. A kind of CFI used to reduce the size of FDEs. 62 The compiler creates a CIE which contains the information common across all 63 the FDEs. Each FDE then points to its CIE. 64 65**CSE** 66 Common Subexpression Elimination. An optimization that removes common 67 subexpression compuation. For example ``(a+b)*(a+b)`` has two subexpressions 68 that are the same: ``(a+b)``. This optimization would perform the addition 69 only once and then perform the multiply (but only if it's computationally 70 correct/safe). 71 72D 73- 74 75**DAG** 76 Directed Acyclic Graph 77 78.. _derived pointer: 79.. _derived pointers: 80 81**Derived Pointer** 82 A pointer to the interior of an object, such that a garbage collector is 83 unable to use the pointer for reachability analysis. While a derived pointer 84 is live, the corresponding object pointer must be kept in a root, otherwise 85 the collector might free the referenced object. With copying collectors, 86 derived pointers pose an additional hazard that they may be invalidated at 87 any `safe point`_. This term is used in opposition to `object pointer`_. 88 89**DSA** 90 Data Structure Analysis 91 92**DSE** 93 Dead Store Elimination 94 95F 96- 97 98**FCA** 99 First Class Aggregate 100 101**FDE** 102 Frame Description Entry. A kind of CFI used to describe the stack frame of 103 one function. 104 105G 106- 107 108**GC** 109 Garbage Collection. The practice of using reachability analysis instead of 110 explicit memory management to reclaim unused memory. 111 112**GEP** 113 ``GetElementPtr``. An LLVM IR instruction that is used to get the address 114 of a subelement of an aggregate data structure. It is documented in detail 115 `here <http://llvm.org/docs/GetElementPtr.html>`_. 116 117**GVN** 118 Global Value Numbering. GVN is a pass that partitions values computed by a 119 function into congruence classes. Values ending up in the same congruence 120 class are guaranteed to be the same for every execution of the program. 121 In that respect, congruency is a compile-time approximation of equivalence 122 of values at runtime. 123 124H 125- 126 127.. _heap: 128 129**Heap** 130 In garbage collection, the region of memory which is managed using 131 reachability analysis. 132 133I 134- 135 136**ICE** 137 Internal Compiler Error. This abbreviation is used to describe errors 138 that occur in LLVM or Clang as they are compiling source code. For example, 139 if a valid C++ source program were to trigger an assert in Clang when 140 compiled, that could be referred to as an "ICE". 141 142**IPA** 143 Inter-Procedural Analysis. Refers to any variety of code analysis that 144 occurs between procedures, functions or compilation units (modules). 145 146**IPO** 147 Inter-Procedural Optimization. Refers to any variety of code optimization 148 that occurs between procedures, functions or compilation units (modules). 149 150**ISel** 151 Instruction Selection 152 153L 154- 155 156**LCSSA** 157 Loop-Closed Static Single Assignment Form 158 159**LGTM** 160 "Looks Good To Me". In a review thread, this indicates that the 161 reviewer thinks that the patch is okay to commit. 162 163**LICM** 164 Loop Invariant Code Motion 165 166**LSDA** 167 Language Specific Data Area. C++ "zero cost" unwinding is built on top a 168 generic unwinding mechanism. As the unwinder walks each frame, it calls 169 a "personality" function to do language specific analysis. Each function's 170 FDE points to an optional LSDA which is passed to the personality function. 171 For C++, the LSDA contain info about the type and location of catch 172 statements in that function. 173 174**Load-VN** 175 Load Value Numbering 176 177**LTO** 178 Link-Time Optimization 179 180M 181- 182 183**MC** 184 Machine Code 185 186N 187- 188.. _nfc: 189 190**NFC** 191 "No functional change". Used in a commit message to indicate that a patch 192 is a pure refactoring/cleanup. 193 Usually used in the first line, so it is visible without opening the 194 actual commit email. 195 196O 197- 198.. _object pointer: 199.. _object pointers: 200 201**Object Pointer** 202 A pointer to an object such that the garbage collector is able to trace 203 references contained within the object. This term is used in opposition to 204 `derived pointer`_. 205 206P 207- 208 209**PR** 210 Problem report. A bug filed on `the LLVM Bug Tracking System 211 <https://bugs.llvm.org/enter_bug.cgi>`_. 212 213**PRE** 214 Partial Redundancy Elimination 215 216R 217- 218 219**RAUW** 220 221 Replace All Uses With. The functions ``User::replaceUsesOfWith()``, 222 ``Value::replaceAllUsesWith()``, and 223 ``Constant::replaceUsesOfWithOnConstant()`` implement the replacement of one 224 Value with another by iterating over its def/use chain and fixing up all of 225 the pointers to point to the new value. See 226 also `def/use chains <ProgrammersManual.html#iterating-over-def-use-use-def-chains>`_. 227 228**Reassociation** 229 Rearranging associative expressions to promote better redundancy elimination 230 and other optimization. For example, changing ``(A+B-A)`` into ``(B+A-A)``, 231 permitting it to be optimized into ``(B+0)`` then ``(B)``. 232 233.. _roots: 234.. _stack roots: 235 236**Root** 237 In garbage collection, a pointer variable lying outside of the `heap`_ from 238 which the collector begins its reachability analysis. In the context of code 239 generation, "root" almost always refers to a "stack root" --- a local or 240 temporary variable within an executing function. 241 242**RPO** 243 Reverse postorder 244 245S 246- 247 248.. _safe point: 249 250**Safe Point** 251 In garbage collection, it is necessary to identify `stack roots`_ so that 252 reachability analysis may proceed. It may be infeasible to provide this 253 information for every instruction, so instead the information may is 254 calculated only at designated safe points. With a copying collector, 255 `derived pointers`_ must not be retained across safe points and `object 256 pointers`_ must be reloaded from stack roots. 257 258**SDISel** 259 Selection DAG Instruction Selection. 260 261**SCC** 262 Strongly Connected Component 263 264**SCCP** 265 Sparse Conditional Constant Propagation 266 267**SLP** 268 Superword-Level Parallelism, same as :ref:`Basic-Block Vectorization 269 <lexicon-bb-vectorization>`. 270 271**Splat** 272 Splat refers to a vector of identical scalar elements. 273 274 The term is based on the PowerPC Altivec instructions that provided 275 this functionality in hardware. For example, "vsplth" and the corresponding 276 software intrinsic "vec_splat()". Examples of other hardware names for this 277 action include "duplicate" (ARM) and "broadcast" (x86). 278 279**SRoA** 280 Scalar Replacement of Aggregates 281 282**SSA** 283 Static Single Assignment 284 285**Stack Map** 286 In garbage collection, metadata emitted by the code generator which 287 identifies `roots`_ within the stack frame of an executing function. 288 289T 290- 291 292**TBAA** 293 Type-Based Alias Analysis 294 295