The LLVM Lexicon

Note

This document is a work in progress!

Definitions

A

ADCE
Aggressive Dead Code Elimination
AST

Abstract Syntax Tree.

Due to Clang’s influence (mostly the fact that parsing and semantic analysis are so intertwined for C and especially C++), the typical working definition of AST in the LLVM community is roughly “the compiler’s first complete symbolic (as opposed to textual) representation of an input program”. As such, an “AST” might be a more general graph instead of a “tree” (consider the symbolic representation for the type of a typical “linked list node”). This working definition is closer to what some authors call an “annotated abstract syntax tree”.

Consult your favorite compiler book or search engine for more details.

B

BB Vectorization
Basic-Block Vectorization
BURS
Bottom Up Rewriting System — A method of instruction selection for code generation. An example is the BURG tool.

C

CFI
Call Frame Information. Used in DWARF debug info and in C++ unwind info to show how the function prolog lays out the stack frame.
CIE
Common Information Entry. A kind of CFI used to reduce the size of FDEs. The compiler creates a CIE which contains the information common across all the FDEs. Each FDE then points to its CIE.
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 computationally correct/safe).

D

DAG
Directed Acyclic Graph
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
FDE
Frame Description Entry. A kind of CFI used to describe the stack frame of one function.

G

GC
Garbage Collection. The practice of using reachability analysis instead of explicit memory management to reclaim unused memory.

H

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
LSDA
Language Specific Data Area. C++ “zero cost” unwinding is built on top a generic unwinding mechanism. As the unwinder walks each frame, it calls a “personality” function to do language specific analysis. Each function’s FDE points to an optional LSDA which is passed to the personality function. For C++, the LSDA contain info about the type and location of catch statements in that function.
Load-VN
Load Value Numbering
LTO
Link-Time Optimization

M

MC
Machine Code

N

NFC
“No functional change”. Used in a commit message to indicate that a patch is a pure refactoring/cleanup. Usually used in the first line, so it is visible without opening the actual commit email.

O

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.
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).
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.
RPO
Reverse postorder

S

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
SLP
Superword-Level Parallelism, same as Basic-Block Vectorization.
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.

T

TBAA
Type-Based Alias Analysis