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

BDCE

Bit-tracking dead code elimination. Some bit-wise instructions (shifts, ands, ors, etc.) “kill” some of their input bits – that is, they make it such that those bits can be either zero or one without affecting control or data flow of a program. The BDCE pass removes instructions that only compute these dead bits.

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 computation. 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.

GEP

GetElementPtr. An LLVM IR instruction that is used to get the address of a subelement of an aggregate data structure. It is documented in detail here.

GVN

Global Value Numbering. GVN is a pass that partitions values computed by a function into congruence classes. Values ending up in the same congruence class are guaranteed to be the same for every execution of the program. In that respect, congruency is a compile-time approximation of equivalence of values at runtime.

H

Heap

In garbage collection, the region of memory which is managed using reachability analysis.

I

ICE

Internal Compiler Error. This abbreviation is used to describe errors that occur in LLVM or Clang as they are compiling source code. For example, if a valid C++ source program were to trigger an assert in Clang when compiled, that could be referred to as an “ICE”.

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

LGTM

“Looks Good To Me”. In a review thread, this indicates that the reviewer thinks that the patch is okay to commit.

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

PR

Problem report. A bug filed on the LLVM Bug Tracking System.

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.

Splat

Splat refers to a vector of identical scalar elements.

The term is based on the PowerPC Altivec instructions that provided this functionality in hardware. For example, “vsplth” and the corresponding software intrinsic “vec_splat()”. Examples of other hardware names for this action include “duplicate” (ARM) and “broadcast” (x86).

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