The LLVM Lexicon¶
This document is a work in progress!
Aggressive Dead Code Elimination
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.
- BB Vectorization
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.
Bottom Up Rewriting System — A method of instruction selection for code generation. An example is the BURG tool.
This abbreviation has two meanings. Either: Call Frame Information. Used in DWARF debug info and in C++ unwind info to show how the function prolog lays out the stack frame.
Or: Control Flow Integrity. A general term for computer security techniques that prevent a wide variety of malware attacks from redirecting the flow of execution (the control flow) of a program.
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.
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).
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.
Data Structure Analysis
Dead Store Elimination
First Class Aggregate
Frame Description Entry. A kind of CFI used to describe the stack frame of one function.
Garbage Collection. The practice of using reachability analysis instead of explicit memory management to reclaim unused memory.
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.
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.
In garbage collection, the region of memory which is managed using reachability analysis.
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”.
Identical Code Folding
Indirect Call Promotion
Inter-Procedural Analysis. Refers to any variety of code analysis that occurs between procedures, functions or compilation units (modules).
Inter-Procedural Optimization. Refers to any variety of code optimization that occurs between procedures, functions or compilation units (modules).
Loop-Closed Static Single Assignment Form
“Looks Good To Me”. In a review thread, this indicates that the reviewer thinks that the patch is okay to commit.
Loop Invariant Code Motion
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 Value Numbering
“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.
- 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.
Problem report. A bug filed on the LLVM Bug Tracking System.
Partial Redundancy Elimination
Replace All Uses With. The functions
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.
Rearranging associative expressions to promote better redundancy elimination and other optimization. For example, changing
(B+A-A), permitting it to be optimized into
Request for Comment. An email sent to a project mailing list in order to solicit feedback on a proposed change.
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.
Run-time Type Information
- 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 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.
Selection DAG Instruction Selection.
Strongly Connected Component
Sparse Conditional Constant Propagation
Superword-Level Parallelism, same as Basic-Block Vectorization.
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).
Scalar Replacement of Aggregates
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.
Type-Based Alias Analysis
Whole Program Devirtualization