The LLVM Lexicon
NOTE: This document is a work in progress!
Table Of Contents
- Aggressive Dead Code Elimination
- Bottom Up Rewriting System—A method of instruction selection for
code generation. An example is the BURG tool.
- 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 compulationally 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
- Garbage Collection. The practice of using reachability analysis instead
of explicit memory management to reclaim unused memory.
- In garbage collection, the region of memory which is managed using
- 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
- Instruction Selection.
- Loop-Closed Static Single Assignment Form
- Loop Invariant Code Motion
- Load Value Numbering
- Link-Time Optimization
- 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
- Partial Redundancy Elimination
- An abbreviation for 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.
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).
- 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.
- 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.
- Selection DAG Instruction Selection.
- Strongly Connected Component
- Sparse Conditional Constant Propagation
- 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
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The LLVM Compiler Infrastructure
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