The LLVM Lexicon

BURS: Bottom Up Rewriting System—A method of instruction selection for code generation. An example is the BURG tool.

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 compulationally correct/safe).

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

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

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

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.

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.

RAUW: 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.
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.

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
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.