Index: docs/SourceLevelDebugging.rst
===================================================================
--- docs/SourceLevelDebugging.rst
+++ docs/SourceLevelDebugging.rst
@@ -531,6 +531,7 @@
 variable to their machine locations over ranges of instructions.
 From IR to object emission, the major transformations which affect variable
 location fidelity are:
+
 1. Instruction Selection
 2. Register allocation
 3. Block layout
@@ -539,6 +540,14 @@
 significantly change the ordering of the program, and occurs in a number of
 different passes.
 
+Some variable locations are not transformed during CodeGen. Stack locations
+specified by ``llvm.dbg.declare`` are valid and unchanging for the entire
+duration of the function, and are recorded in a simple MachineFunction table.
+Location changes in the prologue and epilogue of a function are also ignored:
+frame setup and destruction may take several instructions, require a
+disproportionate amount of debugging information in the output binary to
+describe, and should be stepped over by debuggers anyway.
+
 Variable locations in Instruction Selection and MIR
 ---------------------------------------------------
 
@@ -573,7 +582,8 @@
   DBG_VALUE %1, $noreg, !123, !DIExpression()
 
 And have the following operands:
- * The first operand can record the variable location as a register, an
+ * The first operand can record the variable location as a register,
+   a frame index, an
    immediate, or the base address register if the original debug intrinsic
    referred to memory. ``$noreg`` indicates the variable location is undefined,
    equivalent to an ``undef`` dbg.value operand.