[PATCH] D63083: [DebugInfo][Docs] Document that prologue/epilogue variable location changes are ignored

[Jeremy Morse via Phabricator via llvm-commits]

jmorse updated this revision to Diff 204012.
jmorse edited the summary of this revision.
jmorse added a comment.

Whitespace change to ease reading of .rst, split summary into being an actual commit message


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D63083/new/

https://reviews.llvm.org/D63083

Files:
  docs/SourceLevelDebugging.rst


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
 ---------------------------------------------------
 
@@ -573,10 +582,10 @@
   DBG_VALUE %1, $noreg, !123, !DIExpression()
 
 And have the following operands:
- * The first operand can record the variable location as a register, an
-   immediate, or the base address register if the original debug intrinsic
-   referred to memory. ``$noreg`` indicates the variable location is undefined,
-   equivalent to an ``undef`` dbg.value operand.
+ * The first operand can record the variable location as a register,
+   a frame index, an immediate, or the base address register if the original
+   debug intrinsic referred to memory. ``$noreg`` indicates the variable
+   location is undefined, equivalent to an ``undef`` dbg.value operand.
  * The type of the second operand indicates whether the variable location is
    directly referred to by the DBG_VALUE, or whether it is indirect. The
    ``$noreg`` register signifies the former, an immediate operand (0) the


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