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

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This patch documents that LLVM does not describe all changes in variable
locations during the prologue and the epilogue. The debugger doesn't /
shouldn't step through that portion of the function anyway, and describing
every location through such stages would bloat location lists.

Perform some minor cleanup at the same time,
 * Fix an enumerated list
 * Document that dbg.declare intrinsics have their variable location recorded
   in a MachineFunction table, not with DBG_VALUE meta-insts
 * Adds frame-indexes to the list of things that can be operands to
   DBG_VALUEs.

Differential Revision: https://reviews.llvm.org/D63083

llvm-svn: 363654
This commit is contained in:
Jeremy Morse 2019-06-18 08:52:38 +00:00
parent 71f592d2bb
commit 9e238559b7
1 changed files with 13 additions and 4 deletions
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17
docs/SourceLevelDebugging.rst
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@ -531,6 +531,7 @@ within the LLVM IR. By the end of CodeGen, this becomes a mapping from each
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 @@ each of which are discussed below. In addition, instruction scheduling can
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 @@ inserted. These ``DBG_VALUE`` instructions appear thus:
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