llvm-mirror/docs/HowToUpdateDebugInfo.rst

=======================================================
How to Update Debug Info: A Guide for LLVM Pass Authors
=======================================================

.. contents::
   :local:

Introduction
============

Certain kinds of code transformations can inadvertently result in a loss of
debug info, or worse, make debug info misrepresent the state of a program.

This document specifies how to correctly update debug info in various kinds of
code transformations, and offers suggestions for how to create targeted debug
info tests for arbitrary transformations.

For more on the philosophy behind LLVM debugging information, see
:doc:`SourceLevelDebugging`.

Rules for updating debug locations
==================================

.. _WhenToPreserveLocation:

When to preserve an instruction location
----------------------------------------

A transformation should preserve the debug location of an instruction if the
instruction either remains in its basic block, or if its basic block is folded
into a predecessor that branches unconditionally. The APIs to use are
``IRBuilder``, or ``Instruction::setDebugLoc``.

The purpose of this rule is to ensure that common block-local optimizations
preserve the ability to set breakpoints on source locations corresponding to
the instructions they touch. Debugging, crash logs, and SamplePGO accuracy
would be severely impacted if that ability were lost.

Examples of transformations that should follow this rule include:

* Instruction scheduling. Block-local instruction reordering should not drop
  source locations, even though this may lead to jumpy single-stepping
  behavior.

* Simple jump threading. For example, if block ``B1`` unconditionally jumps to
  ``B2``, *and* is its unique predecessor, instructions from ``B2`` can be
  hoisted into ``B1``. Source locations from ``B2`` should be preserved.

* Peephole optimizations that replace or expand an instruction, like ``(add X
  X) => (shl X 1)``. The location of the ``shl`` instruction should be the same
  as the location of the ``add`` instruction.

* Tail duplication. For example, if blocks ``B1`` and ``B2`` both
  unconditionally branch to ``B3`` and ``B3`` can be folded into its
  predecessors, source locations from ``B3`` should be preserved.

Examples of transformations for which this rule *does not* apply include:

* LICM. E.g., if an instruction is moved from the loop body to the preheader,
  the rule for :ref:`dropping locations<WhenToDropLocation>` applies.

.. _WhenToMergeLocation:

When to merge instruction locations
-----------------------------------

A transformation should merge instruction locations if it replaces multiple
instructions with a single merged instruction, *and* that merged instruction
does not correspond to any of the original instructions' locations. The API to
use is ``Instruction::applyMergedLocation``.

The purpose of this rule is to ensure that a) the single merged instruction
has a location with an accurate scope attached, and b) to prevent misleading
single-stepping (or breakpoint) behavior. Often, merged instructions are memory
accesses which can trap: having an accurate scope attached greatly assists in
crash triage by identifying the (possibly inlined) function where the bad
memory access occurred. This rule is also meant to assist SamplePGO by banning
scenarios in which a sample of a block containing a merged instruction is
misattributed to a block containing one of the instructions-to-be-merged.

Examples of transformations that should follow this rule include:

* Merging identical loads/stores which occur on both sides of a CFG diamond
  (see the ``MergedLoadStoreMotion`` pass).

* Merging identical loop-invariant stores (see the LICM utility
  ``llvm::promoteLoopAccessesToScalars``).

* Peephole optimizations which combine multiple instructions together, like
  ``(add (mul A B) C) => llvm.fma.f32(A, B, C)``.  Note that the location of
  the ``fma`` does not exactly correspond to the locations of either the
  ``mul`` or the ``add`` instructions.

Examples of transformations for which this rule *does not* apply include:

* Block-local peepholes which delete redundant instructions, like
  ``(sext (zext i8 %x to i16) to i32) => (zext i8 %x to i32)``. The inner
  ``zext`` is modified but remains in its block, so the rule for
  :ref:`preserving locations<WhenToPreserveLocation>` should apply.

* Converting an if-then-else CFG diamond into a ``select``. Preserving the
  debug locations of speculated instructions can make it seem like a condition
  is true when it's not (or vice versa), which leads to a confusing
  single-stepping experience. The rule for
  :ref:`dropping locations<WhenToDropLocation>` should apply here.

* Hoisting identical instructions which appear in several successor blocks into
  a predecessor block (see ``BranchFolder::HoistCommonCodeInSuccs``). In this
  case there is no single merged instruction. The rule for
  :ref:`dropping locations<WhenToDropLocation>` applies.

.. _WhenToDropLocation:

When to drop an instruction location
------------------------------------

A transformation should drop debug locations if the rules for
:ref:`preserving<WhenToPreserveLocation>` and
:ref:`merging<WhenToMergeLocation>` debug locations do not apply. The API to
use is ``Instruction::setDebugLoc()``.

The purpose of this rule is to prevent erratic or misleading single-stepping
behavior in situations in which an instruction has no clear, unambiguous
relationship to a source location.

To handle an instruction without a location, the DWARF generator
defaults to allowing the last-set location after a label to cascade forward, or
to setting a line 0 location with viable scope information if no previous
location is available.

See the discussion in the section about
:ref:`merging locations<WhenToMergeLocation>` for examples of when the rule for
dropping locations applies.

Rules for updating debug values
===============================

Deleting an IR-level Instruction
--------------------------------

When an ``Instruction`` is deleted, its debug uses change to ``undef``. This is
a loss of debug info: the value of one or more source variables becomes
unavailable, starting with the ``llvm.dbg.value(undef, ...)``. When there is no
way to reconstitute the value of the lost instruction, this is the best
possible outcome. However, it's often possible to do better:

* If the dying instruction can be RAUW'd, do so. The
  ``Value::replaceAllUsesWith`` API transparently updates debug uses of the
  dying instruction to point to the replacement value.

* If the dying instruction cannot be RAUW'd, call ``llvm::salvageDebugInfo`` on
  it. This makes a best-effort attempt to rewrite debug uses of the dying
  instruction by describing its effect as a ``DIExpression``.

* If one of the **operands** of a dying instruction would become trivially
  dead, use ``llvm::replaceAllDbgUsesWith`` to rewrite the debug uses of that
  operand. Consider the following example function:

.. code-block:: llvm

  define i16 @foo(i16 %a) {
    %b = sext i16 %a to i32
    %c = and i32 %b, 15
    call void @llvm.dbg.value(metadata i32 %c, ...)
    %d = trunc i32 %c to i16
    ret i16 %d
  }

Now, here's what happens after the unnecessary truncation instruction ``%d`` is
replaced with a simplified instruction:

.. code-block:: llvm

  define i16 @foo(i16 %a) {
    call void @llvm.dbg.value(metadata i32 undef, ...)
    %simplified = and i16 %a, 15
    ret i16 %simplified
  }

Note that after deleting ``%d``, all uses of its operand ``%c`` become
trivially dead. The debug use which used to point to ``%c`` is now ``undef``,
and debug info is needlessly lost.

To solve this problem, do:

.. code-block:: cpp

  llvm::replaceAllDbgUsesWith(%c, theSimplifiedAndInstruction, ...)

This results in better debug info because the debug use of ``%c`` is preserved:

.. code-block:: llvm

  define i16 @foo(i16 %a) {
    %simplified = and i16 %a, 15
    call void @llvm.dbg.value(metadata i16 %simplified, ...)
    ret i16 %simplified
  }

You may have noticed that ``%simplified`` is narrower than ``%c``: this is not
a problem, because ``llvm::replaceAllDbgUsesWith`` takes care of inserting the
necessary conversion operations into the DIExpressions of updated debug uses.

Deleting a MIR-level MachineInstr
---------------------------------

TODO

How to automatically convert tests into debug info tests
========================================================

.. _IRDebugify:

Mutation testing for IR-level transformations
---------------------------------------------

An IR test case for a transformation can, in many cases, be automatically
mutated to test debug info handling within that transformation. This is a
simple way to test for proper debug info handling.

The ``debugify`` utility
^^^^^^^^^^^^^^^^^^^^^^^^

The ``debugify`` testing utility is just a pair of passes: ``debugify`` and
``check-debugify``.

The first applies synthetic debug information to every instruction of the
module, and the second checks that this DI is still available after an
optimization has occurred, reporting any errors/warnings while doing so.

The instructions are assigned sequentially increasing line locations, and are
immediately used by debug value intrinsics everywhere possible.

For example, here is a module before:

.. code-block:: llvm

   define void @f(i32* %x) {
   entry:
     %x.addr = alloca i32*, align 8
     store i32* %x, i32** %x.addr, align 8
     %0 = load i32*, i32** %x.addr, align 8
     store i32 10, i32* %0, align 4
     ret void
   }

and after running ``opt -debugify``:

.. code-block:: llvm

   define void @f(i32* %x) !dbg !6 {
   entry:
     %x.addr = alloca i32*, align 8, !dbg !12
     call void @llvm.dbg.value(metadata i32** %x.addr, metadata !9, metadata !DIExpression()), !dbg !12
     store i32* %x, i32** %x.addr, align 8, !dbg !13
     %0 = load i32*, i32** %x.addr, align 8, !dbg !14
     call void @llvm.dbg.value(metadata i32* %0, metadata !11, metadata !DIExpression()), !dbg !14
     store i32 10, i32* %0, align 4, !dbg !15
     ret void, !dbg !16
   }

   !llvm.dbg.cu = !{!0}
   !llvm.debugify = !{!3, !4}
   !llvm.module.flags = !{!5}

   !0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
   !1 = !DIFile(filename: "debugify-sample.ll", directory: "/")
   !2 = !{}
   !3 = !{i32 5}
   !4 = !{i32 2}
   !5 = !{i32 2, !"Debug Info Version", i32 3}
   !6 = distinct !DISubprogram(name: "f", linkageName: "f", scope: null, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: true, unit: !0, retainedNodes: !8)
   !7 = !DISubroutineType(types: !2)
   !8 = !{!9, !11}
   !9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
   !10 = !DIBasicType(name: "ty64", size: 64, encoding: DW_ATE_unsigned)
   !11 = !DILocalVariable(name: "2", scope: !6, file: !1, line: 3, type: !10)
   !12 = !DILocation(line: 1, column: 1, scope: !6)
   !13 = !DILocation(line: 2, column: 1, scope: !6)
   !14 = !DILocation(line: 3, column: 1, scope: !6)
   !15 = !DILocation(line: 4, column: 1, scope: !6)
   !16 = !DILocation(line: 5, column: 1, scope: !6)

Using ``debugify``
^^^^^^^^^^^^^^^^^^

A simple way to use ``debugify`` is as follows:

.. code-block:: bash

  $ opt -debugify -pass-to-test -check-debugify sample.ll

This will inject synthetic DI to ``sample.ll`` run the ``pass-to-test`` and
then check for missing DI. The ``-check-debugify`` step can of course be
omitted in favor of more customizable FileCheck directives.

Some other ways to run debugify are available:

.. code-block:: bash

   # Same as the above example.
   $ opt -enable-debugify -pass-to-test sample.ll

   # Suppresses verbose debugify output.
   $ opt -enable-debugify -debugify-quiet -pass-to-test sample.ll

   # Prepend -debugify before and append -check-debugify -strip after
   # each pass on the pipeline (similar to -verify-each).
   $ opt -debugify-each -O2 sample.ll

In order for ``check-debugify`` to work, the DI must be coming from
``debugify``. Thus, modules with existing DI will be skipped.

``debugify`` can be used to test a backend, e.g:

.. code-block:: bash

   $ opt -debugify < sample.ll | llc -o -

There is also a MIR-level debugify pass that can be run before each backend
pass, see:
:ref:`Mutation testing for MIR-level transformations<MIRDebugify>`.

``debugify`` in regression tests
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The output of the ``debugify`` pass must be stable enough to use in regression
tests. Changes to this pass are not allowed to break existing tests.

.. note::

   Regression tests must be robust. Avoid hardcoding line/variable numbers in
   check lines. In cases where this can't be avoided (say, if a test wouldn't
   be precise enough), moving the test to its own file is preferred.

.. _MIRDebugify:

Mutation testing for MIR-level transformations
----------------------------------------------

A variant of the ``debugify`` utility described in
:ref:`Mutation testing for IR-level transformations<IRDebugify>` can be used
for MIR-level transformations as well: much like the IR-level pass,
``mir-debugify`` inserts sequentially increasing line locations to each
``MachineInstr`` in a ``Module`` (although there is no equivalent MIR-level
``check-debugify`` pass).

For example, here is a snippet before:

.. code-block:: llvm

  name:            test
  body:             |
    bb.1 (%ir-block.0):
      %0:_(s32) = IMPLICIT_DEF
      %1:_(s32) = IMPLICIT_DEF
      %2:_(s32) = G_CONSTANT i32 2
      %3:_(s32) = G_ADD %0, %2
      %4:_(s32) = G_SUB %3, %1

and after running ``llc -run-pass=mir-debugify``:

.. code-block:: llvm

  name:            test
  body:             |
    bb.0 (%ir-block.0):
      %0:_(s32) = IMPLICIT_DEF debug-location !12
      DBG_VALUE %0(s32), $noreg, !9, !DIExpression(), debug-location !12
      %1:_(s32) = IMPLICIT_DEF debug-location !13
      DBG_VALUE %1(s32), $noreg, !11, !DIExpression(), debug-location !13
      %2:_(s32) = G_CONSTANT i32 2, debug-location !14
      DBG_VALUE %2(s32), $noreg, !9, !DIExpression(), debug-location !14
      %3:_(s32) = G_ADD %0, %2, debug-location !DILocation(line: 4, column: 1, scope: !6)
      DBG_VALUE %3(s32), $noreg, !9, !DIExpression(), debug-location !DILocation(line: 4, column: 1, scope: !6)
      %4:_(s32) = G_SUB %3, %1, debug-location !DILocation(line: 5, column: 1, scope: !6)
      DBG_VALUE %4(s32), $noreg, !9, !DIExpression(), debug-location !DILocation(line: 5, column: 1, scope: !6)

By default, ``mir-debugify`` inserts ``DBG_VALUE`` instructions **everywhere**
it is legal to do so.  In particular, every (non-PHI) machine instruction that
defines a register must be followed by a ``DBG_VALUE`` use of that def.  If
an instruction does not define a register, but can be followed by a debug inst,
MIRDebugify inserts a ``DBG_VALUE`` that references a constant.  Insertion of
``DBG_VALUE``'s can be disabled by setting ``-debugify-level=locations``.

To run MIRDebugify once, simply insert ``mir-debugify`` into your ``llc``
invocation, like:

.. code-block:: bash

  # Before some other pass.
  $ llc -run-pass=mir-debugify,other-pass ...

  # After some other pass.
  $ llc -run-pass=other-pass,mir-debugify ...

To run MIRDebugify before each pass in a pipeline, use
``-debugify-and-strip-all-safe``. This can be combined with ``-start-before``
and ``-start-after``. For example:

.. code-block:: bash

  $ llc -debugify-and-strip-all-safe -run-pass=... <other llc args>
  $ llc -debugify-and-strip-all-safe -O1 <other llc args>

To strip out all debug info from a test, use ``mir-strip-debug``, like:

.. code-block:: bash

  $ llc -run-pass=mir-debugify,other-pass,mir-strip-debug

It can be useful to combine ``mir-debugify`` and ``mir-strip-debug`` to
identify backend transformations which break in the presence of debug info.
For example, to run the AArch64 backend tests with all normal passes
"sandwiched" in between MIRDebugify and MIRStripDebugify mutation passes, run:

.. code-block:: bash

  $ llvm-lit test/CodeGen/AArch64 -Dllc="llc -debugify-and-strip-all-safe"

Using LostDebugLocObserver
--------------------------

TODO