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

## 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. Debug info availability is also essential for SamplePGO.

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 Source Level Debugging with LLVM.

## Rules for updating debug locations

### 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 dropping locations applies.

In addition to the rule above, a transformation should also preserve the debug location of an instruction that is moved between basic blocks, if the destination block already contains an instruction with an identical debug location.

Examples of transformations that should follow this rule include:

- Moving instructions between basic blocks. For example, if instruction `I1` in `BB1` is moved before `I2` in `BB2`, the source location of `I1` can be preserved if it has the same source location as `I2`.

### When to merge instruction locations

A transformation should merge instruction locations if it replaces multiple instructions with one or more new instructions, and the new instruction(s) produce the output of more than one of the original instructions. The API to use is `Instruction::applyMergedLocation`. For each new instruction I, its new location should be a merge of the locations of all instructions whose output is produced by I. Typically, this includes any instruction being RAUWed by a new instruction, and excludes any instruction that only produces an intermediate value used by the RAUWed instruction.

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.

To maintain distinct source locations for SamplePGO, it is often beneficial to retain an arbitrary but deterministic location instead of discarding line and column information as part of merging. In particular, loss of location information for calls inhibit optimizations such as indirect call promotion. This behavior can be optionally enabled until support for accurately representing merged instructions in the line table is implemented.

Examples of transformations that should follow this rule include:

- Hoisting identical instructions from all successors of a conditional branch or sinking those from all paths to a postdominating block. For example, merging identical loads/stores which occur on both sides of a CFG diamond (see the `MergedLoadStoreMotion` pass). For each group of identical instructions being hoisted/sunk, the merge of all their locations should be applied to the merged instruction.

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

- Scalar instructions being combined into a vector instruction, like `(add A1, B1), (add A2, B2) => (add (A1, A2), (B1, B2))`. As the new vector `add` computes the result of both original `add` instructions simultaneously, it should use a merge of the two locations. Similarly, if prior optimizations have already produced vectors `(A1, A2)` and `(B2, B1)`, then we might create a `(shufflevector (1, 0), (B2, B1))` instruction to produce `(B1, B2)` for the vector `add`; in this case we’ve created two instructions to replace the original `adds`, so both new instructions should use the merged location.

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 preserving locations should apply.

- Peephole optimizations which combine multiple instructions together, like `(add (mul A B) C) => llvm.fma.f32(A, B, C)`. Note that the result of the `mul` no longer appears in the program, while the result of the `add` is now produced by the `fma`, so the `add`’s location should be used.

- 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 dropping locations should apply here.

- Hoisting/sinking that would make a location reachable when it previously wasn’t. Consider hoisting two identical instructions with the same location from first two cases of a switch that has three cases. Merging their locations would make the location from the first two cases reachable when the third case is taken. The rule for dropping locations applies.

### When to drop an instruction location

A transformation should drop debug locations if the rules for preserving and merging debug locations do not apply. The API to use is `Instruction::dropLocation()`.

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 merging locations for examples of when the rule for dropping locations applies.

### When to remap a debug location

When code paths are duplicated, during passes such as loop unrolling or jump threading, DILocation attachments need to be remapped using mapAtomInstance and RemapSourceAtom. This is to support the Key Instructions debug info feature. See Key Instructions debug info in LLVM and Clang for information.

### Setting locations for new instructions

Whenever a new instruction is created and there is no suitable location for that instruction, that instruction should be annotated accordingly. There are a set of special `DebugLoc` values that can be set on an instruction to annotate the reason that it does not have a valid location. These are as follows:

- `DebugLoc::getCompilerGenerated()`: This indicates that the instruction is a compiler-generated instruction, i.e. it is not associated with any user source code.

- `DebugLoc::getDropped()`: This indicates that the instruction has intentionally had its source location removed, according to the rules for dropping locations; this is set automatically by `Instruction::dropLocation()`.

- `DebugLoc::getUnknown()`: This indicates that the instruction does not have a known or currently knowable source location, e.g. that it is infeasible to determine the correct source location, or that the source location is ambiguous in a way that LLVM cannot currently represent.

- `DebugLoc::getTemporary()`: This is used for instructions that we don’t expect to be emitted (e.g. `UnreachableInst`), and so should not need a valid location; if we ever try to emit a temporary location into an object/asm file, this indicates that something has gone wrong.

Where applicable, these should be used instead of leaving an instruction without an assigned location or explicitly setting the location as `DebugLoc()`. Ordinarily these special locations are identical to an absent location, but LLVM built with coverage-tracking (`-DLLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING="COVERAGE"`) will keep track of these special locations in order to detect unintentionally-missing locations; for this reason, the most important rule is to not apply any of these if it isn’t clear which, if any, is appropriate - an absent location can be detected and fixed, while an incorrectly annotated instruction is much harder to detect. On the other hand, if any of these clearly apply, then they should be used to prevent false positives from being flagged up.

## 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 `#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:

    define i16 @foo(i16 %a) {
      %b = sext i16 %a to i32
      %c = and i32 %b, 15
        #dbg_value(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:

    define i16 @foo(i16 %a) {
        #dbg_value(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:

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

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

    define i16 @foo(i16 %a) {
      %simplified = and i16 %a, 15
        #dbg_value(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

## Rules for updating `DIAssignID` Attachments

`DIAssignID` metadata attachments are used by Assignment Tracking, which is currently an experimental debug mode.

See Debug Info Assignment Tracking for how to update them and for more info on Assignment Tracking.

## How to automatically convert tests into debug info tests

### 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 pass

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 records everywhere possible.

For example, here is a module before:

    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`:

    define void @f(i32* %x) !dbg !6 {
    entry:
      %x.addr = alloca i32*, align 8, !dbg !12
        #dbg_value(i32** %x.addr, !9, !DIExpression(), !12)
      store i32* %x, i32** %x.addr, align 8, !dbg !13
      %0 = load i32*, i32** %x.addr, align 8, !dbg !14
        #dbg_value(i32* %0, !11, !DIExpression(), !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:

    $ 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:

    # 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:

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

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

#### `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.

#### Using Coverage Tracking to remove false positives

As described above, there are valid reasons for instructions to not have source locations. Therefore, when detecting dropped or not-generated source locations, it may be preferable to avoid detecting cases where the missing source location is intentional. For this, you can use the “coverage tracking” feature in LLVM to prevent these from appearing in the `debugify` output. This is enabled in a build of LLVM by setting the CMake flag `-DLLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING=COVERAGE`. When this has been set, LLVM will enable runtime tracking of DebugLoc annotations, allowing `debugify` to ignore instructions that have an explicitly recorded reason given for not having a source location.

For triaging source location bugs detected with `debugify`, you may find it helpful to instead set the CMake flag to enable “origin tracking”, `-DLLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING=COVERAGE_AND_ORIGIN`. This flag adds more detail to `debugify`’s output, by including one or more stacktraces with every missing source location, capturing the point at which the empty source location was created, and every point at which it was copied to an instruction, making it trivial in most cases to find the origin of the underlying bug. If using origin tracking, it is recommended to also build LLVM with debug info enabled, so that the stacktrace can be accurately symbolized.

Note

The coverage tracking feature has been designed primarily for use with the original debug info preservation mode of `debugify`, and so may not be reliable in other settings. When using this mode, the stacktraces produced by the `COVERAGE_AND_ORIGIN` setting will be printed in an easy-to-read format as part of the reports generated by the `llvm-original-di-preservation.py` script.

### Test original debug info preservation in optimizations

In addition to automatically generating debug info, the checks provided by the `debugify` utility pass can also be used to test the preservation of pre-existing debug info metadata. It could be run as follows:

    # Run the pass by checking original Debug Info preservation.
    $ opt -verify-debuginfo-preserve -pass-to-test sample.ll

    # Check the preservation of original Debug Info after each pass.
    $ opt -verify-each-debuginfo-preserve -O2 sample.ll

Limit number of observed functions to speed up the analysis:

    # Test up to 100 functions (per compile unit) per pass.
    $ opt -verify-each-debuginfo-preserve -O2 -debugify-func-limit=100 sample.ll

Please do note that running `-verify-each-debuginfo-preserve` on big projects could be heavily time consuming. Therefore, we suggest using `-debugify-func-limit` with a suitable limit number to prevent extremely long builds.

Furthermore, there is a way to export the issues that have been found into a JSON file as follows:

    $ opt -verify-debuginfo-preserve -verify-di-preserve-export=sample.json -pass-to-test sample.ll

and then use the `llvm/utils/llvm-original-di-preservation.py` script to generate an HTML page with the issues reported in a more human readable form as follows:

    $ llvm-original-di-preservation.py sample.json sample.html

Testing of original debug info preservation can be invoked from front-end level as follows:

    # Test each pass.
    $ clang -Xclang -fverify-debuginfo-preserve -g -O2 sample.c

    # Test each pass and export the issues report into the JSON file.
    $ clang -Xclang -fverify-debuginfo-preserve -Xclang -fverify-debuginfo-preserve-export=sample.json -g -O2 sample.c

Please do note that there are some known false positives, for source locations and debug record checking, so that will be addressed as a future work.

### Mutation testing for MIR-level transformations

A variant of the `debugify` utility described in Mutation testing for IR-level transformations 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`. And the MIR-level `mir-check-debugify` is similar to IR-level `check-debugify` pass.

For example, here is a snippet before:

    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`:

    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:

    # 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:

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

If you want to check it after each pass in a pipeline, use `-debugify-check-and-strip-all-safe`. This can also be combined with `-start-before` and `-start-after`. For example:

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

To check all debug info from a test, use `mir-check-debugify`, like:

    $ llc -run-pass=mir-debugify,other-pass,mir-check-debugify

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

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

It can be useful to combine `mir-debugify`, `mir-check-debugify` and/or `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:

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

### Using LostDebugLocObserver

TODO