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.

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. 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:

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.

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.

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

Documentation

Getting Involved

Additional Links

This Page

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

Introduction ¶

Rules for updating debug locations ¶

When to preserve an instruction location ¶

When to merge instruction locations ¶

When to drop an instruction location ¶

Rules for updating debug values ¶

Deleting an IR-level Instruction ¶

Deleting a MIR-level MachineInstr ¶

Rules for updating `DIAssignID` Attachments ¶

How to automatically convert tests into debug info tests ¶

Mutation testing for IR-level transformations ¶

The `debugify` utility pass ¶

Using `debugify`¶

`debugify` in regression tests ¶

Test original debug info preservation in optimizations ¶

Mutation testing for MIR-level transformations ¶

Using LostDebugLocObserver ¶