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 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 preserving locations 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 dropping locations 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 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 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:
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:

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:

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
  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.

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

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:

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
  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:

$ 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.

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"