Debug info migration: From intrinsics to records¶

We’re planning on removing debug info intrinsics from LLVM, as they’re slow, unwieldy and can confuse optimisation passes if they’re not expecting them. Instead of having a sequence of instructions that looks like this:

    %add = add i32 %foo, %bar
    call void @llvm.dbg.value(metadata %add, ...
    %sub = sub i32 %add, %tosub
    call void @llvm.dbg.value(metadata %sub, ...
    call void @a_normal_function()

with dbg.value intrinsics representing debug info records, it would instead be printed as:

    %add = add i32 %foo, %bar
      #dbg_value(%add, ...
    %sub = sub i32 %add, %tosub
      #dbg_value(%sub, ...
    call void @a_normal_function()

The debug records are not instructions, do not appear in the instruction list, and won’t appear in your optimisation passes unless you go digging for them deliberately.

Great, what do I need to do!¶

Very little – we’ve already instrumented all of LLVM to handle these new records (”DbgRecords”) and behave identically to past LLVM behaviour. This is currently being turned on by default, so that DbgRecords will be used by default in memory, IR, and bitcode.

API Changes¶

There are two significant changes to be aware of. Firstly, we’re adding a single bit of debug relevant data to the BasicBlock::iterator class (it’s so that we can determine whether ranges intend on including debug info at the beginning of a block or not). That means when writing passes that insert LLVM IR instructions, you need to identify positions with BasicBlock::iterator rather than just a bare Instruction *. Most of the time this means that after identifying where you intend on inserting something, you must also call getIterator on the instruction position – however when inserting at the start of a block you must use getFirstInsertionPt, getFirstNonPHIIt or begin and use that iterator to insert, rather than just fetching a pointer to the first instruction.

The second matter is that if you transfer sequences of instructions from one place to another manually, i.e. repeatedly using moveBefore where you might have used splice, then you should instead use the method moveBeforePreserving. moveBeforePreserving will transfer debug info records with the instruction they’re attached to. This is something that happens automatically today – if you use moveBefore on every element of an instruction sequence, then debug intrinsics will be moved in the normal course of your code, but we lose this behaviour with non-instruction debug info.

For a more in-depth overview of how to update existing code to support debug records, see the guide below.

Textual IR Changes¶

As we change from using debug intrinsics to debug records, any tools that depend on parsing IR produced by LLVM will need to handle the new format. For the most part, the difference between the printed form of a debug intrinsic call and a debug record is trivial:

An extra 2 spaces of indentation are added.
The text (tail|notail|musttail)? call void @llvm.dbg.<type> is replaced with #dbg_<type>.
The leading metadata is removed from each argument to the intrinsic.
The DILocation changes from being an instruction attachment with the format !dbg !<Num>, to being an ordinary argument, i.e. !<Num>, that is passed as the final argument to the debug record.

Following these rules, we have this example of a debug intrinsic and the equivalent debug record:

; Debug Intrinsic:
  call void @llvm.dbg.value(metadata i32 %add, metadata !10, metadata !DIExpression()), !dbg !20
; Debug Record:
    #dbg_value(i32 %add, !10, !DIExpression(), !20)

Test updates¶

Any tests downstream of the main LLVM repo that test the IR output of LLVM may break as a result of the change to using records. Updating an individual test to expect records instead of intrinsics should be trivial, given the update rules above. Updating many tests may be burdensome however; to update the lit tests in the main repository, the following steps were used:

Collect the list of failing lit tests into a single file, failing-tests.txt, separated by (and ending with) newlines.

Use the following line to split the failing tests into tests that use update_test_checks and tests that don’t:

$ while IFS= read -r f; do grep -q "Assertions have been autogenerated by" "$f" && echo "$f" >> update-checks-tests.txt || echo "$f" >> manual-tests.txt; done < failing-tests.txt

For the tests that use update_test_checks, run the appropriate update_test_checks script - for the main LLVM repo, this was achieved with:

$ xargs ./llvm/utils/update_test_checks.py --opt-binary ./build/bin/opt < update-checks-tests.txt
$ xargs ./llvm/utils/update_cc_test_checks.py --llvm-bin ./build/bin/ < update-checks-tests.txt

The remaining tests can be manually updated, although if there is a large number of tests then the following scripts may be useful; firstly, a script used to extract the check-line prefixes from a file:

$ cat ./get-checks.sh
#!/bin/bash

# Always add CHECK, since it's more effort than it's worth to filter files where
# every RUN line uses other check prefixes.
# Then detect every instance of "check-prefix(es)=..." and add the
# comma-separated arguments as extra checks.
for filename in "$@"
do
    echo "$filename,CHECK"
    allchecks=$(grep -Eo 'check-prefix(es)?[ =][A-Z0-9_,-]+' $filename | sed -E 's/.+[= ]([A-Z0-9_,-]+).*/\1/g; s/,/\n/g')
    for check in $allchecks; do
        echo "$filename,$check"
    done
done

Then a second script to perform the work of actually updating the check-lines in each of the failing tests, with a series of simple substitution patterns:

$ cat ./substitute-checks.sh
#!/bin/bash

file="$1"
check="$2"

# Any test that explicitly tests debug intrinsic output is not suitable to
# update by this script.
if grep -q "write-experimental-debuginfo=false" "$file"; then
    exit 0
fi

sed -i -E -e "
/(#|;|\/\/).*$check[A-Z0-9_\-]*:/!b
/DIGlobalVariableExpression/b
/!llvm.dbg./bpostcall
s/((((((no|must)?tail )?call.*)?void )?@)?llvm.)?dbg\.([a-z]+)/#dbg_\7/
:postcall
/declare #dbg_/d
s/metadata //g
s/metadata\{/{/g
s/DIExpression\(([^)]*)\)\)(,( !dbg)?)?/DIExpression(\1),/
/#dbg_/!b
s/((\))?(,) )?!dbg (![0-9]+)/\3\4\2/
s/((\))?(, ))?!dbg/\3/
" "$file"

Both of these scripts combined can be used on the list in manual-tests.txt as follows:

$ cat manual-tests.txt | xargs ./get-checks.sh | sort | uniq | awk -F ',' '{ system("./substitute-checks.sh " $1 " " $2) }'

These scripts dealt successfully with the vast majority of checks in clang/test and llvm/test.

Verify the resulting tests pass, and detect any failing tests:

$ xargs ./build/bin/llvm-lit -q < failing-tests.txt
********************
Failed Tests (5):
LLVM :: DebugInfo/Generic/dbg-value-lower-linenos.ll
LLVM :: Transforms/HotColdSplit/transfer-debug-info.ll
LLVM :: Transforms/ObjCARC/basic.ll
LLVM :: Transforms/ObjCARC/ensure-that-exception-unwind-path-is-visited.ll
LLVM :: Transforms/SafeStack/X86/debug-loc2.ll


Total Discovered Tests: 295
Failed: 5 (1.69%)

Some tests may have failed - the update scripts are simplistic and preserve no context across lines, and so there are cases that they will not handle; the remaining cases must be manually updated (or handled by further scripts).

C-API changes¶

Some new functions that have been added are temporary and will be deprecated in the future. The intention is that they’ll help downstream projects adapt during the transition period.

Deleted functions
-----------------
LLVMDIBuilderInsertDeclareBefore   # Insert a debug record (new debug info format) instead of a debug intrinsic (old debug info format).
LLVMDIBuilderInsertDeclareAtEnd    # Same as above.
LLVMDIBuilderInsertDbgValueBefore  # Same as above.
LLVMDIBuilderInsertDbgValueAtEnd   # Same as above.

New functions (to be deprecated)
--------------------------------
LLVMIsNewDbgInfoFormat     # Returns true if the module is in the new non-instruction mode.
LLVMSetIsNewDbgInfoFormat  # Convert to the requested debug info format.

New functions (no plans to deprecate)
-------------------------------------
LLVMGetFirstDbgRecord                    # Obtain the first debug record attached to an instruction.
LLVMGetLastDbgRecord                     # Obtain the last debug record attached to an instruction.
LLVMGetNextDbgRecord                     # Get next debug record or NULL.
LLVMGetPreviousDbgRecord                 # Get previous debug record or NULL.
LLVMDIBuilderInsertDeclareRecordBefore   # Insert a debug record (new debug info format).
LLVMDIBuilderInsertDeclareRecordAtEnd    # Same as above. See info below.
LLVMDIBuilderInsertDbgValueRecordBefore  # Same as above. See info below.
LLVMDIBuilderInsertDbgValueRecordAtEnd   # Same as above. See info below.

LLVMPositionBuilderBeforeDbgRecords          # See info below.
LLVMPositionBuilderBeforeInstrAndDbgRecords  # See info below.

LLVMDIBuilderInsertDeclareRecordBefore, LLVMDIBuilderInsertDeclareRecordAtEnd, LLVMDIBuilderInsertDbgValueRecordBefore and LLVMDIBuilderInsertDbgValueRecordAtEnd are replacing the deleted LLVMDIBuilderInsertDeclareBefore-style functions.

LLVMPositionBuilderBeforeDbgRecords and LLVMPositionBuilderBeforeInstrAndDbgRecords behave the same as LLVMPositionBuilder and LLVMPositionBuilderBefore except the insertion position is set before the debug records that precede the target instruction. Note that this doesn’t mean that debug intrinsics before the chosen instruction are skipped, only debug records (which unlike debug records are not themselves instructions).

If you don’t know which function to call then follow this rule: If you are trying to insert at the start of a block, or purposfully skip debug intrinsics to determine the insertion point for any other reason, then call the new functions.

LLVMPositionBuilder and LLVMPositionBuilderBefore are unchanged. They insert before the indicated instruction but after any attached debug records.

LLVMGetFirstDbgRecord, LLVMGetLastDbgRecord, LLVMGetNextDbgRecord and LLVMGetPreviousDbgRecord can be used for iterating over debug records attached to instructions (provided as LLVMValueRef).

LLVMDbgRecordRef DbgRec;
for (DbgRec = LLVMGetFirstDbgRecord(Inst); DbgRec;
     DbgRec = LLVMGetNextDbgRecord(DbgRec)) {
  // do something with DbgRec
}

LLVMDbgRecordRef DbgRec;
for (DbgRec = LLVMGetLastDbgRecord(Inst); DbgRec;
     DbgRec = LLVMGetPreviousDbgRecord(DbgRec)) {
  // do something with DbgRec
}

The new “Debug Record” model¶

Below is a brief overview of the new representation that replaces debug intrinsics; for an instructive guide on updating old code, see here.

What exactly have you replaced debug intrinsics with?¶

We’re using a dedicated C++ class called DbgRecord to store debug info, with a one-to-one relationship between each instance of a debug intrinsic and each DbgRecord object in any LLVM IR program; these DbgRecords are represented in the IR as non-instruction debug records, as described in the [Source Level Debugging](project:SourceLevelDebugging.rst#Debug Records) document. This class has a set of subclasses that store exactly the same information as is stored in debugging intrinsics. Each one also has almost entirely the same set of methods, that behave in the same way:

https://llvm.org/docs/doxygen/classllvm_1_1DbgRecord.html https://llvm.org/docs/doxygen/classllvm_1_1DbgVariableRecord.html https://llvm.org/docs/doxygen/classllvm_1_1DbgLabelRecord.html

This allows you to treat a DbgVariableRecord as if it’s a dbg.value/dbg.declare/dbg.assign intrinsic most of the time, for example in generic (auto-param) lambdas, and the same for DbgLabelRecord and dbg.labels.

How do these `DbgRecords` fit into the instruction stream?¶

Like so:

                 +---------------+          +---------------+
---------------->|  Instruction  +--------->|  Instruction  |
                 +-------+-------+          +---------------+
                         |
                         |
                         |
                         |
                         v
                  +-------------+
          <-------+  DbgMarker  |<-------
         /        +-------------+        \
        /                                 \
       /                                   \
      v                                     ^
 +-------------+    +-------------+   +-------------+
 |  DbgRecord  +--->|  DbgRecord  +-->|  DbgRecord  |
 +-------------+    +-------------+   +-------------+

Each instruction has a pointer to a DbgMarker (which will become optional), that contains a list of DbgRecord objects. No debugging records appear in the instruction list at all. DbgRecords have a parent pointer to their owning DbgMarker, and each DbgMarker has a pointer back to it’s owning instruction.

Not shown are the links from DbgRecord to other parts of the Value/Metadata hierachy: DbgRecord subclasses have tracking pointers to the DIMetadata that they use, and DbgVariableRecord has references to Values that are stored in a DebugValueUser base class. This refers to a ValueAsMetadata object referring to Values, via the TrackingMetadata facility.

The various kinds of debug intrinsic (value, declare, assign, label) are all stored in DbgRecord subclasses, with a “RecordKind” field distinguishing DbgLabelRecords from DbgVariableRecords, and a LocationType field in the DbgVariableRecord class further disambiguating the various debug variable intrinsics it can represent.

How to update existing code¶

Any existing code that interacts with debug intrinsics in some way will need to be updated to interact with debug records in the same way. A few quick rules to keep in mind when updating code:

Debug records will not be seen when iterating over instructions; to find the debug records that appear immediately before an instruction, you’ll need to iterate over Instruction::getDbgRecordRange().
Debug records have interfaces that are identical to those of debug intrinsics, meaning that any code that operates on debug intrinsics can be trivially applied to debug records as well. The exceptions for this are Instruction or CallInst methods that don’t logically apply to debug records, and isa/cast/dyn_cast methods, are replaced by methods on the DbgRecord class itself.
Debug records cannot appear in a module that also contains debug intrinsics; the two are mutually exclusive. As debug records are the future format, handling records correctly should be prioritized in new code.
Until support for intrinsics is no longer present, a valid hotfix for code that only handles debug intrinsics and is non-trivial to update is to convert the module to the intrinsic format using Module::setIsNewDbgInfoFormat, and convert it back afterwards.
- This can also be performed within a lexical scope for a module or an individual function using the class ScopedDbgInfoFormatSetter:
```
void handleModule(Module &M) {
  {
    ScopedDbgInfoFormatSetter FormatSetter(M, false);
    handleModuleWithDebugIntrinsics(M);
  }
  // Module returns to previous debug info format after exiting the above block.
}
```

Below is a rough guide on how existing code that currently supports debug intrinsics can be updated to support debug records.

Creating debug records¶

Debug records will automatically be created by the DIBuilder class when the new format is enabled. As with instructions, it is also possible to call DbgRecord::clone to create an unattached copy of an existing record.

Skipping debug records, ignoring debug-uses of `Values`, stably counting instructions, etc.¶

This will all happen transparently without needing to think about it!

for (Instruction &I : BB) {
  // Old: Skips debug intrinsics
  if (isa<DbgInfoIntrinsic>(&I))
    continue;
  // New: No extra code needed, debug records are skipped by default.
  ...
}

Finding debug records¶

Utilities such as findDbgUsers and the like now have an optional argument that will return the set of DbgVariableRecord records that refer to a Value. You should be able to treat them the same as intrinsics.

// Old:
  SmallVector<DbgVariableIntrinsic *> DbgUsers;
  findDbgUsers(DbgUsers, V);
  for (auto *DVI : DbgUsers) {
    if (DVI->getParent() != BB)
      DVI->replaceVariableLocationOp(V, New);
  }
// New:
  SmallVector<DbgVariableIntrinsic *> DbgUsers;
  SmallVector<DbgVariableRecord *> DVRUsers;
  findDbgUsers(DbgUsers, V, &DVRUsers);
  for (auto *DVI : DbgUsers)
    if (DVI->getParent() != BB)
      DVI->replaceVariableLocationOp(V, New);
  for (auto *DVR : DVRUsers)
    if (DVR->getParent() != BB)
      DVR->replaceVariableLocationOp(V, New);

Examining debug records at positions¶

Call Instruction::getDbgRecordRange() to get the range of DbgRecord objects that are attached to an instruction.

for (Instruction &I : BB) {
  // Old: Uses a data member of a debug intrinsic, and then skips to the next
  // instruction.
  if (DbgInfoIntrinsic *DII = dyn_cast<DbgInfoIntrinsic>(&I)) {
    recordDebugLocation(DII->getDebugLoc());
    continue;
  }
  // New: Iterates over the debug records that appear before `I`, and treats
  // them identically to the intrinsic block above.
  // NB: This should always appear at the top of the for-loop, so that we
  // process the debug records preceding `I` before `I` itself.
  for (DbgRecord &DR = I.getDbgRecordRange()) {
    recordDebugLocation(DR.getDebugLoc());
  }
  processInstruction(I);
}

This can also be passed through the function filterDbgVars to specifically iterate over DbgVariableRecords, which are more commonly used.

for (Instruction &I : BB) {
  // Old: If `I` is a DbgVariableIntrinsic we record the variable, and apply
  // extra logic if it is an `llvm.dbg.declare`.
  if (DbgVariableIntrinsic *DVI = dyn_cast<DbgVariableIntrinsic>(&I)) {
    recordVariable(DVI->getVariable());
    if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(DVI))
      recordDeclareAddress(DDI->getAddress());
    continue;
  }
  // New: `filterDbgVars` is used to iterate over only DbgVariableRecords.
  for (DbgVariableRecord &DVR = filterDbgVars(I.getDbgRecordRange())) {
    recordVariable(DVR.getVariable());
    // Debug variable records are not cast to subclasses; simply call the
    // appropriate `isDbgX()` check, and use the methods as normal.
    if (DVR.isDbgDeclare())
      recordDeclareAddress(DVR.getAddress());
  }
  // ...
}

Processing individual debug records¶

In most cases, any code that operates on debug intrinsics can be extracted to a template function or auto lambda (if it is not already in one) that can be applied to both debug intrinsics and debug records - though keep in mind the main exception that isa/cast/dyn_cast do not apply to DbgVariableRecord types.

// Old: Function that operates on debug variable intrinsics in a BasicBlock, and
// collects llvm.dbg.declares.
void processDbgInfoInBlock(BasicBlock &BB,
                           SmallVectorImpl<DbgDeclareInst*> &DeclareIntrinsics) {
  for (Instruction &I : BB) {
    if (DbgVariableIntrinsic *DVI = dyn_cast<DbgVariableIntrinsic>(&I)) {
      processVariableValue(DebugVariable(DVI), DVI->getValue());
      if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(DVI))
        Declares.push_back(DDI);
      else if (!isa<Constant>(DVI->getValue()))
        DVI->setKillLocation();
    }
  }
}

// New: Template function is used to deduplicate handling of intrinsics and
// records.
// An overloaded function is also used to handle isa/cast/dyn_cast operations
// for intrinsics and records, since those functions cannot be directly applied
// to DbgRecords.
DbgDeclareInst *DynCastToDeclare(DbgVariableIntrinsic *DVI) {
  return dyn_cast<DbgDeclareInst>(DVI);
}
DbgVariableRecord *DynCastToDeclare(DbgVariableRecord *DVR) {
  return DVR->isDbgDeclare() ? DVR : nullptr;
}

template<typename DbgVarTy, DbgDeclTy>
void processDbgVariable(DbgVarTy *DbgVar,
                       SmallVectorImpl<DbgDeclTy*> &Declares) {
    processVariableValue(DebugVariable(DbgVar), DbgVar->getValue());
    if (DbgDeclTy *DbgDeclare = DynCastToDeclare(DbgVar))
      Declares.push_back(DbgDeclare);
    else if (!isa<Constant>(DbgVar->getValue()))
      DbgVar->setKillLocation();
};

void processDbgInfoInBlock(BasicBlock &BB,
                           SmallVectorImpl<DbgDeclareInst*> &DeclareIntrinsics,
                           SmallVectorImpl<DbgVariableRecord*> &DeclareRecords) {
  for (Instruction &I : BB) {
    if (DbgVariableIntrinsic *DVI = dyn_cast<DbgVariableIntrinsic>(&I))
      processDbgVariable(DVI, DeclareIntrinsics);
    for (DbgVariableRecord *DVR : filterDbgVars(I.getDbgRecordRange()))
      processDbgVariable(DVR, DeclareRecords);
  }
}

Moving and deleting debug records¶

You can use DbgRecord::removeFromParent to unlink a DbgRecord from it’s marker, and then BasicBlock::insertDbgRecordBefore or BasicBlock::insertDbgRecordAfter to re-insert the DbgRecord somewhere else. You cannot insert a DbgRecord at an arbitary point in a list of DbgRecords (if you’re doing this with llvm.dbg.values then it’s unlikely to be correct).

Erase DbgRecords by calling eraseFromParent.

// Old: Move a debug intrinsic to the start of the block, and delete all other intrinsics for the same variable in the block.
void moveDbgIntrinsicToStart(DbgVariableIntrinsic *DVI) {
  BasicBlock *ParentBB = DVI->getParent();
  DVI->removeFromParent();
  for (Instruction &I : ParentBB) {
    if (auto *BlockDVI = dyn_cast<DbgVariableIntrinsic>(&I))
      if (BlockDVI->getVariable() == DVI->getVariable())
        BlockDVI->eraseFromParent();
  }
  DVI->insertBefore(ParentBB->getFirstInsertionPt());
}

// New: Perform the same operation, but for a debug record.
void moveDbgRecordToStart(DbgVariableRecord *DVR) {
  BasicBlock *ParentBB = DVR->getParent();
  DVR->removeFromParent();
  for (Instruction &I : ParentBB) {
    for (auto &BlockDVR : filterDbgVars(I.getDbgRecordRange()))
      if (BlockDVR->getVariable() == DVR->getVariable())
        BlockDVR->eraseFromParent();
  }
  DVR->insertBefore(ParentBB->getFirstInsertionPt());
}

What about dangling debug records?¶

If you have a block like so:

    foo:
      %bar = add i32 %baz...
      dbg.value(metadata i32 %bar,...
      br label %xyzzy

your optimisation pass may wish to erase the terminator and then do something to the block. This is easy to do when debug info is kept in instructions, but with DbgRecords there is no trailing instruction to attach the variable information to in the block above, once the terminator is erased. For such degenerate blocks, DbgRecords are stored temporarily in a map in LLVMContext, and are re-inserted when a terminator is reinserted to the block or other instruction inserted at end().

This can technically lead to trouble in the vanishingly rare scenario where an optimisation pass erases a terminator and then decides to erase the whole block. (We recommend not doing that).

Anything else?¶

The above guide does not comprehensively cover every pattern that could apply to debug intrinsics; as mentioned at the start of the guide, you can temporarily convert the target module from debug records to intrinsics as a stopgap measure. Most operations that can be performed on debug intrinsics have exact equivalents for debug records, but if you encounter any exceptions, reading the class docs (linked here) may give some insight, there may be examples in the existing codebase, and you can always ask for help on the forums.

Documentation

Getting Involved

Additional Links

This Page

Debug info migration: From intrinsics to records¶

Great, what do I need to do!¶

API Changes¶

Textual IR Changes¶

Test updates¶

C-API changes¶

The new “Debug Record” model¶

What exactly have you replaced debug intrinsics with?¶

How do these `DbgRecords` fit into the instruction stream?¶

How to update existing code¶

Creating debug records¶

Skipping debug records, ignoring debug-uses of `Values`, stably counting instructions, etc.¶

Finding debug records¶

Examining debug records at positions¶

Processing individual debug records¶

Moving and deleting debug records¶

What about dangling debug records?¶

Anything else?¶

Debug info migration: From intrinsics to records¶

Great, what do I need to do!¶

API Changes¶

Textual IR Changes¶

Test updates¶

C-API changes¶

The new “Debug Record” model¶

What exactly have you replaced debug intrinsics with?¶

How do these DbgRecords fit into the instruction stream?¶

How to update existing code¶

Creating debug records¶

Skipping debug records, ignoring debug-uses of Values, stably counting instructions, etc.¶

Finding debug records¶

Examining debug records at positions¶

Processing individual debug records¶

Moving and deleting debug records¶

What about dangling debug records?¶

Anything else?¶

How do these `DbgRecords` fit into the instruction stream?¶

Skipping debug records, ignoring debug-uses of `Values`, stably counting instructions, etc.¶