Debug Info Assignment Tracking

Assignment Tracking is an alternative technique for tracking variable location debug info through optimisations in LLVM. It provides accurate variable locations for assignments where a local variable (or a field of one) is the LHS. In rare and complicated circumstances indirect assignments might be optimized away without being tracked, but otherwise we make our best effort to track all variable locations.

The core idea is to track more information about source assignments in order and preserve enough information to be able to defer decisions about whether to use non-memory locations (register, constant) or memory locations until after middle end optimisations have run. This is in opposition to using llvm.dbg.declare and llvm.dbg.value, which is to make the decision for most variables early on, which can result in suboptimal variable locations that may be either incorrect or incomplete.

A secondary goal of assignment tracking is to cause minimal additional work for LLVM pass writers, and minimal disruption to LLVM in general.

Status and usage

Status: Experimental work in progress. Enabling is strongly advised against except for development and testing.

Enable in Clang: -Xclang -fexperimental-assignment-tracking

That causes Clang to get LLVM to run the pass declare-to-assign. The pass converts conventional debug intrinsics to assignment tracking metadata and sets the module flag debug-info-assignment-tracking to the value i1 true. To check whether assignment tracking is enabled for a module call isAssignmentTrackingEnabled(const Module &M) (from llvm/IR/DebugInfo.h).

Design and implementation

Assignment markers: llvm.dbg.assign

llvm.dbg.value, a conventional debug intrinsic, marks out a position in the IR where a variable takes a particular value. Similarly, Assignment Tracking marks out the position of assignments with a new intrinsic called llvm.dbg.assign.

In order to know where in IR it is appropriate to use a memory location for a variable, each assignment marker must in some way refer to the store, if any (or multiple!), that performs the assignment. That way, the position of the store and marker can be considered together when making that choice. Another important benefit of referring to the store is that we can then build a two-way mapping of stores<->markers that can be used to find markers that need to be updated when stores are modified.

An llvm.dbg.assign marker that is not linked to any instruction signals that the store that performed the assignment has been optimised out, and therefore the memory location will not be valid for at least some part of the program.

Here's the llvm.dbg.assign signature. Each parameter is wrapped in MetadataAsValue, and Value * type parameters are first wrapped in ValueAsMetadata:

void @llvm.dbg.assign(Value *Value,
                      DIExpression *ValueExpression,
                      DILocalVariable *Variable,
                      DIAssignID *ID,
                      Value *Address,
                      DIExpression *AddressExpression)

The first three parameters look and behave like an llvm.dbg.value. ID is a reference to a store (see next section). Address is the destination address of the store and it is modified by AddressExpression. An empty/undef/poison address means the address component has been killed (the memory address is no longer a valid location). LLVM currently encodes variable fragment information in DIExpressions, so as an implementation quirk the FragmentInfo for Variable is contained within ValueExpression only.

The formal LLVM-IR signature is:

void @llvm.dbg.assign(metadata, metadata, metadata, metadata, metadata, metadata)

Instruction link: DIAssignID

DIAssignID metadata is the mechanism that is currently used to encode the store<->marker link. The metadata node has no operands and all instances are distinct; equality is checked for by comparing addresses.

llvm.dbg.assign intrinsics use a DIAssignID metadata node instance as an operand. This way it refers to any store-like instruction that has the same DIAssignID attachment. E.g. For this test.cpp,

int fun(int a) {
  return a;

compiled without optimisations:

$ clang++ test.cpp -o test.ll -emit-llvm -S -g -O0 -Xclang -fexperimental-assignment-tracking

we get:

define dso_local noundef i32 @_Z3funi(i32 noundef %a) #0 !dbg !8 {
  %a.addr = alloca i32, align 4, !DIAssignID !13
  call void @llvm.dbg.assign(metadata i1 undef, metadata !14, metadata !DIExpression(), metadata !13, metadata i32* %a.addr, metadata !DIExpression()), !dbg !15
  store i32 %a, i32* %a.addr, align 4, !DIAssignID !16
  call void @llvm.dbg.assign(metadata i32 %a, metadata !14, metadata !DIExpression(), metadata !16, metadata i32* %a.addr, metadata !DIExpression()), !dbg !15
  %0 = load i32, i32* %a.addr, align 4, !dbg !17
  ret i32 %0, !dbg !18

!13 = distinct !DIAssignID()
!14 = !DILocalVariable(name: "a", ...)
!16 = distinct !DIAssignID()

The first llvm.dbg.assign refers to the alloca through !DIAssignID !13, and the second refers to the store through !DIAssignID !16.

Store-like instructions

In the absence of a linked llvm.dbg.assign, a store to an address that is known to be the backing storage for a variable is considered to represent an assignment to that variable.

This gives us a safe fall-back in cases where llvm.dbg.assign intrinsics have been deleted, the DIAssignID attachment on the store has been dropped, or the optimiser has made a once-indirect store (not tracked with Assignment Tracking) direct.

Middle-end: Considerations for pass-writers

Non-debug instruction updates

Cloning an instruction: nothing new to do. Cloning automatically clones a DIAssignID attachment. Multiple instructions may have the same DIAssignID instruction. In this case, the assignment is considered to take place in multiple positions in the program.

Moving a non-debug instruction: nothing new to do. Instructions linked to an llvm.dbg.assign have their initial IR position marked by the position of the llvm.dbg.assign.

Deleting a non-debug instruction: nothing new to do. Simple DSE does not require any change; it’s safe to delete an instruction with a DIAssignID attachment. An llvm.dbg.assign that uses a DIAssignID that is not attached to any instruction indicates that the memory location isn’t valid.

Merging stores: In many cases no change is required as DIAssignID attachments are automatically merged if combineMetadata is called. One way or another, the DIAssignID attachments must be merged such that new store becomes linked to all the llvm.dbg.assign intrinsics that the merged stores were linked to. This can be achieved simply by calling a helper function Instruction::mergeDIAssignID.

Inlining stores: As stores are inlined we generate llvm.dbg.assign intrinsics and DIAssignID attachments as if the stores represent source assignments, just like the in frontend. This isn’t perfect, as stores may have been moved, modified or deleted before inlining, but it does at least keep the information about the variable correct within the non-inlined scope.

Splitting stores: SROA and passes that split stores treat llvm.dbg.assign intrinsics similarly to llvm.dbg.declare intrinsics. Clone the llvm.dbg.assign intrinsics linked to the store, update the FragmentInfo in the ValueExpression, and give the split stores (and cloned intrinsics) new DIAssignID attachments each. In other words, treat the split stores as separate assignments. For partial DSE (e.g. shortening a memset), we do the same except that llvm.dbg.assign for the dead fragment gets an Undef Address.

Promoting allocas and store/loads: llvm.dbg.assign intrinsics implicitly describe joined values in memory locations at CFG joins, but this is not necessarily the case after promoting (or partially promoting) the variable. Passes that promote variables are responsible for inserting llvm.dbg.assign intrinsics after the resultant PHIs generated during promotion. mem2reg already has to do this (with llvm.dbg.value) for llvm.dbg.declares. Where a store has no linked intrinsic, the store is assumed to represent an assignment for variables stored at the destination address.

Debug intrinsic updates

Moving a debug intrinsic: avoid moving llvm.dbg.assign intrinsics where possible, as they represent a source-level assignment, whose position in the program should not be affected by optimization passes.

Deleting a debug intrinsic: Nothing new to do. Just like for conventional debug intrinsics, unless it is unreachable, it’s almost always incorrect to delete a llvm.dbg.assign intrinsic.

Lowering llvm.dbg.assign to MIR

To begin with only SelectionDAG ISel will be supported. llvm.dbg.assign intrinsics are lowered to MIR DBG_INSTR_REF instructions. Before this happens we need to decide where it is appropriate to use memory locations and where we must use a non-memory location (or no location) for each variable. In order to make those decisions we run a standard fixed-point dataflow analysis that makes the choice at each instruction, iteratively joining the results for each block.

TODO list

As this is an experimental work in progress so there are some items we still need to tackle:

  • As mentioned in test llvm/test/DebugInfo/assignment-tracking/X86/diamond-3.ll, the analysis should treat escaping calls like untagged stores.

  • The system expects locals to be backed by a local alloca. This isn't always the case - sometimes a pointer to storage is passed into a function (e.g. sret, byval). We need to be able to handle those cases. See llvm/test/DebugInfo/Generic/assignment-tracking/track-assignments.ll and clang/test/CodeGen/assignment-tracking/assignment-tracking.cpp for examples.

  • trackAssignments doesn't yet work for variables that have their llvm.dbg.declare location modified by a DIExpression, e.g. when the address of the variable is itself stored in an alloca with the llvm.dbg.declare using DIExpression(DW_OP_deref). See indirectReturn in llvm/test/DebugInfo/Generic/assignment-tracking/track-assignments.ll and in clang/test/CodeGen/assignment-tracking/assignment-tracking.cpp for an example.

  • In order to solve the first bullet-point we need to be able to specify that a memory location is available without using a DIAssignID. This is because the storage address is not computed by an instruction (it‘s an argument value) and therefore we have nowhere to put the metadata attachment. To solve this we probably need another marker intrinsic to denote "the variable’s stack home is X address" - similar to llvm.dbg.declare and llvm.dbg.addr except that it needs to compose with llvm.dbg.assign intrinsics such that the stack home address is only selected as a location for the variable when the llvm.dbg.assign intrinsics agree it should be.

  • Given the above (a special “the stack home is X” intrinsic), and the fact that we can only track assignments with fixed offsets and sizes, I think we can probably get rid of the address and address-expression part, since it will always be computable with the info we have.