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
DIExpression
s, 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 {
entry:
%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.declare
s. 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 theirllvm.dbg.declare
location modified by aDIExpression
, e.g. when the address of the variable is itself stored in analloca
with thellvm.dbg.declare
usingDIExpression(DW_OP_deref)
. SeeindirectReturn
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 tollvm.dbg.declare
except that it needs to compose withllvm.dbg.assign
intrinsics such that the stack home address is only selected as a location for the variable when thellvm.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.