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.