What’s all this then?¶
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!¶
Approximately nothing – we’ve already instrumented all of LLVM to handle
these new records (“DPValues”) and behave identically to past LLVM
behaviour. We plan on turning this on by default some time soon, with IR
converted to the intrinsic form of debug info at terminals (textual IR,
bitcode) for a short while, before then changing the textual IR and
bitcode formats.
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.
Anything else?¶
Not really, but here’s an “old vs new” comparison of how to do certain things and quickstart for how this “new” debug info is structured.
Skipping debug records, ignoring debug-uses of Values, stably counting instructions…¶
This will all happen transparently without needing to think about it!
What exactly have you replaced debug intrinsics with?¶
We’re using a dedicated C++ class called DPValue to store debug
info, with a one-to-one relationship between each instance of a debug
intrinsic and each DPValue object in any LLVM IR program. This class
stores exactly the same information as is stored in debugging
intrinsics. It also has almost entirely the same set of methods, that
behave in the same way:
https://llvm.org/docs/doxygen/classllvm_1_1DPValue.html
This allows you to treat a DPValue as if it’s a dbg.value
intrinsic most of the time, for example in generic (auto-param) lambdas.
How do these DPValues fit into the instruction stream?¶
Like so:
+---------------+ +---------------+
---------------->| Instruction +--------->| Instruction |
+-------+-------+ +---------------+
|
|
|
|
v
+------------+
<-----+ DPMarker |<----
/ +------------+ \
/ \
/ \
v ^
+-----------+ +-----------+ +-----------+
| DPValue +--->| DPValue +-->| DPValue |
+-----------+ +-----------+ +-----------+
Each instruction has a pointer to a DPMarker (which will become
optional), that contains a list of DPValue objects. No debugging
records appear in the instruction list at all. DPValues have a
parent pointer to their owning DPMarker, and each DPMarker has a
pointer back to it’s owning instruction.
Not shown are the links from DPValues to other parts of the
Value/Metadata hierachy: DPValues have raw pointers to
DILocalVariable, DIExpression and DILocation objects, and
references to Values 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) are all
stored in the DPValue object, with a “Type” field disamgibuating
which is which.
Finding debug info records¶
Utilities such as findDbgUsers and the like now have an optional
argument that will return the set of DPValue records that refer to a
Value. You should be able to treat them the same as intrinsics.
Examining debug info records at positions¶
Call Instruction::getDbgValueRange() to get the range of DPValue
objects that are attached to an instruction.
Moving around, deleting¶
You can use DPValue::removeFromParent to unlink a DPValue from
it’s marker, and then BasicBlock::insertDPValueBefore or
BasicBlock::insertDPValueAfter to re-insert the DPValue
somewhere else. You cannot insert a DPValue at an arbitary point in
a list of DPValues (if you’re doing this with dbg.values
then it’s unlikely to be correct).
Erase DPValues by calling eraseFromParent or deleteInstr
if it’s already been removed.
What about dangling DPValues?¶
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 DPValues there is no trailing instruction
to attach the variable information to in the block above, once the
terminator is erased. For such degenerate blocks, DPValues 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).