[llvm-dev] Proposal for multi location debug info support in LLVM IR
Keno Fischer via llvm-dev
llvm-dev at lists.llvm.org
Sat Dec 26 13:23:27 PST 2015
Hi folks,
I've discussed my desire to have multi location debug info support in LLVM
IR with some of you before (and others - apologies if I forgot to CC you).
I finally found some time to write down my thoughts about how this should
work at the IR level and worked out the proposal below. Please let me know
if this a) also does what you want out of it, b) seems like a sane way to
encode things in the IR c) you see any obstacles in implementation.
This is a very rough draft, so I expect there may be several iterations, so
I'll try to keep https://gist.github.com/Keno/480b8057df1b7c63c321 updated
with the current iteration. Have at it:
===============================================================
# What is it? / Why do we want this?
At any given time, the value of a source variable may be available in more
than
one place. The most common case is that the variable is available in memory
as
well as loaded in a register, but esp. in higher level languages where the
notion of a variable is more disconnected from the physical realities,
there can
also be situations where you can find the same value in multiple places in
memory, or perhaps more commonly, their being multiple ways to get to the
same
value in memory (e.g. through the GC frame and the argument register).
One can represent this in DWARF (i.e. ranges can overlap), but perhaps more
importantly one can avoid having to make a choice about which value to
track in
mid level optimizations. The answer will generally depend on which value
will
live longer, but at that stage we do not know that information yet. As a
concrete example, InstCombine will currently replace llvm.dbg.declare but
llvm.dbg.values on every load and store, expecting the alloca to get
removed,
but if that assumption is wrong, we get worse debug info than we would have
without replacing the declare. With multiple location support, both
locations
can be described and either both emitted to DWARF, or we can chose the one
that
is live longer and emit that.
As such, there is two separate but related goals:
1. Allow frontends to describe complex situations where variables may be
available in more than one location.
2. Provide a coherent framework for describing the locations of source
variables in the optimization pipeline to improve debug info quality.
This proposal concerns the IR format for encoding this information.
Separately,
getting this info into DWARF will require additional work, some of which has
already been done in D11933 and D11986. The backend work is outside the
scope
of this proposal.
# Goals of this design
I tried to come up with a scheme that is a minimal modification of the
existing
mechanism to ease upgrading for both frontends and optimizers, but still
separates the three concerns I think are required for multiple locations
support
- Indicating that a value changed at source level (e.g. because an
assignment occurred)
- Indicating that the same value is now available in a new location
- Indicating that a value is no longer available in some location
The last one is required in order to be able describe e.g. stack slot
coloring,
where a memory location may cease to describe a variable even though the
value
remained the same at source level.
# The Proposal
I propose changing the llvm.dbg.value intrinsic from (note I'm ignoring the
i64
offset argument which is already essentially dead and I expect it to be
removed
soon)
void @llvm.dbg.value(metadata, metadata, metadata)
to
token @llvm.dbg.value(token, metadata, metadata, metadata)
with the semantics being the following:
- A change of value of a variable is indicated by (pseudeo-IR)
%first = call token @llvm.dbg.value(token undef, metadata %val,
metadata !var, metadata !expr)
for the purpose of this proposal, I'll denote such a call (with undef
first argument) as a `key call`.
- To add a location with the same value for the same variable, you pass
the
token of the FIRST llvm.dbg.value, as this llvm.dbg.value's first
argument
E.g. to add another location for the variable above:
%second = call token @llvm.dbg.value(token %first, metadata %val2,
metadata !var, metadata !expr2)
- To indicate that a location will no longer hold a value, you do the
following:
call token @llvm.dbg.value(token %second, metadata token undef,
metadata !var, metadata !())
- The current set of locations for a variable at a given instruction
are all
those llvm.dbg.value instructions that dominate this location (
equivalently all those llvm.dbg.value calls whose token you could use
at
that location without upsetting the Verifier), except that if more
than
one key call is dominating, only the most recent one and all calls
associated to it by first argument count.
I think that should encapsulate the semantics, but here are some
consequences
of and comments on the above that I think would be useful to discuss:
- The upgrade path for existing IR is very simple and just consists of
adding token undef as the first argument to any call in the IR.
- In general, if a value gets removed by an optimization, the
corresponding
llvm.dbg.value call can be removed, unless that call is a key call, in
which case the value should be undefed out. This is necessary both to
be
able to keep it around as the first argument to the other calls, and
more
importantly to mark the end point of a previous set of locations.
- I'm, not sure I like the location removal incantation, since it
doesn't
seem super intuitive, however, I did not want to introduce an extra
intrinsic just for this purpose. The second argument being a token
guarantees that just undefing out an instruction will not turn a
location
add into a location remove of the key call.
- It should be noted that for optimized (pseudo-C) source like:
if (foo) {
x = a;
} else {
x = b;
}
the IR would have to look like:
if.true:
%xtrue = ... (a)
call token llvm.dbg.value(token undef, %xtrue, !var, !())
br cont
if.false:
%xfalse = ... (b)
call token llvm.dbg.value(token undef, %xfalse, !var, !())
br cont
cont:
%x = phi [%xtrue, %if.true], [%xfalse, %if.false]
call token llvm.dbg.value(token undef, %x, !var, !())
as the live range of the debug value would end at the end of the
respective basic block.
- A related concern is what the following:
call token llvm.dbg.value(token undef, %xold, !var, !())
if.true:
%xtrue = ... (a)
call token llvm.dbg.value(token undef, %xtrue, !var, !())
br cont
if.false:
%xfalse = ... (b)
call token llvm.dbg.value(token undef, %xfalse, !var, !())
br cont
cont:
%x = phi [%xtrue, %if.true], [%xfalse, %if.false]
(i.e. the above but with a forgotten llvm.dbg.value in the cont
block).
By the semantics I have written above, `cont` would again have %xold
as
the value for %x, even though there was an intermediate assignment. I
am
not sure if this represents a problem, but it might at the very least
be
unexpected.
- Do we run into problems in whatever MSVC's equivalent for debug info
is.
- I think llvm.dbg.declare can be deprecated and it's uses replaced by
llvm.dbg.value with an DW_OP_deref. That would also clarify the
semantics
of the operation which have caused some confusion in the past.
- We may want to add an extra pass that does debug info inference (some
of
which is done in InstCombine right now)
Here are some of the invariants, the verifier would enforce (included in the
hope that they can clarify anything in the above):
1. If the first argument is not token undef, then
a. If the second argument is not token undef,
I. the first argument must be a call to llvm.dbg.value whose
first
argument is token undef
b. If the second argument is token undef
II. the first argument must be a call to llvm.dbg.value whose
second
argument is not token undef
III. the expression argument must be empty
c. In either case, the variable described must be the same as the
one
described by the call that is the first argument.
d. There may not be another call to llvm.dbg.value with token undef
that dominates this instruction, is not the one passed as the
first
argument and is dominated by the one passed as the first
argument.
2. All other invariants regarding calls to llvm.dbg.value carry over
unchanged
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