| a3d6e407 | 06-Oct-2025 |
Chris Fallin <[email protected]> |
Cranelift: add debug tag infrastructure. (#11768)
* Cranelift: add debug tag infrastructure.
This PR adds *debug tags*, a kind of metadata that can attach to CLIF
instructions and be lowered to VCo
Cranelift: add debug tag infrastructure. (#11768)
* Cranelift: add debug tag infrastructure.
This PR adds *debug tags*, a kind of metadata that can attach to CLIF
instructions and be lowered to VCode instructions and as metadata on
the produced compiled code. It also adds opaque descriptor blobs
carried with stackslots. Together, these two features allow decorating
IR with first-class debug instrumentation that is properly preserved
by the compiler, including across optimizations and
inlining. (Wasmtime's use of these features will come in followup
PRs.)
The key idea of a "debug tag" is to allow the Cranelift embedder to
express whatever information it needs to, in a format that is opaque
to Cranelift itself, except for the parts that need translation during
lowering. In particular, the `DebugTag::StackSlot` variant gets
translated to a physical offset into the stackframe in the compiled
metadata output. So, for example, the embedder can emit a tag
referring to a stackslot, and another describing an offset in that
stackslot.
The debug tags exist as a *sequence* on any given instruction; the
meaning of the sequence is known only to the embedder, *except* that
during inlining, the tags for the inlining call instruction are
prepended to the tags of inlined instructions. In this way, a
canonical use-case of tags as describing original source-language
frames can preserve the source-language view even when multiple
functions are inlined into one.
The descriptor on a stackslot may look a little odd at first, but its
purpose is to allow serializing some description of
stackslot-contained runtime user-program data, in a way that is firmly
attached to the stackslot. In particular, in the face of inlining,
this descriptor is copied into the inlining (parent) function from the
inlined function when the stackslot entity is copied; no other
metadata outside Cranelift needs to track the identity of stackslots
and know about that motion. This fits nicely with the ability of tags
to refer to stackslots; together, the embedder can annotate
instructions as having certain state in stackslots, and describe the
format of that state per stackslot.
This infrastructure is tested with some compile-tests now;
testing of the interpretation of the metadata output will come with
end-to-end debug instrumentation tests in a followup PR.
* Review feedback: add back sequence points and enforce tags only on sequence points or calls.
* Use Vecs for debug metadata in MachBuffer to avoid SmallVec size penalty in not-used case.
* Review feedback: switch from inlined stackslot descriptor blobs to u64 keys.
show more ...
|
| 59de3a32 | 07-Jun-2024 |
Nick Fitzgerald <[email protected]> |
Cranelift: Allow CLIF frontends to define their own stack maps (#8728)
Tracking GC references and producing stack maps is a significant amount of
complexity in `regalloc2`.
At the same time, GC ref
Cranelift: Allow CLIF frontends to define their own stack maps (#8728)
Tracking GC references and producing stack maps is a significant amount of
complexity in `regalloc2`.
At the same time, GC reference value types are pretty annoying to deal with in
Cranelift itself. We know our `r64` is "actually" just an `i64` pointer, and we
want to do `i64`-y things with it, such as an `iadd` to compute a derived
pointer, but `iadd` only takes integer types and not `r64`s. We investigated
loosening that restriction and it was way too painful given the way that CLIF
type inference and its controlling type vars work. So to compute those derived
pointers, we have to first `bitcast` the `r64` into an `i64`. This is
unfortunate in two ways. First, because of arcane interactions between register
allocation constraints, stack maps, and ABIs this involves inserting unnecessary
register-to-register moves in our generated code which hurts binary size and
performance ever so slightly. Second, and much more seriously, this is a serious
footgun. If a GC reference isn't an `r64` right now, then it will not appear in
stack maps, and failure to record a live GC reference in a stack map means that
the collector could reclaim the object while you are still using it, leading to
use-after-free bugs! Very bad. And the mid-end needs to know
*not* to GVN these bitcasts or else we get similar bugs (see
https://github.com/bytecodealliance/wasmtime/pull/8317).
Overall GC references are a painful situation for us today.
This commit is the introduction of an alternative. (Note, though, that we aren't
quite ready to remove the old stack maps infrastructure just yet.)
Instead of preserving GC references all the way through the whole pipeline and
computing live GC references and inserting spills at safepoints for stack maps
all the way at the end of that pipeline in register allocation, the
CLIF-producing frontend explicitly generates its own stack slots and spills for
safepoints. The only thing the rest of the compiler pipeline needs to know is
the metadata required to produce the stack map for the associated safepoint. We
can completely remove `r32` and `r64` from Cranelift and just use plain `i32`
and `i64` values. Or `f64` if the runtime uses NaN-boxing, which the old stack
maps system did not support at all. Or 32-bit GC references on a 64-bit target,
which was also not supported by the old system. Furthermore, we *cannot* get
miscompiles due to GVN'ing bitcasts that shouldn't be GVN'd because there aren't
any bitcasts hiding GC references from stack maps anymore. And in the case of a
moving GC, we don't need to worry about the mid-end doing illegal code motion
across calls that could have triggered a GC that invalidated the moved GC
reference because frontends will reload their GC references from the stack slots
after the call, and that loaded value simply isn't a candidate for GVN with the
previous version. We don't have to worry about those bugs by construction.
So everything gets a lot easier under this new system.
But this commit doesn't mean we are 100% done and ready to transition to the new
system, so what is actually in here?
* CLIF producers can mark values as needing to be present in a stack map if they
are live across a safepoint in `cranelift-frontend`. This is the
`FunctionBuilder::declare_needs_stack_map` method.
* When we finalize the function we are building, we do a simple, single-pass
liveness analysis to determine the set of GC references that are live at each
safepoint, and then we insert spills to explicit stack slots just before the
safepoint. We intentionally trade away the precision of a fixed-point liveness
analysis for the speed and simplicity of a single-pass implementation.
* We annotate the safepoint with the metadata necessary to construct its
associated stack map. This is the new
`cranelift_codegen::ir::DataFlowGraph::append_user_stack_map_entry` method and
all that stuff.
* These stack map entries are part of the CLIF and can be roundtripped through
printing and parsing CLIF.
* Each stack map entry describes a GC-managed value that is on the stack and how
to locate it: its type, the stack slot it is located within, and the offset
within that stack slot where it resides. Different stack map entries for the
same safepoint may have different types or a different width from the target's
pointer.
Here is what is *not* handled yet, and left for future follow up commits:
* Lowering the stack map entries' locations from symbolic stack slot and offset
pairs to physical stack frame offsets after register allocation.
* Coalescing and aggregating the safepoints and their raw stack map entries into
a compact PC-to-stack-map table during emission.
* Supporting moving GCs. Right now we generate spills into stack slots for live
GC references just before safepoints, but we don't reload the GC references from
the stack upon their next use after the safepoint. This involves rewriting uses
of the old, spilled values which could be a little finicky, but we think we have
a good approach.
* Port Wasmtime over to using this new stack maps system.
* Removing the old stack map system, including `r{32,64}` from Cranelift and GC
reference handling from `regalloc2`. (For the time being, the new system
generally refers to "user stack maps" to disambiguate from the old system where
it might otherwise be confusing.) If we wanted to remove the old system now,
that would require us to also port Wasmtime to the new system now, and we'd end
up with a monolithic PR. Better to do this incrementally and temporarily have
the old and in-progress new system overlap for a short period of time.
Co-authored-by: Trevor Elliott <[email protected]>
show more ...
|