Add the concept of "backend intrinsics" to Cranelift #9651
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This commit is an initial stab at implementing interpreter-to-host
communication in Pulley. The basic problem is that Pulley needs the
ability to call back into Wasmtime to implement tasks such as
`memory.grow`, imported functions, etc. For native platforms this is a
simple `call_indirect` operation in Cranelift but the story for Pulley
must be different because it's effectively switching from interpreted
code to native code.
The solution I've ended up settling on looks pretty similar to native
platforms but with a few important tweaks:
* A new `call_indirect_host` opcode is added to Pulley.
* Function signatures that can be called from Pulley bytecode are
statically enumerated at build-time.
* This enables the implementation of `call_indirect_host` to take an
immediate of which signature is being used and cast the function
pointer to the right type.
* A new "backend intrinsic" concept is added to Cranelift.
* This is a new variant of `ExternalName`.
* The intention is that this has backend-specific meaning.
* For Pulley, this means that the Nth function signature is being called.
* Code generation for Pulley in `wasmtime-cranelift` now has
Pulley-specific handling of the wasm-to-host transition where all
previous `call_indirect` instructions are replaced with a call to a
"backend intrinsic" which gets lowered to a `call_indirect_host`.
Note that most of this still isn't hooked up everywhere in Wasmtime.
That means that the testing here is pretty light at this time. It'll
require a fair bit more work to get everything fully integrated from
Wasmtime in Pulley. This is expected to be one of the significant
remaining chunks of work and should help unblock future testing (or make
those diffs smaller ideally).
alexcrichton
requested review from
abrown and
pchickey
and removed request for
a team
|
cc @cfallin as you're also no doubt interested in this as well |
cfallin
reviewed
Nov 21, 2024
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This commit is an initial stab at implementing interpreter-to-host communication in Pulley. The basic problem is that Pulley needs the ability to call back into Wasmtime to implement tasks such as
memory.grow, imported functions, etc. For native platforms this is a simplecall_indirectoperation in Cranelift but the story for Pulley must be different because it's effectively switching from interpreted code to native code.The solution I've ended up settling on looks pretty similar to native platforms but with a few important tweaks:
call_indirect_hostopcode is added to Pulley.call_indirect_hostto take an immediate of which signature is being used and cast the function pointer to the right type.ExternalName.wasmtime-craneliftnow has Pulley-specific handling of the wasm-to-host transition where all previouscall_indirectinstructions are replaced with a call to a "backend intrinsic" which gets lowered to acall_indirect_host.Note that most of this still isn't hooked up everywhere in Wasmtime. That means that the testing here is pretty light at this time. It'll require a fair bit more work to get everything fully integrated from Wasmtime in Pulley. This is expected to be one of the significant remaining chunks of work and should help unblock future testing (or make those diffs smaller ideally).