Add the concept of "backend intrinsics" to Cranelift #9651
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alexcrichton
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abrown and
pchickey
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cc @cfallin as you're also no doubt interested in this as well |
cfallin
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Nov 21, 2024
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Instead of adding a whole new instruction, would it be possible to use a UserExternalName with a reserved namespace like |
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I was hoping originally to do that yeah but one part I couldn't figure out was how to translate a |
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Each Function consists of a FunctionStencil with things like all instructions and tables in FunctionParameters to map eg UserExternalNameRef to UserExternalName. The backend is only supposed to look at the stencil such that the stencil can be used as cache key for a compilation cache. The user of the generated machine code then uses these tables to map the relocations emitted together with the machine code into the actual symbols that are referenced. For example using |
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
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The backend itself has no way of knowing whether to generate such a relocation though, right? In that it can't look at the To confirm though you're thinking that something could look like:
And that'd skip the idea of intrinsics altogether? |
All calls to other functions should result in relocations whether they target another pulley function or a host function. It is then up to whatever consumes the MachBuffer to apply the right relocations as indicated, which in the case call relocations would include filling in the actual index of the UserExternalName and patch a call instruction to call_indirect_host instruction in case the target is a host function. |
|
Ah ok that makes sense to me yeah. That requires that the opcodes have the same length but I think that's reasonable to arrange. I'll work on that route! |
alexcrichton
added a commit
to alexcrichton/wasmtime
that referenced
this pull request
Nov 22, 2024
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 initial idea for this in bytecodealliance#9651 is replaced here and looks mostly similar but with a few changes. The overall structure of how this works is: * 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 pulley-specific relocation is added to Cranelift for this opcode. * `RelocDistance::Far` calls to a name trigger the use of `call_indirect_host`. * The relocation is filled in by Wasmtime after compilation where the signature number is inserted. * A new `NS_*` value for user-function namespaces is reserved in `wasmtime-cranelift` for this new namespace of functions. * 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
added a commit
to alexcrichton/wasmtime
that referenced
this pull request
Nov 22, 2024
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 initial idea for this in bytecodealliance#9651 is replaced here and looks mostly similar but with a few changes. The overall structure of how this works is: * 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 pulley-specific relocation is added to Cranelift for this opcode. * `RelocDistance::Far` calls to a name trigger the use of `call_indirect_host`. * The relocation is filled in by Wasmtime after compilation where the signature number is inserted. * A new `NS_*` value for user-function namespaces is reserved in `wasmtime-cranelift` for this new namespace of functions. * 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
added a commit
to alexcrichton/wasmtime
that referenced
this pull request
Nov 22, 2024
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 initial idea for this in bytecodealliance#9651 is replaced here and looks mostly similar but with a few changes. The overall structure of how this works is: * 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 pulley-specific relocation is added to Cranelift for this opcode. * `RelocDistance::Far` calls to a name trigger the use of `call_indirect_host`. * The relocation is filled in by Wasmtime after compilation where the signature number is inserted. * A new `NS_*` value for user-function namespaces is reserved in `wasmtime-cranelift` for this new namespace of functions. * 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
added a commit
to alexcrichton/wasmtime
that referenced
this pull request
Nov 22, 2024
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 initial idea for this in bytecodealliance#9651 is replaced here and looks mostly similar but with a few changes. The overall structure of how this works is: * 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 pulley-specific relocation is added to Cranelift for this opcode. * `RelocDistance::Far` calls to a name trigger the use of `call_indirect_host`. * The relocation is filled in by Wasmtime after compilation where the signature number is inserted. * A new `NS_*` value for user-function namespaces is reserved in `wasmtime-cranelift` for this new namespace of functions. * 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).
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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).