pulley: Implement interpreter-to-host calls

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).

cranelift/codegen/src/binemit/mod.rs

@@ -119,6 +119,10 @@ pub enum Reloc {
     S390xTlsGd64,
     /// s390x TLS GDCall - marker to enable optimization of TLS calls
     S390xTlsGdCall,
+
+    /// Pulley - call a host function indirectly where the embedder resolving
+    /// this relocation needs to fill in the expected signature.
+    PulleyCallIndirectHost,
 }
 
 impl fmt::Display for Reloc {
@@ -152,6 +156,7 @@ impl fmt::Display for Reloc {
             Self::Aarch64Ld64GotLo12Nc => write!(f, "Aarch64AdrGotLo12Nc"),
             Self::S390xTlsGd64 => write!(f, "TlsGd64"),
             Self::S390xTlsGdCall => write!(f, "TlsGdCall"),
+            Self::PulleyCallIndirectHost => write!(f, "PulleyCallIndirectHost"),
         }
     }
 }

cranelift/codegen/src/isa/pulley_shared/abi.rs

@@ -541,36 +541,29 @@ where
         insts
     }
 
-    fn gen_call(dest: &CallDest, tmp: Writable<Reg>, info: CallInfo<()>) -> SmallVec<[Self::I; 2]> {
-        if info.callee_conv == isa::CallConv::Tail || info.callee_conv == isa::CallConv::Fast {
-            match &dest {
-                &CallDest::ExtName(ref name, RelocDistance::Near) => smallvec![Inst::Call {
-                    info: Box::new(info.map(|()| name.clone()))
-                }
-                .into()],
-                &CallDest::ExtName(ref name, RelocDistance::Far) => smallvec![
-                    Inst::LoadExtName {
-                        dst: WritableXReg::try_from(tmp).unwrap(),
-                        name: Box::new(name.clone()),
-                        offset: 0,
-                    }
-                    .into(),
-                    Inst::IndirectCall {
-                        info: Box::new(info.map(|()| XReg::new(tmp.to_reg()).unwrap()))
-                    }
-                    .into(),
-                ],
-                &CallDest::Reg(reg) => smallvec![Inst::IndirectCall {
-                    info: Box::new(info.map(|()| XReg::new(*reg).unwrap()))
-                }
-                .into()],
+    fn gen_call(
+        dest: &CallDest,
+        _tmp: Writable<Reg>,
+        info: CallInfo<()>,
+    ) -> SmallVec<[Self::I; 2]> {
+        match dest {
+            // "near" calls are pulley->pulley calls so they use a normal "call"
+            // opcode
+            CallDest::ExtName(name, RelocDistance::Near) => smallvec![Inst::Call {
+                info: Box::new(info.map(|()| name.clone()))
             }
-        } else {
-            todo!(
-                "host calls? callee_conv = {:?}; caller_conv = {:?}",
-                info.callee_conv,
-                info.caller_conv,
-            )
+            .into()],
+            // "far" calls are pulley->host calls so they use a different opcode
+            // which is lowered with a special relocation in the backend.
+            CallDest::ExtName(name, RelocDistance::Far) => smallvec![Inst::IndirectCallHost {
+                info: Box::new(info.map(|()| name.clone()))
+            }
+            .into()],
+            // Indirect calls are all assumed to be pulley->pulley calls
+            CallDest::Reg(reg) => smallvec![Inst::IndirectCall {
+                info: Box::new(info.map(|()| XReg::new(*reg).unwrap()))
+            }
+            .into()],
         }
     }
 

cranelift/codegen/src/isa/pulley_shared/inst.isle

@@ -59,6 +59,10 @@
     ;; An indirect call to an unknown callee.
     (IndirectCall (info BoxCallIndInfo))
 
+    ;; An indirect call out to a host-defined function. The host function
+    ;; pointer is the first "argument" of this function call.
+    (IndirectCallHost (info BoxCallInfo))
+
     ;; Unconditional jumps.
     (Jump (label MachLabel))
 

cranelift/codegen/src/isa/pulley_shared/inst/emit.rs

@@ -229,6 +229,23 @@ fn pulley_emit<P>(
             state.adjust_virtual_sp_offset(-callee_pop_size);
         }
 
+        Inst::IndirectCallHost { info } => {
+            // Emit a relocation to fill in the actual immediate argument here
+            // in `call_indirect_host`.
+            sink.add_reloc(Reloc::PulleyCallIndirectHost, &info.dest, 0);
+            enc::call_indirect_host(sink, 0_u8);
+
+            if let Some(s) = state.take_stack_map() {
+                let offset = sink.cur_offset();
+                sink.push_user_stack_map(state, offset, s);
+            }
+            sink.add_call_site();
+
+            // If a callee pop is happening here that means that something has
+            // messed up, these are expected to be "very simple" signatures.
+            assert!(info.callee_pop_size == 0);
+        }
+
         Inst::Jump { label } => {
             sink.use_label_at_offset(*start_offset + 1, *label, LabelUse::Jump(1));
             sink.add_uncond_branch(*start_offset, *start_offset + 5, *label);

cranelift/codegen/src/isa/pulley_shared/inst/mod.rs

@@ -95,7 +95,7 @@ fn pulley_get_operands(inst: &mut Inst, collector: &mut impl OperandVisitor) {
             collector.reg_def(dst);
         }
 
-        Inst::Call { info } => {
+        Inst::Call { info } | Inst::IndirectCallHost { info } => {
             let CallInfo { uses, defs, .. } = &mut **info;
             for CallArgPair { vreg, preg } in uses {
                 collector.reg_fixed_use(vreg, *preg);
@@ -592,6 +592,10 @@ impl Inst {
                 format!("indirect_call {callee}, {info:?}")
             }
 
+            Inst::IndirectCallHost { info } => {
+                format!("indirect_call_host {info:?}")
+            }
+
             Inst::Jump { label } => format!("jump {}", label.to_string()),
 
             Inst::BrIf {

cranelift/codegen/src/machinst/buffer.rs

@@ -2037,6 +2037,10 @@ impl<I: VCodeInst> TextSectionBuilder for MachTextSectionBuilder<I> {
         self.force_veneers = ForceVeneers::Yes;
     }
 
+    fn write(&mut self, offset: u64, data: &[u8]) {
+        self.buf.data[offset.try_into().unwrap()..][..data.len()].copy_from_slice(data);
+    }
+
     fn finish(&mut self, ctrl_plane: &mut ControlPlane) -> Vec<u8> {
         // Double-check all functions were pushed.
         assert_eq!(self.next_func, self.buf.label_offsets.len());

cranelift/codegen/src/machinst/mod.rs

@@ -554,6 +554,10 @@ pub trait TextSectionBuilder {
     /// A debug-only option which is used to for
     fn force_veneers(&mut self);
 
+    /// Write the `data` provided at `offset`, for example when resolving a
+    /// relocation.
+    fn write(&mut self, offset: u64, data: &[u8]);
+
     /// Completes this text section, filling out any final details, and returns
     /// the bytes of the text section.
     fn finish(&mut self, ctrl_plane: &mut ControlPlane) -> Vec<u8>;

cranelift/filetests/filetests/isa/pulley64/call_indirect_host.clif

@@ -0,0 +1,37 @@
+test compile precise-output
+target pulley64
+
+function %call_indirect_host() {
+  fn0 = u10:0() system_v
+block0:
+  call fn0()
+  return
+}
+
+; VCode:
+;   x30 = xconst8 -16
+;   x27 = xadd32 x27, x30
+;   store64 sp+8, x28 // flags =  notrap aligned
+;   store64 sp+0, x29 // flags =  notrap aligned
+;   x29 = xmov x27
+; block0:
+;   indirect_call_host CallInfo { dest: User(userextname0), uses: [], defs: [], clobbers: PRegSet { bits: [65535, 65279, 4294967295, 0] }, callee_conv: SystemV, caller_conv: Fast, callee_pop_size: 0 }
+;   x28 = load64_u sp+8 // flags = notrap aligned
+;   x29 = load64_u sp+0 // flags = notrap aligned
+;   x30 = xconst8 16
+;   x27 = xadd32 x27, x30
+;   ret
+;
+; Disassembled:
+; xconst8 spilltmp0, -16
+; xadd32 sp, sp, spilltmp0
+; store64_offset8 sp, 8, lr
+; store64 sp, fp
+; xmov fp, sp
+; call_indirect_host 0
+; load64_offset8 lr, sp, 8
+; load64 fp, sp
+; xconst8 spilltmp0, 16
+; xadd32 sp, sp, spilltmp0
+; ret
+

crates/cranelift/Cargo.toml

@@ -32,11 +32,12 @@ thiserror = { workspace = true }
 cfg-if = { workspace = true }
 wasmtime-versioned-export-macros = { workspace = true }
 itertools = "0.12"
+pulley-interpreter = { workspace = true, optional = true }
 
 [features]
 all-arch = ["cranelift-codegen/all-arch"]
 host-arch = ["cranelift-codegen/host-arch"]
-pulley = ["cranelift-codegen/pulley"]
+pulley = ["cranelift-codegen/pulley", "dep:pulley-interpreter"]
 trace-log = ["cranelift-codegen/trace-log"]
 component-model = ["wasmtime-environ/component-model"]
 incremental-cache = ["cranelift-codegen/incremental-cache"]

crates/cranelift/src/compiler.rs

@@ -119,6 +119,105 @@ impl Compiler {
             wmemcheck,
         }
     }
+
+    /// Peform an indirect call from Cranelift-generated code to native code in
+    /// Wasmtime itself.
+    ///
+    /// For native platforms this is a simple `call_indirect` instruction but
+    /// for the Pulley backend this is special as it's transitioning from
+    /// Cranelift-generated bytecode to native code on the host. That requires a
+    /// special opcode in the interpreter and is modeled slightly differently in
+    /// Cranelift IR.
+    fn call_indirect_host(
+        &self,
+        builder: &mut FunctionBuilder<'_>,
+        sig: ir::SigRef,
+        addr: Value,
+        args: &[Value],
+    ) -> ir::Inst {
+        let signature = &builder.func.dfg.signatures[sig];
+
+        // When calling the host we should always be using the platform's
+        // default calling convention since it'll be calling Rust code in
+        // Wasmtime itself.
+        assert_eq!(signature.call_conv, self.isa.default_call_conv());
+
+        #[cfg(feature = "pulley")]
+        {
+            use cranelift_codegen::ir::types::{I32, I64, I8};
+
+            // If pulley is enabled, even if we're not targetting it, determine
+            // what pulley signature that the input `signature` maps to. This is
+            // done to ensure that even on native platforms we've always got a
+            // signature listed in pulley for all platform intrinsics. In theory
+            // the set of signatures here doesn't change over time all that
+            // much. If a new signature is added then the `pulley/src/lib.rs`
+            // file and the `for_each_host_signature!` macro need to be updated.
+            // In theory that's all that needs to happen as well...
+            macro_rules! pulley_signum {
+                ($(fn($($args:ident),*) $(-> $ret:ident)?;)*) => {'outer: {
+
+                    let mut ret = 0;
+
+                    $(
+                        let mut params = signature.params.iter().map(|p| p.value_type);
+                        let mut results = signature.returns.iter().map(|p| p.value_type);
+                        if true
+                            $(&& params.next() == Some($args))*
+                            && params.next().is_none()
+                            $(&& results.next() == Some($ret))?
+                            && results.next().is_none()
+                        {
+                            break 'outer ret;
+                        }
+                        ret += 1;
+                    )*
+
+                    let _ = ret;
+                    unimplemented!("no pulley host signature found for {signature:?}");
+                }};
+            }
+
+            let pulley_signum = pulley_interpreter::for_each_host_signature!(pulley_signum);
+
+            let is_pulley = match self.isa.triple().architecture {
+                target_lexicon::Architecture::Pulley32 => true,
+                target_lexicon::Architecture::Pulley64 => true,
+                _ => false,
+            };
+
+            // If this target is actually pulley then a custom `call`
+            // instruction is emitted. This will generate a new function with
+            // the Cranelift-name of a "backend intrinsic" which is how the
+            // Pulley backend models this special opcode that doesn't otherwise
+            // map into the Cranelift set of opcodes.
+            if is_pulley {
+                let mut new_signature = signature.clone();
+                new_signature
+                    .params
+                    .insert(0, ir::AbiParam::new(self.isa.pointer_type()));
+                let new_sig = builder.func.import_signature(new_signature);
+                let name = ir::ExternalName::User(builder.func.declare_imported_user_function(
+                    ir::UserExternalName {
+                        namespace: crate::NS_PULLEY_HOSTCALL,
+                        index: pulley_signum,
+                    },
+                ));
+                let func = builder.func.import_function(ir::ExtFuncData {
+                    name,
+                    signature: new_sig,
+                    // This is the signal that a special `call_indirect_host`
+                    // opcode is used to jump from pulley to the host.
+                    colocated: false,
+                });
+                let mut raw_args = vec![addr];
+                raw_args.extend_from_slice(args);
+                return builder.ins().call(func, &raw_args);
+            }
+        }
+
+        builder.ins().call_indirect(sig, addr, args)
+    }
 }
 
 impl wasmtime_environ::Compiler for Compiler {
@@ -360,7 +459,8 @@ impl wasmtime_environ::Compiler for Compiler {
 
         // Do an indirect call to the callee.
         let callee_signature = builder.func.import_signature(array_call_sig);
-        builder.ins().call_indirect(
+        self.call_indirect_host(
+            &mut builder,
             callee_signature,
             callee,
             &[callee_vmctx, caller_vmctx, args_base, args_len],
@@ -557,9 +657,7 @@ impl wasmtime_environ::Compiler for Compiler {
         // all the same results as the libcall.
         let block_params = builder.block_params(block0).to_vec();
         let host_sig = builder.func.import_signature(host_sig);
-        let call = builder
-            .ins()
-            .call_indirect(host_sig, func_addr, &block_params);
+        let call = self.call_indirect_host(&mut builder, host_sig, func_addr, &block_params);
         let results = builder.func.dfg.inst_results(call).to_vec();
         builder.ins().return_(&results);
         builder.finalize();