wasmtime/crates/api/src/trampoline/func.rs

//! Support for a calling of an imported function.

use super::create_handle::create_handle;
use crate::r#ref::HostRef;
use crate::{Callable, FuncType, Store, Trap, Val};
use alloc::{boxed::Box, rc::Rc, string::ToString, vec::Vec};
use anyhow::Result;
use core::cmp;
use cranelift_codegen::ir::{types, InstBuilder, StackSlotData, StackSlotKind, TrapCode};
use cranelift_codegen::{binemit, ir, isa, Context};
use cranelift_entity::{EntityRef, PrimaryMap};
use cranelift_frontend::{FunctionBuilder, FunctionBuilderContext};
use cranelift_wasm::{DefinedFuncIndex, FuncIndex};
use wasmtime_environ::{CompiledFunction, Export, Module};
use wasmtime_jit::CodeMemory;
use wasmtime_runtime::{InstanceHandle, VMContext, VMFunctionBody};

struct TrampolineState {
    func: Rc<dyn Callable + 'static>,
    trap: Option<HostRef<Trap>>,
    #[allow(dead_code)]
    code_memory: CodeMemory,
}

unsafe extern "C" fn stub_fn(vmctx: *mut VMContext, call_id: u32, values_vec: *mut i64) -> u32 {
    let mut instance = InstanceHandle::from_vmctx(vmctx);

    let (args, returns_len) = {
        let module = instance.module_ref();
        let signature = &module.signatures[module.functions[FuncIndex::new(call_id as usize)]];

        let mut args = Vec::new();
        for i in 1..signature.params.len() {
            args.push(Val::read_value_from(
                values_vec.offset(i as isize - 1),
                signature.params[i].value_type,
            ))
        }
        (args, signature.returns.len())
    };

    let mut returns = vec![Val::default(); returns_len];
    let func = &instance
        .host_state()
        .downcast_mut::<TrampolineState>()
        .expect("state")
        .func;

    match func.call(&args, &mut returns) {
        Ok(()) => {
            for i in 0..returns_len {
                // TODO check signature.returns[i].value_type ?
                returns[i].write_value_to(values_vec.add(i));
            }
            0
        }
        Err(trap) => {
            // TODO read custom exception
            InstanceHandle::from_vmctx(vmctx)
                .host_state()
                .downcast_mut::<TrampolineState>()
                .expect("state")
                .trap = Some(trap);
            1
        }
    }
}

/// Create a trampoline for invoking a Callable.
fn make_trampoline(
    isa: &dyn isa::TargetIsa,
    code_memory: &mut CodeMemory,
    fn_builder_ctx: &mut FunctionBuilderContext,
    call_id: u32,
    signature: &ir::Signature,
) -> *const VMFunctionBody {
    // Mostly reverse copy of the similar method from wasmtime's
    // wasmtime-jit/src/compiler.rs.
    let pointer_type = isa.pointer_type();
    let mut stub_sig = ir::Signature::new(isa.frontend_config().default_call_conv);

    // Add the `vmctx` parameter.
    stub_sig.params.push(ir::AbiParam::special(
        pointer_type,
        ir::ArgumentPurpose::VMContext,
    ));

    // Add the `call_id` parameter.
    stub_sig.params.push(ir::AbiParam::new(types::I32));

    // Add the `values_vec` parameter.
    stub_sig.params.push(ir::AbiParam::new(pointer_type));

    // Add error/trap return.
    stub_sig.returns.push(ir::AbiParam::new(types::I32));

    let values_vec_len = 8 * cmp::max(signature.params.len() - 1, signature.returns.len()) as u32;

    let mut context = Context::new();
    context.func =
        ir::Function::with_name_signature(ir::ExternalName::user(0, 0), signature.clone());

    let ss = context.func.create_stack_slot(StackSlotData::new(
        StackSlotKind::ExplicitSlot,
        values_vec_len,
    ));
    let value_size = 8;

    {
        let mut builder = FunctionBuilder::new(&mut context.func, fn_builder_ctx);
        let block0 = builder.create_ebb();

        builder.append_ebb_params_for_function_params(block0);
        builder.switch_to_block(block0);
        builder.seal_block(block0);

        let values_vec_ptr_val = builder.ins().stack_addr(pointer_type, ss, 0);
        let mflags = ir::MemFlags::trusted();
        for i in 1..signature.params.len() {
            if i == 0 {
                continue;
            }

            let val = builder.func.dfg.ebb_params(block0)[i];
            builder.ins().store(
                mflags,
                val,
                values_vec_ptr_val,
                ((i - 1) * value_size) as i32,
            );
        }

        let vmctx_ptr_val = builder.func.dfg.ebb_params(block0)[0];
        let call_id_val = builder.ins().iconst(types::I32, call_id as i64);

        let callee_args = vec![vmctx_ptr_val, call_id_val, values_vec_ptr_val];

        let new_sig = builder.import_signature(stub_sig.clone());

        let callee_value = builder
            .ins()
            .iconst(pointer_type, stub_fn as *const VMFunctionBody as i64);
        let call = builder
            .ins()
            .call_indirect(new_sig, callee_value, &callee_args);

        let call_result = builder.func.dfg.inst_results(call)[0];
        builder.ins().trapnz(call_result, TrapCode::User(0));

        let mflags = ir::MemFlags::trusted();
        let mut results = Vec::new();
        for (i, r) in signature.returns.iter().enumerate() {
            let load = builder.ins().load(
                r.value_type,
                mflags,
                values_vec_ptr_val,
                (i * value_size) as i32,
            );
            results.push(load);
        }
        builder.ins().return_(&results);
        builder.finalize()
    }

    let mut code_buf: Vec<u8> = Vec::new();
    let mut reloc_sink = RelocSink {};
    let mut trap_sink = binemit::NullTrapSink {};
    let mut stackmap_sink = binemit::NullStackmapSink {};
    context
        .compile_and_emit(
            isa,
            &mut code_buf,
            &mut reloc_sink,
            &mut trap_sink,
            &mut stackmap_sink,
        )
        .expect("compile_and_emit");

    let mut unwind_info = Vec::new();
    context.emit_unwind_info(isa, &mut unwind_info);

    code_memory
        .allocate_for_function(&CompiledFunction {
            body: code_buf,
            jt_offsets: context.func.jt_offsets,
            unwind_info,
        })
        .expect("allocate_for_function")
        .as_ptr()
}

pub fn create_handle_with_function(
    ft: &FuncType,
    func: &Rc<dyn Callable + 'static>,
    store: &HostRef<Store>,
) -> Result<InstanceHandle> {
    let sig = ft.get_cranelift_signature().clone();

    let isa = {
        let isa_builder =
            cranelift_native::builder().expect("host machine is not a supported target");
        let flag_builder = cranelift_codegen::settings::builder();
        isa_builder.finish(cranelift_codegen::settings::Flags::new(flag_builder))
    };

    let mut fn_builder_ctx = FunctionBuilderContext::new();
    let mut module = Module::new();
    let mut finished_functions: PrimaryMap<DefinedFuncIndex, *const VMFunctionBody> =
        PrimaryMap::new();
    let mut code_memory = CodeMemory::new();

    //let pointer_type = types::Type::triple_pointer_type(&HOST);
    //let call_conv = isa::CallConv::triple_default(&HOST);

    let sig_id = module.signatures.push(sig.clone());
    let func_id = module.functions.push(sig_id);
    module
        .exports
        .insert("trampoline".to_string(), Export::Function(func_id));
    let trampoline = make_trampoline(
        isa.as_ref(),
        &mut code_memory,
        &mut fn_builder_ctx,
        func_id.index() as u32,
        &sig,
    );
    code_memory.publish();

    finished_functions.push(trampoline);

    let trampoline_state = TrampolineState {
        func: func.clone(),
        trap: None,
        code_memory,
    };

    create_handle(
        module,
        Some(store.borrow_mut()),
        finished_functions,
        Box::new(trampoline_state),
    )
}

/// We don't expect trampoline compilation to produce any relocations, so
/// this `RelocSink` just asserts that it doesn't recieve any.
struct RelocSink {}

impl binemit::RelocSink for RelocSink {
    fn reloc_ebb(
        &mut self,
        _offset: binemit::CodeOffset,
        _reloc: binemit::Reloc,
        _ebb_offset: binemit::CodeOffset,
    ) {
        panic!("trampoline compilation should not produce ebb relocs");
    }
    fn reloc_external(
        &mut self,
        _offset: binemit::CodeOffset,
        _reloc: binemit::Reloc,
        _name: &ir::ExternalName,
        _addend: binemit::Addend,
    ) {
        panic!("trampoline compilation should not produce external symbol relocs");
    }
    fn reloc_constant(
        &mut self,
        _code_offset: binemit::CodeOffset,
        _reloc: binemit::Reloc,
        _constant_offset: ir::ConstantOffset,
    ) {
        panic!("trampoline compilation should not produce constant relocs");
    }
    fn reloc_jt(
        &mut self,
        _offset: binemit::CodeOffset,
        _reloc: binemit::Reloc,
        _jt: ir::JumpTable,
    ) {
        panic!("trampoline compilation should not produce jump table relocs");
    }
}