rust/src/vtable.rs

//! See librustc_codegen_llvm/meth.rs for reference

use crate::prelude::*;

const DROP_FN_INDEX: usize = 0;
const SIZE_INDEX: usize = 1;
const ALIGN_INDEX: usize = 2;

pub fn size_of_obj<'a, 'tcx: 'a>(
    fx: &mut FunctionCx<'a, 'tcx, impl Backend>,
    val: CValue<'tcx>,
) -> Value {
    let (_ptr, vtable) = val.load_value_pair(fx);
    let usize_size = fx.layout_of(fx.tcx.types.usize).size.bytes() as usize;
    fx.bcx.ins().load(
        pointer_ty(fx.tcx),
        MemFlags::new(),
        vtable,
        (SIZE_INDEX * usize_size) as i32,
    )
}

pub fn get_ptr_and_method_ref<'a, 'tcx: 'a>(
    fx: &mut FunctionCx<'a, 'tcx, impl Backend>,
    arg: CValue<'tcx>,
    idx: usize,
) -> (Value, Value) {
    let (ptr, vtable) = arg.load_value_pair(fx);
    let usize_size = fx.layout_of(fx.tcx.types.usize).size.bytes();
    let func_ref = fx.bcx.ins().load(
        pointer_ty(fx.tcx),
        MemFlags::new(),
        vtable,
        ((idx + 3) * usize_size as usize) as i32,
    );
    (ptr, func_ref)
}

pub fn get_vtable<'a, 'tcx: 'a>(
    fx: &mut FunctionCx<'a, 'tcx, impl Backend>,
    ty: Ty<'tcx>,
    trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
) -> Value {
    let data_id = if let Some(data_id) = fx.caches.vtables.get(&(ty, trait_ref)) {
        *data_id
    } else {
        let data_id = build_vtable(fx, ty, trait_ref);
        fx.caches.vtables.insert((ty, trait_ref), data_id);
        data_id
    };

    let local_data_id = fx.module.declare_data_in_func(data_id, &mut fx.bcx.func);
    fx.bcx
        .ins()
        .global_value(fx.module.pointer_type(), local_data_id)
}

fn build_vtable<'a, 'tcx: 'a>(
    fx: &mut FunctionCx<'a, 'tcx, impl Backend>,
    ty: Ty<'tcx>,
    trait_ref: Option<ty::PolyExistentialTraitRef<'tcx>>,
) -> DataId {
    let tcx = fx.tcx;
    let usize_size = fx.layout_of(fx.tcx.types.usize).size.bytes() as usize;

    let (size, align) = tcx
        .layout_of(ParamEnv::reveal_all().and(ty))
        .unwrap()
        .size_and_align();
    let drop_in_place_fn =
        fx.get_function_id(::rustc_mir::monomorphize::resolve_drop_in_place(tcx, ty));

    let mut components: Vec<_> = vec![Some(drop_in_place_fn), None, None];

    if let Some(trait_ref) = trait_ref {
        let trait_ref = trait_ref.with_self_ty(tcx, ty);
        let methods = tcx.vtable_methods(trait_ref);
        let methods = methods.iter().cloned().map(|opt_mth| {
            opt_mth.map_or(None, |(def_id, substs)| {
                Some(fx.get_function_id(
                    Instance::resolve(tcx, ParamEnv::reveal_all(), def_id, substs).unwrap(),
                ))
            })
        });
        components.extend(methods);
    }

    let mut data_ctx = DataContext::new();
    let mut data = ::std::iter::repeat(0u8)
        .take(components.len() * usize_size)
        .collect::<Vec<u8>>()
        .into_boxed_slice();
    write_usize(fx.tcx, &mut data, SIZE_INDEX, size.bytes());
    write_usize(fx.tcx, &mut data, ALIGN_INDEX, align.abi());
    data_ctx.define(data);

    for (i, component) in components.into_iter().enumerate() {
        if let Some(func_id) = component {
            let func_ref = fx.module.declare_func_in_data(func_id, &mut data_ctx);
            data_ctx.write_function_addr((i * usize_size) as u32, func_ref);
        }
    }

    let data_id = fx
        .module
        .declare_data(
            &format!("vtable.{:?}.for.{:?}", trait_ref, ty),
            Linkage::Local,
            false,
        ).unwrap();
    fx.module.define_data(data_id, &data_ctx).unwrap();
    data_id
}

fn write_usize(tcx: TyCtxt, buf: &mut [u8], idx: usize, num: u64) {
    use byteorder::{BigEndian, LittleEndian, WriteBytesExt};

    let usize_size = tcx
        .layout_of(ParamEnv::reveal_all().and(tcx.types.usize))
        .unwrap()
        .size
        .bytes() as usize;
    let mut target = &mut buf[idx * usize_size..(idx + 1) * usize_size];

    match tcx.data_layout.endian {
        layout::Endian::Little => target.write_uint::<LittleEndian>(num, usize_size),
        layout::Endian::Big => target.write_uint::<BigEndian>(num, usize_size),
    }.unwrap()
}