rust/src/librustc_codegen_ssa/mir/operand.rs

use rustc::mir::interpret::{ConstValue, ErrorHandled, Pointer, Scalar};
use rustc::mir;
use rustc::ty;
use rustc::ty::layout::{self, Align, LayoutOf, TyLayout, Size};

use crate::base;
use crate::MemFlags;
use crate::glue;

use crate::traits::*;

use std::fmt;

use super::{FunctionCx, LocalRef};
use super::place::PlaceRef;

/// The representation of a Rust value. The enum variant is in fact
/// uniquely determined by the value's type, but is kept as a
/// safety check.
#[derive(Copy, Clone, Debug)]
pub enum OperandValue<V> {
    /// A reference to the actual operand. The data is guaranteed
    /// to be valid for the operand's lifetime.
    /// The second value, if any, is the extra data (vtable or length)
    /// which indicates that it refers to an unsized rvalue.
    Ref(V, Option<V>, Align),
    /// A single LLVM value.
    Immediate(V),
    /// A pair of immediate LLVM values. Used by fat pointers too.
    Pair(V, V)
}

/// An `OperandRef` is an "SSA" reference to a Rust value, along with
/// its type.
///
/// NOTE: unless you know a value's type exactly, you should not
/// generate LLVM opcodes acting on it and instead act via methods,
/// to avoid nasty edge cases. In particular, using `Builder::store`
/// directly is sure to cause problems -- use `OperandRef::store`
/// instead.
#[derive(Copy, Clone)]
pub struct OperandRef<'tcx, V> {
    // The value.
    pub val: OperandValue<V>,

    // The layout of value, based on its Rust type.
    pub layout: TyLayout<'tcx>,
}

impl<V: CodegenObject> fmt::Debug for OperandRef<'tcx, V> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "OperandRef({:?} @ {:?})", self.val, self.layout)
    }
}

impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
    pub fn new_zst<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        bx: &mut Bx,
        layout: TyLayout<'tcx>
    ) -> OperandRef<'tcx, V> {
        assert!(layout.is_zst());
        OperandRef {
            val: OperandValue::Immediate(bx.const_undef(bx.immediate_backend_type(layout))),
            layout
        }
    }

    pub fn from_const<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        bx: &mut Bx,
        val: &'tcx ty::Const<'tcx>
    ) -> Self {
        let layout = bx.layout_of(val.ty);

        if layout.is_zst() {
            return OperandRef::new_zst(bx, layout);
        }

        let val = match val.val {
            ConstValue::Unevaluated(..) => bug!("unevaluated constant in `OperandRef::from_const`"),
            ConstValue::Param(_) => bug!("encountered a ConstValue::Param in codegen"),
            ConstValue::Infer(_) => bug!("encountered a ConstValue::Infer in codegen"),
            ConstValue::Placeholder(_) => bug!("encountered a ConstValue::Placeholder in codegen"),
            ConstValue::Scalar(x) => {
                let scalar = match layout.abi {
                    layout::Abi::Scalar(ref x) => x,
                    _ => bug!("from_const: invalid ByVal layout: {:#?}", layout)
                };
                let llval = bx.scalar_to_backend(
                    x,
                    scalar,
                    bx.immediate_backend_type(layout),
                );
                OperandValue::Immediate(llval)
            },
            ConstValue::Slice { data, start, end } => {
                let a_scalar = match layout.abi {
                    layout::Abi::ScalarPair(ref a, _) => a,
                    _ => bug!("from_const: invalid ScalarPair layout: {:#?}", layout)
                };
                let a = Scalar::from(Pointer::new(
                    bx.tcx().alloc_map.lock().create_memory_alloc(data),
                    Size::from_bytes(start as u64),
                )).into();
                let a_llval = bx.scalar_to_backend(
                    a,
                    a_scalar,
                    bx.scalar_pair_element_backend_type(layout, 0, true),
                );
                let b_llval = bx.const_usize((end - start) as u64);
                OperandValue::Pair(a_llval, b_llval)
            },
            ConstValue::ByRef { offset, align, alloc } => {
                return bx.load_operand(bx.from_const_alloc(layout, align, alloc, offset));
            },
        };

        OperandRef {
            val,
            layout
        }
    }

    /// Asserts that this operand refers to a scalar and returns
    /// a reference to its value.
    pub fn immediate(self) -> V {
        match self.val {
            OperandValue::Immediate(s) => s,
            _ => bug!("not immediate: {:?}", self)
        }
    }

    pub fn deref<Cx: LayoutTypeMethods<'tcx>>(
        self,
        cx: &Cx
    ) -> PlaceRef<'tcx, V> {
        let projected_ty = self.layout.ty.builtin_deref(true)
            .unwrap_or_else(|| bug!("deref of non-pointer {:?}", self)).ty;
        let (llptr, llextra) = match self.val {
            OperandValue::Immediate(llptr) => (llptr, None),
            OperandValue::Pair(llptr, llextra) => (llptr, Some(llextra)),
            OperandValue::Ref(..) => bug!("Deref of by-Ref operand {:?}", self)
        };
        let layout = cx.layout_of(projected_ty);
        PlaceRef {
            llval: llptr,
            llextra,
            layout,
            align: layout.align.abi,
        }
    }

    /// If this operand is a `Pair`, we return an aggregate with the two values.
    /// For other cases, see `immediate`.
    pub fn immediate_or_packed_pair<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        self,
        bx: &mut Bx
    ) -> V {
        if let OperandValue::Pair(a, b) = self.val {
            let llty = bx.cx().backend_type(self.layout);
            debug!("Operand::immediate_or_packed_pair: packing {:?} into {:?}",
                   self, llty);
            // Reconstruct the immediate aggregate.
            let mut llpair = bx.cx().const_undef(llty);
            let imm_a = base::from_immediate(bx, a);
            let imm_b = base::from_immediate(bx, b);
            llpair = bx.insert_value(llpair, imm_a, 0);
            llpair = bx.insert_value(llpair, imm_b, 1);
            llpair
        } else {
            self.immediate()
        }
    }

    /// If the type is a pair, we return a `Pair`, otherwise, an `Immediate`.
    pub fn from_immediate_or_packed_pair<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        bx: &mut Bx,
        llval: V,
        layout: TyLayout<'tcx>
    ) -> Self {
        let val = if let layout::Abi::ScalarPair(ref a, ref b) = layout.abi {
            debug!("Operand::from_immediate_or_packed_pair: unpacking {:?} @ {:?}",
                    llval, layout);

            // Deconstruct the immediate aggregate.
            let a_llval = bx.extract_value(llval, 0);
            let a_llval = base::to_immediate_scalar(bx, a_llval, a);
            let b_llval = bx.extract_value(llval, 1);
            let b_llval = base::to_immediate_scalar(bx, b_llval, b);
            OperandValue::Pair(a_llval, b_llval)
        } else {
            OperandValue::Immediate(llval)
        };
        OperandRef { val, layout }
    }

    pub fn extract_field<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        &self,
        bx: &mut Bx,
        i: usize
    ) -> Self {
        let field = self.layout.field(bx.cx(), i);
        let offset = self.layout.fields.offset(i);

        let mut val = match (self.val, &self.layout.abi) {
            // If the field is ZST, it has no data.
            _ if field.is_zst() => {
                return OperandRef::new_zst(bx, field);
            }

            // Newtype of a scalar, scalar pair or vector.
            (OperandValue::Immediate(_), _) |
            (OperandValue::Pair(..), _) if field.size == self.layout.size => {
                assert_eq!(offset.bytes(), 0);
                self.val
            }

            // Extract a scalar component from a pair.
            (OperandValue::Pair(a_llval, b_llval), &layout::Abi::ScalarPair(ref a, ref b)) => {
                if offset.bytes() == 0 {
                    assert_eq!(field.size, a.value.size(bx.cx()));
                    OperandValue::Immediate(a_llval)
                } else {
                    assert_eq!(offset, a.value.size(bx.cx())
                        .align_to(b.value.align(bx.cx()).abi));
                    assert_eq!(field.size, b.value.size(bx.cx()));
                    OperandValue::Immediate(b_llval)
                }
            }

            // `#[repr(simd)]` types are also immediate.
            (OperandValue::Immediate(llval), &layout::Abi::Vector { .. }) => {
                OperandValue::Immediate(
                    bx.extract_element(llval, bx.cx().const_usize(i as u64)))
            }

            _ => bug!("OperandRef::extract_field({:?}): not applicable", self)
        };

        // HACK(eddyb) have to bitcast pointers until LLVM removes pointee types.
        // Bools in union fields needs to be truncated.
        let to_immediate_or_cast = |bx: &mut Bx, val, ty| {
            if ty == bx.cx().type_i1() {
                bx.trunc(val, ty)
            } else {
                bx.bitcast(val, ty)
            }
        };

        match val {
            OperandValue::Immediate(ref mut llval) => {
                *llval = to_immediate_or_cast(bx, *llval, bx.cx().immediate_backend_type(field));
            }
            OperandValue::Pair(ref mut a, ref mut b) => {
                *a = to_immediate_or_cast(bx, *a, bx.cx()
                    .scalar_pair_element_backend_type(field, 0, true));
                *b = to_immediate_or_cast(bx, *b, bx.cx()
                    .scalar_pair_element_backend_type(field, 1, true));
            }
            OperandValue::Ref(..) => bug!()
        }

        OperandRef {
            val,
            layout: field
        }
    }
}

impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
    pub fn store<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        self,
        bx: &mut Bx,
        dest: PlaceRef<'tcx, V>
    ) {
        self.store_with_flags(bx, dest, MemFlags::empty());
    }

    pub fn volatile_store<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        self,
        bx: &mut Bx,
        dest: PlaceRef<'tcx, V>
    ) {
        self.store_with_flags(bx, dest, MemFlags::VOLATILE);
    }

    pub fn unaligned_volatile_store<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        self,
        bx: &mut Bx,
        dest: PlaceRef<'tcx, V>,
    ) {
        self.store_with_flags(bx, dest, MemFlags::VOLATILE | MemFlags::UNALIGNED);
    }

    pub fn nontemporal_store<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        self,
        bx: &mut Bx,
        dest: PlaceRef<'tcx, V>
    ) {
        self.store_with_flags(bx, dest, MemFlags::NONTEMPORAL);
    }

    fn store_with_flags<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        self,
        bx: &mut Bx,
        dest: PlaceRef<'tcx, V>,
        flags: MemFlags,
    ) {
        debug!("OperandRef::store: operand={:?}, dest={:?}", self, dest);
        // Avoid generating stores of zero-sized values, because the only way to have a zero-sized
        // value is through `undef`, and store itself is useless.
        if dest.layout.is_zst() {
            return;
        }
        match self {
            OperandValue::Ref(r, None, source_align) => {
                base::memcpy_ty(bx, dest.llval, dest.align, r, source_align,
                                dest.layout, flags)
            }
            OperandValue::Ref(_, Some(_), _) => {
                bug!("cannot directly store unsized values");
            }
            OperandValue::Immediate(s) => {
                let val = base::from_immediate(bx, s);
                bx.store_with_flags(val, dest.llval, dest.align, flags);
            }
            OperandValue::Pair(a, b) => {
                let (a_scalar, b_scalar) = match dest.layout.abi {
                    layout::Abi::ScalarPair(ref a, ref b) => (a, b),
                    _ => bug!("store_with_flags: invalid ScalarPair layout: {:#?}", dest.layout)
                };
                let b_offset = a_scalar.value.size(bx).align_to(b_scalar.value.align(bx).abi);

                let llptr = bx.struct_gep(dest.llval, 0);
                let val = base::from_immediate(bx, a);
                let align = dest.align;
                bx.store_with_flags(val, llptr, align, flags);

                let llptr = bx.struct_gep(dest.llval, 1);
                let val = base::from_immediate(bx, b);
                let align = dest.align.restrict_for_offset(b_offset);
                bx.store_with_flags(val, llptr, align, flags);
            }
        }
    }
    pub fn store_unsized<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
        self,
        bx: &mut Bx,
        indirect_dest: PlaceRef<'tcx, V>
    ) {
        debug!("OperandRef::store_unsized: operand={:?}, indirect_dest={:?}", self, indirect_dest);
        let flags = MemFlags::empty();

        // `indirect_dest` must have `*mut T` type. We extract `T` out of it.
        let unsized_ty = indirect_dest.layout.ty.builtin_deref(true)
            .unwrap_or_else(|| bug!("indirect_dest has non-pointer type: {:?}", indirect_dest)).ty;

        let (llptr, llextra) =
            if let OperandValue::Ref(llptr, Some(llextra), _) = self {
                (llptr, llextra)
            } else {
                bug!("store_unsized called with a sized value")
            };

        // FIXME: choose an appropriate alignment, or use dynamic align somehow
        let max_align = Align::from_bits(128).unwrap();
        let min_align = Align::from_bits(8).unwrap();

        // Allocate an appropriate region on the stack, and copy the value into it
        let (llsize, _) = glue::size_and_align_of_dst(bx, unsized_ty, Some(llextra));
        let lldst = bx.array_alloca(bx.cx().type_i8(), llsize, "unsized_tmp", max_align);
        bx.memcpy(lldst, max_align, llptr, min_align, llsize, flags);

        // Store the allocated region and the extra to the indirect place.
        let indirect_operand = OperandValue::Pair(lldst, llextra);
        indirect_operand.store(bx, indirect_dest);
    }
}

impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
    fn maybe_codegen_consume_direct(
        &mut self,
        bx: &mut Bx,
        place: &mir::Place<'tcx>
    ) -> Option<OperandRef<'tcx, Bx::Value>> {
        debug!("maybe_codegen_consume_direct(place={:?})", place);

        place.iterate(|place_base, place_projection| {
            if let mir::PlaceBase::Local(index) = place_base {
                match self.locals[*index] {
                    LocalRef::Operand(Some(mut o)) => {
                        // Moves out of scalar and scalar pair fields are trivial.
                        for proj in place_projection {
                            match proj.elem {
                                mir::ProjectionElem::Field(ref f, _) => {
                                    o = o.extract_field(bx, f.index());
                                }
                                mir::ProjectionElem::Index(_) |
                                mir::ProjectionElem::ConstantIndex { .. } => {
                                    // ZSTs don't require any actual memory access.
                                    // FIXME(eddyb) deduplicate this with the identical
                                    // checks in `codegen_consume` and `extract_field`.
                                    let elem = o.layout.field(bx.cx(), 0);
                                    if elem.is_zst() {
                                        o = OperandRef::new_zst(bx, elem);
                                    } else {
                                        return None;
                                    }
                                }
                                _ => return None,
                            }
                        }

                        Some(o)
                    }
                    LocalRef::Operand(None) => {
                        bug!("use of {:?} before def", place);
                    }
                    LocalRef::Place(..) | LocalRef::UnsizedPlace(..) => {
                        // watch out for locals that do not have an
                        // alloca; they are handled somewhat differently
                        None
                    }
                }
            } else {
                None
            }
        })
    }

    pub fn codegen_consume(
        &mut self,
        bx: &mut Bx,
        place: &mir::Place<'tcx>
    ) -> OperandRef<'tcx, Bx::Value> {
        debug!("codegen_consume(place={:?})", place);

        let ty = self.monomorphized_place_ty(place);
        let layout = bx.cx().layout_of(ty);

        // ZSTs don't require any actual memory access.
        if layout.is_zst() {
            return OperandRef::new_zst(bx, layout);
        }

        if let Some(o) = self.maybe_codegen_consume_direct(bx, place) {
            return o;
        }

        // for most places, to consume them we just load them
        // out from their home
        let place = self.codegen_place(bx, place);
        bx.load_operand(place)
    }

    pub fn codegen_operand(
        &mut self,
        bx: &mut Bx,
        operand: &mir::Operand<'tcx>
    ) -> OperandRef<'tcx, Bx::Value> {
        debug!("codegen_operand(operand={:?})", operand);

        match *operand {
            mir::Operand::Copy(ref place) |
            mir::Operand::Move(ref place) => {
                self.codegen_consume(bx, place)
            }

            mir::Operand::Constant(ref constant) => {
                let ty = self.monomorphize(&constant.ty);
                self.eval_mir_constant(constant)
                    .map(|c| OperandRef::from_const(bx, c))
                    .unwrap_or_else(|err| {
                        match err {
                            // errored or at least linted
                            ErrorHandled::Reported => {},
                            ErrorHandled::TooGeneric => {
                                bug!("codgen encountered polymorphic constant")
                            },
                        }
                        // Allow RalfJ to sleep soundly knowing that even refactorings that remove
                        // the above error (or silence it under some conditions) will not cause UB
                        bx.abort();
                        // We've errored, so we don't have to produce working code.
                        let layout = bx.cx().layout_of(ty);
                        bx.load_operand(PlaceRef::new_sized(
                            bx.cx().const_undef(bx.cx().type_ptr_to(bx.cx().backend_type(layout))),
                            layout,
                            layout.align.abi,
                        ))
                    })
            }
        }
    }
}