memory.rs

use byteorder::{ReadBytesExt, WriteBytesExt, LittleEndian, BigEndian};
use std::collections::{btree_map, BTreeMap, HashMap, HashSet, VecDeque, BTreeSet};
use std::{fmt, iter, ptr, mem, io};

use rustc::hir::def_id::DefId;
use rustc::ty::{self, BareFnTy, ClosureTy, ClosureSubsts, TyCtxt};
use rustc::ty::subst::Substs;
use rustc::ty::layout::{self, TargetDataLayout};

use syntax::abi::Abi;

use error::{EvalError, EvalResult};
use value::PrimVal;

////////////////////////////////////////////////////////////////////////////////
// Allocations and pointers
////////////////////////////////////////////////////////////////////////////////

#[derive(Copy, Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub struct AllocId(pub u64);

impl fmt::Display for AllocId {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.0)
    }
}

#[derive(Debug)]
pub struct Allocation {
    /// The actual bytes of the allocation.
    /// Note that the bytes of a pointer represent the offset of the pointer
    pub bytes: Vec<u8>,
    /// Maps from byte addresses to allocations.
    /// Only the first byte of a pointer is inserted into the map.
    pub relocations: BTreeMap<u64, AllocId>,
    /// Denotes undefined memory. Reading from undefined memory is forbidden in miri
    pub undef_mask: UndefMask,
    /// The alignment of the allocation to detect unaligned reads.
    pub align: u64,
    /// Whether the allocation may be modified.
    /// Use the `mark_static_initalized` method of `Memory` to ensure that an error occurs, if the memory of this
    /// allocation is modified or deallocated in the future.
    pub static_kind: StaticKind,
}

#[derive(Debug, PartialEq, Copy, Clone)]
pub enum StaticKind {
    /// may be deallocated without breaking miri's invariants
    NotStatic,
    /// may be modified, but never deallocated
    Mutable,
    /// may neither be modified nor deallocated
    Immutable,
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct Pointer {
    pub alloc_id: AllocId,
    pub offset: u64,
}

impl Pointer {
    pub fn new(alloc_id: AllocId, offset: u64) -> Self {
        Pointer { alloc_id, offset }
    }

    pub fn signed_offset(self, i: i64) -> Self {
        // FIXME: is it possible to over/underflow here?
        if i < 0 {
            // trickery to ensure that i64::min_value() works fine
            // this formula only works for true negative values, it panics for zero!
            let n = u64::max_value() - (i as u64) + 1;
            Pointer::new(self.alloc_id, self.offset - n)
        } else {
            self.offset(i as u64)
        }
    }

    pub fn offset(self, i: u64) -> Self {
        Pointer::new(self.alloc_id, self.offset + i)
    }

    pub fn points_to_zst(&self) -> bool {
        self.alloc_id == ZST_ALLOC_ID
    }

    pub fn to_int<'tcx>(&self) -> EvalResult<'tcx, u64> {
        match self.alloc_id {
            NEVER_ALLOC_ID => Ok(self.offset),
            _ => Err(EvalError::ReadPointerAsBytes),
        }
    }

    pub fn from_int(i: u64) -> Self {
        Pointer::new(NEVER_ALLOC_ID, i)
    }

    pub fn zst_ptr() -> Self {
        Pointer::new(ZST_ALLOC_ID, 0)
    }

    pub fn never_ptr() -> Self {
        Pointer::new(NEVER_ALLOC_ID, 0)
    }
}

#[derive(Debug, Copy, Clone, Hash, Eq, PartialEq)]
/// Identifies a specific monomorphized function
pub struct FunctionDefinition<'tcx> {
    pub def_id: DefId,
    pub substs: &'tcx Substs<'tcx>,
    pub abi: Abi,
    pub sig: &'tcx ty::FnSig<'tcx>,
}

/// Either a concrete function, or a glue function
#[derive(Debug, Copy, Clone, Hash, Eq, PartialEq)]
pub enum Function<'tcx> {
    /// A function or method created by compiling code
    Concrete(FunctionDefinition<'tcx>),
    /// Glue required to call a regular function through a Fn(Mut|Once) trait object
    FnDefAsTraitObject(FunctionDefinition<'tcx>),
    /// A drop glue function only needs to know the real type, and then miri can extract
    /// that type from a vtable's drop pointer.
    /// Instead of storing some drop function, we act as if there are no trait objects, by
    /// mapping trait objects to their real types before acting on them.
    DropGlue(ty::Ty<'tcx>),
    /// Glue required to treat the ptr part of a fat pointer
    /// as a function pointer
    FnPtrAsTraitObject(&'tcx ty::FnSig<'tcx>),
    /// Glue for Closures
    Closure(FunctionDefinition<'tcx>),
}

impl<'tcx> Function<'tcx> {
    pub fn expect_concrete(self) -> EvalResult<'tcx, FunctionDefinition<'tcx>> {
        match self {
            Function::Concrete(fn_def) => Ok(fn_def),
            other => Err(EvalError::ExpectedConcreteFunction(other)),
        }
    }
    pub fn expect_drop_glue_real_ty(self) -> EvalResult<'tcx, ty::Ty<'tcx>> {
        match self {
            Function::DropGlue(real_ty) => Ok(real_ty),
            other => Err(EvalError::ExpectedDropGlue(other)),
        }
    }
}

////////////////////////////////////////////////////////////////////////////////
// Top-level interpreter memory
////////////////////////////////////////////////////////////////////////////////

pub struct Memory<'a, 'tcx> {
    /// Actual memory allocations (arbitrary bytes, may contain pointers into other allocations).
    alloc_map: HashMap<AllocId, Allocation>,

    /// The AllocId to assign to the next new allocation. Always incremented, never gets smaller.
    next_id: AllocId,

    /// Set of statics, constants, promoteds, vtables, ... to prevent `mark_static_initalized` from
    /// stepping out of its own allocations. This set only contains statics backed by an
    /// allocation. If they are ByVal or ByValPair they are not here, but will be inserted once
    /// they become ByRef.
    static_alloc: HashSet<AllocId>,

    /// Number of virtual bytes allocated.
    memory_usage: u64,

    /// Maximum number of virtual bytes that may be allocated.
    memory_size: u64,

    /// Function "allocations". They exist solely so pointers have something to point to, and
    /// we can figure out what they point to.
    functions: HashMap<AllocId, Function<'tcx>>,

    /// Inverse map of `functions` so we don't allocate a new pointer every time we need one
    function_alloc_cache: HashMap<Function<'tcx>, AllocId>,

    /// Target machine data layout to emulate.
    pub layout: &'a TargetDataLayout,

    /// List of memory regions containing packed structures.
    ///
    /// We mark memory as "packed" or "unaligned" for a single statement, and clear the marking
    /// afterwards. In the case where no packed structs are present, it's just a single emptyness
    /// check of a set instead of heavily influencing all memory access code as other solutions
    /// would.
    ///
    /// One disadvantage of this solution is the fact that you can cast a pointer to a packed
    /// struct to a pointer to a normal struct and if you access a field of both in the same MIR
    /// statement, the normal struct access will succeed even though it shouldn't. But even with
    /// mir optimizations, that situation is hard/impossible to produce.
    packed: BTreeSet<Entry>,

    /// A cache for basic byte allocations keyed by their contents. This is used to deduplicate
    /// allocations for string and bytestring literals.
    literal_alloc_cache: HashMap<Vec<u8>, AllocId>,
}

const ZST_ALLOC_ID: AllocId = AllocId(0);
const NEVER_ALLOC_ID: AllocId = AllocId(1);

impl<'a, 'tcx> Memory<'a, 'tcx> {
    pub fn new(layout: &'a TargetDataLayout, max_memory: u64) -> Self {
        Memory {
            alloc_map: HashMap::new(),
            functions: HashMap::new(),
            function_alloc_cache: HashMap::new(),
            next_id: AllocId(2),
            layout,
            memory_size: max_memory,
            memory_usage: 0,
            packed: BTreeSet::new(),
            static_alloc: HashSet::new(),
            literal_alloc_cache: HashMap::new(),
        }
    }

    pub fn allocations(&self) -> ::std::collections::hash_map::Iter<AllocId, Allocation> {
        self.alloc_map.iter()
    }

    pub fn create_closure_ptr(&mut self, tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId, substs: ClosureSubsts<'tcx>, fn_ty: ClosureTy<'tcx>) -> Pointer {
        // FIXME: this is a hack
        let fn_ty = tcx.mk_bare_fn(ty::BareFnTy {
            unsafety: fn_ty.unsafety,
            abi: fn_ty.abi,
            sig: fn_ty.sig,
        });
        self.create_fn_alloc(Function::Closure(FunctionDefinition {
            def_id,
            substs: substs.substs,
            abi: fn_ty.abi,
            // FIXME: why doesn't this compile?
            //sig: tcx.erase_late_bound_regions(&fn_ty.sig),
            sig: fn_ty.sig.skip_binder(),
        }))
    }

    pub fn create_fn_as_trait_glue(&mut self, _tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId, substs: &'tcx Substs<'tcx>, fn_ty: &'tcx BareFnTy<'tcx>) -> Pointer {
        self.create_fn_alloc(Function::FnDefAsTraitObject(FunctionDefinition {
            def_id,
            substs,
            abi: fn_ty.abi,
            // FIXME: why doesn't this compile?
            //sig: tcx.erase_late_bound_regions(&fn_ty.sig),
            sig: fn_ty.sig.skip_binder(),
        }))
    }

    pub fn create_fn_ptr_as_trait_glue(&mut self, fn_ty: &'tcx BareFnTy<'tcx>) -> Pointer {
        self.create_fn_alloc(Function::FnPtrAsTraitObject(fn_ty.sig.skip_binder()))
    }

    pub fn create_drop_glue(&mut self, ty: ty::Ty<'tcx>) -> Pointer {
        self.create_fn_alloc(Function::DropGlue(ty))
    }

    pub fn create_fn_ptr(&mut self, _tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId, substs: &'tcx Substs<'tcx>, fn_ty: &'tcx BareFnTy<'tcx>) -> Pointer {
        self.create_fn_alloc(Function::Concrete(FunctionDefinition {
            def_id,
            substs,
            abi: fn_ty.abi,
            // FIXME: why doesn't this compile?
            //sig: tcx.erase_late_bound_regions(&fn_ty.sig),
            sig: fn_ty.sig.skip_binder(),
        }))
    }

    fn create_fn_alloc(&mut self, def: Function<'tcx>) -> Pointer {
        if let Some(&alloc_id) = self.function_alloc_cache.get(&def) {
            return Pointer::new(alloc_id, 0);
        }
        let id = self.next_id;
        debug!("creating fn ptr: {}", id);
        self.next_id.0 += 1;
        self.functions.insert(id, def);
        self.function_alloc_cache.insert(def, id);
        Pointer::new(id, 0)
    }

    pub fn allocate_cached(&mut self, bytes: &[u8]) -> EvalResult<'tcx, Pointer> {
        if let Some(&alloc_id) = self.literal_alloc_cache.get(bytes) {
            return Ok(Pointer::new(alloc_id, 0));
        }

        let ptr = self.allocate(bytes.len() as u64, 1)?;
        self.write_bytes(ptr, bytes)?;
        self.mark_static_initalized(ptr.alloc_id, false)?;
        self.literal_alloc_cache.insert(bytes.to_vec(), ptr.alloc_id);
        Ok(ptr)
    }

    pub fn allocate(&mut self, size: u64, align: u64) -> EvalResult<'tcx, Pointer> {
        if size == 0 {
            return Ok(Pointer::zst_ptr());
        }
        assert!(align != 0);

        if self.memory_size - self.memory_usage < size {
            return Err(EvalError::OutOfMemory {
                allocation_size: size,
                memory_size: self.memory_size,
                memory_usage: self.memory_usage,
            });
        }
        self.memory_usage += size;
        assert_eq!(size as usize as u64, size);
        let alloc = Allocation {
            bytes: vec![0; size as usize],
            relocations: BTreeMap::new(),
            undef_mask: UndefMask::new(size),
            align,
            static_kind: StaticKind::NotStatic,
        };
        let id = self.next_id;
        self.next_id.0 += 1;
        self.alloc_map.insert(id, alloc);
        Ok(Pointer::new(id, 0))
    }

    // TODO(solson): Track which allocations were returned from __rust_allocate and report an error
    // when reallocating/deallocating any others.
    pub fn reallocate(&mut self, ptr: Pointer, new_size: u64, align: u64) -> EvalResult<'tcx, Pointer> {
        // TODO(solson): Report error about non-__rust_allocate'd pointer.
        if ptr.offset != 0 {
            return Err(EvalError::Unimplemented(format!("bad pointer offset: {}", ptr.offset)));
        }
        if ptr.points_to_zst() {
            return self.allocate(new_size, align);
        }
        if self.get(ptr.alloc_id).ok().map_or(false, |alloc| alloc.static_kind != StaticKind::NotStatic) {
            return Err(EvalError::ReallocatedStaticMemory);
        }

        let size = self.get(ptr.alloc_id)?.bytes.len() as u64;

        if new_size > size {
            let amount = new_size - size;
            self.memory_usage += amount;
            let alloc = self.get_mut(ptr.alloc_id)?;
            assert_eq!(amount as usize as u64, amount);
            alloc.bytes.extend(iter::repeat(0).take(amount as usize));
            alloc.undef_mask.grow(amount, false);
        } else if size > new_size {
            self.memory_usage -= size - new_size;
            self.clear_relocations(ptr.offset(new_size), size - new_size)?;
            let alloc = self.get_mut(ptr.alloc_id)?;
            // `as usize` is fine here, since it is smaller than `size`, which came from a usize
            alloc.bytes.truncate(new_size as usize);
            alloc.bytes.shrink_to_fit();
            alloc.undef_mask.truncate(new_size);
        }

        Ok(Pointer::new(ptr.alloc_id, 0))
    }

    // TODO(solson): See comment on `reallocate`.
    pub fn deallocate(&mut self, ptr: Pointer) -> EvalResult<'tcx> {
        if ptr.points_to_zst() {
            return Ok(());
        }
        if ptr.offset != 0 {
            // TODO(solson): Report error about non-__rust_allocate'd pointer.
            return Err(EvalError::Unimplemented(format!("bad pointer offset: {}", ptr.offset)));
        }
        if self.get(ptr.alloc_id).ok().map_or(false, |alloc| alloc.static_kind != StaticKind::NotStatic) {
            return Err(EvalError::DeallocatedStaticMemory);
        }

        if let Some(alloc) = self.alloc_map.remove(&ptr.alloc_id) {
            self.memory_usage -= alloc.bytes.len() as u64;
        } else {
            debug!("deallocated a pointer twice: {}", ptr.alloc_id);
            // TODO(solson): Report error about erroneous free. This is blocked on properly tracking
            // already-dropped state since this if-statement is entered even in safe code without
            // it.
        }
        debug!("deallocated : {}", ptr.alloc_id);

        Ok(())
    }

    pub fn pointer_size(&self) -> u64 {
        self.layout.pointer_size.bytes()
    }

    pub fn endianess(&self) -> layout::Endian {
        self.layout.endian
    }

    pub fn check_align(&self, ptr: Pointer, align: u64, len: u64) -> EvalResult<'tcx> {
        let alloc = self.get(ptr.alloc_id)?;
        // check whether the memory was marked as packed
        // we select all elements that have the correct alloc_id and are within
        // the range given by the offset into the allocation and the length
        let start = Entry {
            alloc_id: ptr.alloc_id,
            packed_start: 0,
            packed_end: ptr.offset + len,
        };
        let end = Entry {
            alloc_id: ptr.alloc_id,
            packed_start: ptr.offset + len,
            packed_end: 0,
        };
        for &Entry { packed_start, packed_end, .. } in self.packed.range(start..end) {
            // if the region we are checking is covered by a region in `packed`
            // ignore the actual alignment
            if packed_start <= ptr.offset && (ptr.offset + len) <= packed_end {
                return Ok(());
            }
        }
        if alloc.align < align {
            return Err(EvalError::AlignmentCheckFailed {
                has: alloc.align,
                required: align,
            });
        }
        if ptr.offset % align == 0 {
            Ok(())
        } else {
            Err(EvalError::AlignmentCheckFailed {
                has: ptr.offset % align,
                required: align,
            })
        }
    }

    pub(crate) fn mark_packed(&mut self, ptr: Pointer, len: u64) {
        self.packed.insert(Entry {
            alloc_id: ptr.alloc_id,
            packed_start: ptr.offset,
            packed_end: ptr.offset + len,
        });
    }

    pub(crate) fn clear_packed(&mut self) {
        self.packed.clear();
    }
}

// The derived `Ord` impl sorts first by the first field, then, if the fields are the same
// by the second field, and if those are the same, too, then by the third field.
// This is exactly what we need for our purposes, since a range within an allocation
// will give us all `Entry`s that have that `AllocId`, and whose `packed_start` is <= than
// the one we're looking for, but not > the end of the range we're checking.
// At the same time the `packed_end` is irrelevant for the sorting and range searching, but used for the check.
// This kind of search breaks, if `packed_end < packed_start`, so don't do that!
#[derive(Eq, PartialEq, Ord, PartialOrd)]
struct Entry {
    alloc_id: AllocId,
    packed_start: u64,
    packed_end: u64,
}

/// Allocation accessors
impl<'a, 'tcx> Memory<'a, 'tcx> {
    pub fn get(&self, id: AllocId) -> EvalResult<'tcx, &Allocation> {
        match self.alloc_map.get(&id) {
            Some(alloc) => Ok(alloc),
            None => match self.functions.get(&id) {
                Some(_) => Err(EvalError::DerefFunctionPointer),
                None if id == NEVER_ALLOC_ID || id == ZST_ALLOC_ID => Err(EvalError::InvalidMemoryAccess),
                None => Err(EvalError::DanglingPointerDeref),
            }
        }
    }

    pub fn get_mut(&mut self, id: AllocId) -> EvalResult<'tcx, &mut Allocation> {
        match self.alloc_map.get_mut(&id) {
            Some(alloc) => match alloc.static_kind {
                StaticKind::Mutable |
                StaticKind::NotStatic => Ok(alloc),
                StaticKind::Immutable => Err(EvalError::ModifiedConstantMemory),
            },
            None => match self.functions.get(&id) {
                Some(_) => Err(EvalError::DerefFunctionPointer),
                None if id == NEVER_ALLOC_ID || id == ZST_ALLOC_ID => Err(EvalError::InvalidMemoryAccess),
                None => Err(EvalError::DanglingPointerDeref),
            }
        }
    }

    pub fn get_fn(&self, id: AllocId) -> EvalResult<'tcx, Function<'tcx>> {
        debug!("reading fn ptr: {}", id);
        match self.functions.get(&id) {
            Some(&fndef) => Ok(fndef),
            None => match self.alloc_map.get(&id) {
                Some(_) => Err(EvalError::ExecuteMemory),
                None => Err(EvalError::InvalidFunctionPointer),
            }
        }
    }

    /// For debugging, print an allocation and all allocations it points to, recursively.
    pub fn dump_alloc(&self, id: AllocId) {
        self.dump_allocs(vec![id]);
    }

    /// For debugging, print a list of allocations and all allocations they point to, recursively.
    pub fn dump_allocs(&self, mut allocs: Vec<AllocId>) {
        use std::fmt::Write;
        allocs.sort();
        allocs.dedup();
        let mut allocs_to_print = VecDeque::from(allocs);
        let mut allocs_seen = HashSet::new();

        while let Some(id) = allocs_to_print.pop_front() {
            if id == ZST_ALLOC_ID || id == NEVER_ALLOC_ID { continue; }
            let mut msg = format!("Alloc {:<5} ", format!("{}:", id));
            let prefix_len = msg.len();
            let mut relocations = vec![];

            let alloc = match (self.alloc_map.get(&id), self.functions.get(&id)) {
                (Some(a), None) => a,
                (None, Some(&Function::Concrete(fn_def))) => {
                    trace!("{} {}", msg, dump_fn_def(fn_def));
                    continue;
                },
                (None, Some(&Function::DropGlue(real_ty))) => {
                    trace!("{} drop glue for {}", msg, real_ty);
                    continue;
                },
                (None, Some(&Function::FnDefAsTraitObject(fn_def))) => {
                    trace!("{} fn as Fn glue for {}", msg, dump_fn_def(fn_def));
                    continue;
                },
                (None, Some(&Function::FnPtrAsTraitObject(fn_def))) => {
                    trace!("{} fn ptr as Fn glue (signature: {:?})", msg, fn_def);
                    continue;
                },
                (None, Some(&Function::Closure(fn_def))) => {
                    trace!("{} closure glue for {}", msg, dump_fn_def(fn_def));
                    continue;
                },
                (None, None) => {
                    trace!("{} (deallocated)", msg);
                    continue;
                },
                (Some(_), Some(_)) => bug!("miri invariant broken: an allocation id exists that points to both a function and a memory location"),
            };

            for i in 0..(alloc.bytes.len() as u64) {
                if let Some(&target_id) = alloc.relocations.get(&i) {
                    if allocs_seen.insert(target_id) {
                        allocs_to_print.push_back(target_id);
                    }
                    relocations.push((i, target_id));
                }
                if alloc.undef_mask.is_range_defined(i, i + 1) {
                    // this `as usize` is fine, since `i` came from a `usize`
                    write!(msg, "{:02x} ", alloc.bytes[i as usize]).unwrap();
                } else {
                    msg.push_str("__ ");
                }
            }

            let immutable = match alloc.static_kind {
                StaticKind::Mutable => " (static mut)",
                StaticKind::Immutable => " (immutable)",
                StaticKind::NotStatic => "",
            };
            trace!("{}({} bytes){}", msg, alloc.bytes.len(), immutable);

            if !relocations.is_empty() {
                msg.clear();
                write!(msg, "{:1$}", "", prefix_len).unwrap(); // Print spaces.
                let mut pos = 0;
                let relocation_width = (self.pointer_size() - 1) * 3;
                for (i, target_id) in relocations {
                    // this `as usize` is fine, since we can't print more chars than `usize::MAX`
                    write!(msg, "{:1$}", "", ((i - pos) * 3) as usize).unwrap();
                    let target = match target_id {
                        ZST_ALLOC_ID => String::from("zst"),
                        NEVER_ALLOC_ID => String::from("int ptr"),
                        _ => format!("({})", target_id),
                    };
                    // this `as usize` is fine, since we can't print more chars than `usize::MAX`
                    write!(msg, "└{0:─^1$}┘ ", target, relocation_width as usize).unwrap();
                    pos = i + self.pointer_size();
                }
                trace!("{}", msg);
            }
        }
    }
}

fn dump_fn_def<'tcx>(fn_def: FunctionDefinition<'tcx>) -> String {
    let name = ty::tls::with(|tcx| tcx.item_path_str(fn_def.def_id));
    let abi = if fn_def.abi == Abi::Rust {
        format!("")
    } else {
        format!("extern {} ", fn_def.abi)
    };
    format!("function pointer: {}: {}{}", name, abi, fn_def.sig)
}

/// Byte accessors
impl<'a, 'tcx> Memory<'a, 'tcx> {
    fn get_bytes_unchecked(&self, ptr: Pointer, size: u64, align: u64) -> EvalResult<'tcx, &[u8]> {
        if size == 0 {
            return Ok(&[]);
        }
        self.check_align(ptr, align, size)?;
        let alloc = self.get(ptr.alloc_id)?;
        let allocation_size = alloc.bytes.len() as u64;
        if ptr.offset + size > allocation_size {
            return Err(EvalError::PointerOutOfBounds { ptr, size, allocation_size });
        }
        assert_eq!(ptr.offset as usize as u64, ptr.offset);
        assert_eq!(size as usize as u64, size);
        let offset = ptr.offset as usize;
        Ok(&alloc.bytes[offset..offset + size as usize])
    }

    fn get_bytes_unchecked_mut(&mut self, ptr: Pointer, size: u64, align: u64) -> EvalResult<'tcx, &mut [u8]> {
        if size == 0 {
            return Ok(&mut []);
        }
        self.check_align(ptr, align, size)?;
        let alloc = self.get_mut(ptr.alloc_id)?;
        let allocation_size = alloc.bytes.len() as u64;
        if ptr.offset + size > allocation_size {
            return Err(EvalError::PointerOutOfBounds { ptr, size, allocation_size });
        }
        assert_eq!(ptr.offset as usize as u64, ptr.offset);
        assert_eq!(size as usize as u64, size);
        let offset = ptr.offset as usize;
        Ok(&mut alloc.bytes[offset..offset + size as usize])
    }

    fn get_bytes(&self, ptr: Pointer, size: u64, align: u64) -> EvalResult<'tcx, &[u8]> {
        if size == 0 {
            return Ok(&[]);
        }
        if self.relocations(ptr, size)?.count() != 0 {
            return Err(EvalError::ReadPointerAsBytes);
        }
        self.check_defined(ptr, size)?;
        self.get_bytes_unchecked(ptr, size, align)
    }

    fn get_bytes_mut(&mut self, ptr: Pointer, size: u64, align: u64) -> EvalResult<'tcx, &mut [u8]> {
        if size == 0 {
            return Ok(&mut []);
        }
        self.clear_relocations(ptr, size)?;
        self.mark_definedness(ptr, size, true)?;
        self.get_bytes_unchecked_mut(ptr, size, align)
    }
}

/// Reading and writing
impl<'a, 'tcx> Memory<'a, 'tcx> {
    /// mark an allocation as being the entry point to a static (see `static_alloc` field)
    pub fn mark_static(&mut self, alloc_id: AllocId) {
        if alloc_id != NEVER_ALLOC_ID && alloc_id != ZST_ALLOC_ID && !self.static_alloc.insert(alloc_id) {
            bug!("tried to mark an allocation ({:?}) as static twice", alloc_id);
        }
    }

    /// mark an allocation as static and initialized, either mutable or not
    pub fn mark_static_initalized(&mut self, alloc_id: AllocId, mutable: bool) -> EvalResult<'tcx> {
        // do not use `self.get_mut(alloc_id)` here, because we might have already marked a
        // sub-element or have circular pointers (e.g. `Rc`-cycles)
        let relocations = match self.alloc_map.get_mut(&alloc_id) {
            Some(&mut Allocation { ref mut relocations, static_kind: ref mut kind @ StaticKind::NotStatic, .. }) => {
                *kind = if mutable {
                    StaticKind::Mutable
                } else {
                    StaticKind::Immutable
                };
                // take out the relocations vector to free the borrow on self, so we can call
                // mark recursively
                mem::replace(relocations, Default::default())
            },
            None if alloc_id == NEVER_ALLOC_ID || alloc_id == ZST_ALLOC_ID => return Ok(()),
            None if !self.functions.contains_key(&alloc_id) => return Err(EvalError::DanglingPointerDeref),
            _ => return Ok(()),
        };
        // recurse into inner allocations
        for &alloc in relocations.values() {
            // relocations into other statics are not "inner allocations"
            if !self.static_alloc.contains(&alloc) {
                self.mark_static_initalized(alloc, mutable)?;
            }
        }
        // put back the relocations
        self.alloc_map.get_mut(&alloc_id).expect("checked above").relocations = relocations;
        Ok(())
    }

    pub fn copy(&mut self, src: Pointer, dest: Pointer, size: u64, align: u64) -> EvalResult<'tcx> {
        if size == 0 {
            return Ok(());
        }
        self.check_relocation_edges(src, size)?;

        let src_bytes = self.get_bytes_unchecked(src, size, align)?.as_ptr();
        let dest_bytes = self.get_bytes_mut(dest, size, align)?.as_mut_ptr();

        // SAFE: The above indexing would have panicked if there weren't at least `size` bytes
        // behind `src` and `dest`. Also, we use the overlapping-safe `ptr::copy` if `src` and
        // `dest` could possibly overlap.
        unsafe {
            assert_eq!(size as usize as u64, size);
            if src.alloc_id == dest.alloc_id {
                ptr::copy(src_bytes, dest_bytes, size as usize);
            } else {
                ptr::copy_nonoverlapping(src_bytes, dest_bytes, size as usize);
            }
        }

        self.copy_undef_mask(src, dest, size)?;
        self.copy_relocations(src, dest, size)?;

        Ok(())
    }

    pub fn read_c_str(&self, ptr: Pointer) -> EvalResult<'tcx, &[u8]> {
        let alloc = self.get(ptr.alloc_id)?;
        assert_eq!(ptr.offset as usize as u64, ptr.offset);
        let offset = ptr.offset as usize;
        match alloc.bytes[offset..].iter().position(|&c| c == 0) {
            Some(size) => {
                if self.relocations(ptr, (size + 1) as u64)?.count() != 0 {
                    return Err(EvalError::ReadPointerAsBytes);
                }
                self.check_defined(ptr, (size + 1) as u64)?;
                Ok(&alloc.bytes[offset..offset + size])
            },
            None => Err(EvalError::UnterminatedCString(ptr)),
        }
    }

    pub fn read_bytes(&self, ptr: Pointer, size: u64) -> EvalResult<'tcx, &[u8]> {
        self.get_bytes(ptr, size, 1)
    }

    pub fn write_bytes(&mut self, ptr: Pointer, src: &[u8]) -> EvalResult<'tcx> {
        let bytes = self.get_bytes_mut(ptr, src.len() as u64, 1)?;
        bytes.clone_from_slice(src);
        Ok(())
    }

    pub fn write_repeat(&mut self, ptr: Pointer, val: u8, count: u64) -> EvalResult<'tcx> {
        let bytes = self.get_bytes_mut(ptr, count, 1)?;
        for b in bytes { *b = val; }
        Ok(())
    }

    pub fn read_ptr(&self, ptr: Pointer) -> EvalResult<'tcx, Pointer> {
        let size = self.pointer_size();
        self.check_defined(ptr, size)?;
        let endianess = self.endianess();
        let bytes = self.get_bytes_unchecked(ptr, size, size)?;
        let offset = read_target_uint(endianess, bytes).unwrap();
        assert_eq!(offset as u64 as u128, offset);
        let offset = offset as u64;
        let alloc = self.get(ptr.alloc_id)?;
        match alloc.relocations.get(&ptr.offset) {
            Some(&alloc_id) => Ok(Pointer::new(alloc_id, offset)),
            None => Ok(Pointer::from_int(offset)),
        }
    }

    pub fn write_ptr(&mut self, dest: Pointer, ptr: Pointer) -> EvalResult<'tcx> {
        self.write_usize(dest, ptr.offset as u64)?;
        self.get_mut(dest.alloc_id)?.relocations.insert(dest.offset, ptr.alloc_id);
        Ok(())
    }

    pub fn write_primval(
        &mut self,
        dest: Pointer,
        val: PrimVal,
        size: u64,
    ) -> EvalResult<'tcx> {
        match val {
            PrimVal::Ptr(ptr) => {
                assert_eq!(size, self.pointer_size());
                self.write_ptr(dest, ptr)
            }

            PrimVal::Bytes(bytes) => {
                // We need to mask here, or the byteorder crate can die when given a u64 larger
                // than fits in an integer of the requested size.
                let mask = match size {
                    1 => !0u8 as u128,
                    2 => !0u16 as u128,
                    4 => !0u32 as u128,
                    8 => !0u64 as u128,
                    16 => !0,
                    _ => bug!("unexpected PrimVal::Bytes size"),
                };
                self.write_uint(dest, bytes & mask, size)
            }

            PrimVal::Undef => self.mark_definedness(dest, size, false),
        }
    }

    pub fn read_bool(&self, ptr: Pointer) -> EvalResult<'tcx, bool> {
        let bytes = self.get_bytes(ptr, 1, self.layout.i1_align.abi())?;
        match bytes[0] {
            0 => Ok(false),
            1 => Ok(true),
            _ => Err(EvalError::InvalidBool),
        }
    }

    pub fn write_bool(&mut self, ptr: Pointer, b: bool) -> EvalResult<'tcx> {
        let align = self.layout.i1_align.abi();
        self.get_bytes_mut(ptr, 1, align)
            .map(|bytes| bytes[0] = b as u8)
    }

    fn int_align(&self, size: u64) -> EvalResult<'tcx, u64> {
        match size {
            1 => Ok(self.layout.i8_align.abi()),
            2 => Ok(self.layout.i16_align.abi()),
            4 => Ok(self.layout.i32_align.abi()),
            8 => Ok(self.layout.i64_align.abi()),
            16 => Ok(self.layout.i128_align.abi()),
            _ => bug!("bad integer size: {}", size),
        }
    }

    pub fn read_int(&self, ptr: Pointer, size: u64) -> EvalResult<'tcx, i128> {
        let align = self.int_align(size)?;
        self.get_bytes(ptr, size, align).map(|b| read_target_int(self.endianess(), b).unwrap())
    }

    pub fn write_int(&mut self, ptr: Pointer, n: i128, size: u64) -> EvalResult<'tcx> {
        let align = self.int_align(size)?;
        let endianess = self.endianess();
        let b = self.get_bytes_mut(ptr, size, align)?;
        write_target_int(endianess, b, n).unwrap();
        Ok(())
    }

    pub fn read_uint(&self, ptr: Pointer, size: u64) -> EvalResult<'tcx, u128> {
        let align = self.int_align(size)?;
        self.get_bytes(ptr, size, align).map(|b| read_target_uint(self.endianess(), b).unwrap())
    }

    pub fn write_uint(&mut self, ptr: Pointer, n: u128, size: u64) -> EvalResult<'tcx> {
        let align = self.int_align(size)?;
        let endianess = self.endianess();
        let b = self.get_bytes_mut(ptr, size, align)?;
        write_target_uint(endianess, b, n).unwrap();
        Ok(())
    }

    pub fn read_isize(&self, ptr: Pointer) -> EvalResult<'tcx, i64> {
        self.read_int(ptr, self.pointer_size()).map(|i| i as i64)
    }

    pub fn write_isize(&mut self, ptr: Pointer, n: i64) -> EvalResult<'tcx> {
        let size = self.pointer_size();
        self.write_int(ptr, n as i128, size)
    }

    pub fn read_usize(&self, ptr: Pointer) -> EvalResult<'tcx, u64> {
        self.read_uint(ptr, self.pointer_size()).map(|i| i as u64)
    }

    pub fn write_usize(&mut self, ptr: Pointer, n: u64) -> EvalResult<'tcx> {
        let size = self.pointer_size();
        self.write_uint(ptr, n as u128, size)
    }

    pub fn write_f32(&mut self, ptr: Pointer, f: f32) -> EvalResult<'tcx> {
        let endianess = self.endianess();
        let align = self.layout.f32_align.abi();
        let b = self.get_bytes_mut(ptr, 4, align)?;
        write_target_f32(endianess, b, f).unwrap();
        Ok(())
    }

    pub fn write_f64(&mut self, ptr: Pointer, f: f64) -> EvalResult<'tcx> {
        let endianess = self.endianess();
        let align = self.layout.f64_align.abi();
        let b = self.get_bytes_mut(ptr, 8, align)?;
        write_target_f64(endianess, b, f).unwrap();
        Ok(())
    }

    pub fn read_f32(&self, ptr: Pointer) -> EvalResult<'tcx, f32> {
        self.get_bytes(ptr, 4, self.layout.f32_align.abi())
            .map(|b| read_target_f32(self.endianess(), b).unwrap())
    }

    pub fn read_f64(&self, ptr: Pointer) -> EvalResult<'tcx, f64> {
        self.get_bytes(ptr, 8, self.layout.f64_align.abi())
            .map(|b| read_target_f64(self.endianess(), b).unwrap())
    }
}

/// Relocations
impl<'a, 'tcx> Memory<'a, 'tcx> {
    fn relocations(&self, ptr: Pointer, size: u64)
        -> EvalResult<'tcx, btree_map::Range<u64, AllocId>>
    {
        let start = ptr.offset.saturating_sub(self.pointer_size() - 1);
        let end = ptr.offset + size;
        Ok(self.get(ptr.alloc_id)?.relocations.range(start..end))
    }

    fn clear_relocations(&mut self, ptr: Pointer, size: u64) -> EvalResult<'tcx> {
        // Find all relocations overlapping the given range.
        let keys: Vec<_> = self.relocations(ptr, size)?.map(|(&k, _)| k).collect();
        if keys.is_empty() { return Ok(()); }

        // Find the start and end of the given range and its outermost relocations.
        let start = ptr.offset;
        let end = start + size;
        let first = *keys.first().unwrap();
        let last = *keys.last().unwrap() + self.pointer_size();

        let alloc = self.get_mut(ptr.alloc_id)?;

        // Mark parts of the outermost relocations as undefined if they partially fall outside the
        // given range.
        if first < start { alloc.undef_mask.set_range(first, start, false); }
        if last > end { alloc.undef_mask.set_range(end, last, false); }

        // Forget all the relocations.
        for k in keys { alloc.relocations.remove(&k); }

        Ok(())
    }

    fn check_relocation_edges(&self, ptr: Pointer, size: u64) -> EvalResult<'tcx> {
        let overlapping_start = self.relocations(ptr, 0)?.count();
        let overlapping_end = self.relocations(ptr.offset(size), 0)?.count();
        if overlapping_start + overlapping_end != 0 {
            return Err(EvalError::ReadPointerAsBytes);
        }
        Ok(())
    }

    fn copy_relocations(&mut self, src: Pointer, dest: Pointer, size: u64) -> EvalResult<'tcx> {
        let relocations: Vec<_> = self.relocations(src, size)?
            .map(|(&offset, &alloc_id)| {
                // Update relocation offsets for the new positions in the destination allocation.
                (offset + dest.offset - src.offset, alloc_id)
            })
            .collect();
        self.get_mut(dest.alloc_id)?.relocations.extend(relocations);
        Ok(())
    }
}

/// Undefined bytes
impl<'a, 'tcx> Memory<'a, 'tcx> {
    // FIXME(solson): This is a very naive, slow version.
    fn copy_undef_mask(&mut self, src: Pointer, dest: Pointer, size: u64) -> EvalResult<'tcx> {
        // The bits have to be saved locally before writing to dest in case src and dest overlap.
        assert_eq!(size as usize as u64, size);
        let mut v = Vec::with_capacity(size as usize);
        for i in 0..size {
            let defined = self.get(src.alloc_id)?.undef_mask.get(src.offset + i);
            v.push(defined);
        }
        for (i, defined) in v.into_iter().enumerate() {
            self.get_mut(dest.alloc_id)?.undef_mask.set(dest.offset + i as u64, defined);
        }
        Ok(())
    }

    fn check_defined(&self, ptr: Pointer, size: u64) -> EvalResult<'tcx> {
        let alloc = self.get(ptr.alloc_id)?;
        if !alloc.undef_mask.is_range_defined(ptr.offset, ptr.offset + size) {
            return Err(EvalError::ReadUndefBytes);
        }
        Ok(())
    }

    pub fn mark_definedness(
        &mut self,
        ptr: Pointer,
        size: u64,
        new_state: bool
    ) -> EvalResult<'tcx> {
        if size == 0 {
            return Ok(())
        }
        let mut alloc = self.get_mut(ptr.alloc_id)?;
        alloc.undef_mask.set_range(ptr.offset, ptr.offset + size, new_state);
        Ok(())
    }
}

////////////////////////////////////////////////////////////////////////////////
// Methods to access integers in the target endianess
////////////////////////////////////////////////////////////////////////////////

fn write_target_uint(endianess: layout::Endian, mut target: &mut [u8], data: u128) -> Result<(), io::Error> {
    let len = target.len();
    match endianess {
        layout::Endian::Little => target.write_uint128::<LittleEndian>(data, len),
        layout::Endian::Big => target.write_uint128::<BigEndian>(data, len),
    }
}
fn write_target_int(endianess: layout::Endian, mut target: &mut [u8], data: i128) -> Result<(), io::Error> {
    let len = target.len();
    match endianess {
        layout::Endian::Little => target.write_int128::<LittleEndian>(data, len),
        layout::Endian::Big => target.write_int128::<BigEndian>(data, len),
    }
}

fn read_target_uint(endianess: layout::Endian, mut source: &[u8]) -> Result<u128, io::Error> {
    match endianess {
        layout::Endian::Little => source.read_uint128::<LittleEndian>(source.len()),
        layout::Endian::Big => source.read_uint128::<BigEndian>(source.len()),
    }
}
fn read_target_int(endianess: layout::Endian, mut source: &[u8]) -> Result<i128, io::Error> {
    match endianess {
        layout::Endian::Little => source.read_int128::<LittleEndian>(source.len()),
        layout::Endian::Big => source.read_int128::<BigEndian>(source.len()),
    }
}

////////////////////////////////////////////////////////////////////////////////
// Methods to access floats in the target endianess
////////////////////////////////////////////////////////////////////////////////

fn write_target_f32(endianess: layout::Endian, mut target: &mut [u8], data: f32) -> Result<(), io::Error> {
    match endianess {
        layout::Endian::Little => target.write_f32::<LittleEndian>(data),
        layout::Endian::Big => target.write_f32::<BigEndian>(data),
    }
}
fn write_target_f64(endianess: layout::Endian, mut target: &mut [u8], data: f64) -> Result<(), io::Error> {
    match endianess {
        layout::Endian::Little => target.write_f64::<LittleEndian>(data),
        layout::Endian::Big => target.write_f64::<BigEndian>(data),
    }
}

fn read_target_f32(endianess: layout::Endian, mut source: &[u8]) -> Result<f32, io::Error> {
    match endianess {
        layout::Endian::Little => source.read_f32::<LittleEndian>(),
        layout::Endian::Big => source.read_f32::<BigEndian>(),
    }
}
fn read_target_f64(endianess: layout::Endian, mut source: &[u8]) -> Result<f64, io::Error> {
    match endianess {
        layout::Endian::Little => source.read_f64::<LittleEndian>(),
        layout::Endian::Big => source.read_f64::<BigEndian>(),
    }
}

////////////////////////////////////////////////////////////////////////////////
// Undefined byte tracking
////////////////////////////////////////////////////////////////////////////////

type Block = u64;
const BLOCK_SIZE: u64 = 64;

#[derive(Clone, Debug)]
pub struct UndefMask {
    blocks: Vec<Block>,
    len: u64,
}

impl UndefMask {
    fn new(size: u64) -> Self {
        let mut m = UndefMask {
            blocks: vec![],
            len: 0,
        };
        m.grow(size, false);
        m
    }

    /// Check whether the range `start..end` (end-exclusive) is entirely defined.
    pub fn is_range_defined(&self, start: u64, end: u64) -> bool {
        if end > self.len { return false; }
        for i in start..end {
            if !self.get(i) { return false; }
        }
        true
    }

    fn set_range(&mut self, start: u64, end: u64, new_state: bool) {
        let len = self.len;
        if end > len { self.grow(end - len, new_state); }
        self.set_range_inbounds(start, end, new_state);
    }

    fn set_range_inbounds(&mut self, start: u64, end: u64, new_state: bool) {
        for i in start..end { self.set(i, new_state); }
    }

    fn get(&self, i: u64) -> bool {
        let (block, bit) = bit_index(i);
        (self.blocks[block] & 1 << bit) != 0
    }

    fn set(&mut self, i: u64, new_state: bool) {
        let (block, bit) = bit_index(i);
        if new_state {
            self.blocks[block] |= 1 << bit;
        } else {
            self.blocks[block] &= !(1 << bit);
        }
    }

    fn grow(&mut self, amount: u64, new_state: bool) {
        let unused_trailing_bits = self.blocks.len() as u64 * BLOCK_SIZE - self.len;
        if amount > unused_trailing_bits {
            let additional_blocks = amount / BLOCK_SIZE + 1;
            assert_eq!(additional_blocks as usize as u64, additional_blocks);
            self.blocks.extend(iter::repeat(0).take(additional_blocks as usize));
        }
        let start = self.len;
        self.len += amount;
        self.set_range_inbounds(start, start + amount, new_state);
    }

    fn truncate(&mut self, length: u64) {
        self.len = length;
        let truncate = self.len / BLOCK_SIZE + 1;
        assert_eq!(truncate as usize as u64, truncate);
        self.blocks.truncate(truncate as usize);
        self.blocks.shrink_to_fit();
    }
}

fn bit_index(bits: u64) -> (usize, usize) {
    let a = bits / BLOCK_SIZE;
    let b = bits % BLOCK_SIZE;
    assert_eq!(a as usize as u64, a);
    assert_eq!(b as usize as u64, b);
    (a as usize, b as usize)
}