msvc.rs

// SPDX-License-Identifier: MIT OR Apache-2.0
use crate::builder::common::{
    builtin_type_layout, compute_builtin_type_layout, compute_opaque_type_layout,
};
use crate::layout::{
    Annotation, Array, BuiltinType, FieldLayout, Record, RecordField, RecordKind, Type, TypeLayout,
    TypeVariant,
};
use crate::result::{err, ErrorType, Result};
use crate::target::Target;
use crate::util::{
    align_to, annotation_alignment, is_attr_packed, pragma_pack_value, size_add, size_mul,
    MaxAssign, MaxExt, MinAssign, MinExt, BITS_PER_BYTE,
};

pub fn compute_layout(target: Target, ty: &Type<()>) -> Result<Type<TypeLayout>> {
    match &ty.variant {
        TypeVariant::Builtin(bi) => compute_builtin_type_layout(target, *bi),
        TypeVariant::Opaque(layout) => compute_opaque_type_layout(*layout),
        TypeVariant::Record(r) => compute_record_layout(target, r.kind, &ty.annotations, &r.fields),
        TypeVariant::Typedef(dst) => {
            // #pragma pack is ignored on typedefs. See test case 0088.
            let dst_ty = compute_layout(target, dst)?;
            let max_alignment =
                annotation_alignment(target, &ty.annotations).unwrap_or(BITS_PER_BYTE);
            // __declspec(align) increases both the required and the field alignment but
            // never decreases them. It does not affect the size or the pointer alignment.
            // See test case 0014.
            Ok(Type {
                layout: TypeLayout {
                    field_alignment_bits: dst_ty.layout.field_alignment_bits.max(max_alignment),
                    required_alignment_bits: dst_ty
                        .layout
                        .required_alignment_bits
                        .max(max_alignment),
                    ..dst_ty.layout
                },
                annotations: ty.annotations.clone(),
                variant: TypeVariant::Typedef(Box::new(dst_ty)),
            })
        }
        TypeVariant::Array(a) => {
            let ety = compute_layout(target, &a.element_type)?;
            Ok(Type {
                layout: TypeLayout {
                    // The size of an array is the size of the underlying type multiplied by the
                    // number of elements. Since the size might not be a multiple of the field
                    // alignment, the address of the second element might not be properly aligned
                    // for the field alignment. A flexible array has size 0. See test case 0018.
                    size_bits: size_mul(ety.layout.size_bits, a.num_elements.unwrap_or(0))?,
                    // The alignments are inherited from the underlying type.
                    ..ety.layout
                },
                // Pre-validation ensures that arrays do not have annotations.
                annotations: vec![],
                variant: TypeVariant::Array(Array {
                    element_type: Box::new(ety),
                    num_elements: a.num_elements,
                }),
            })
        }
        TypeVariant::Enum(v) => {
            // #pragma pack is ignored. See test case 0054.
            // __attribute__((aligned)) is ignored by clang. See test case 0055.
            let requested_alignment =
                annotation_alignment(target, &ty.annotations).unwrap_or(BITS_PER_BYTE);
            // Enums always have the base type int even if the values do not fit into int. The
            // values are silently truncated if necessary. See test case 0019.
            let mut layout = builtin_type_layout(target, BuiltinType::Int);
            // The alignment requested by __declspec(align)) does not affect the size and therefore
            // also not the pointer alignment. See test case 0051.
            layout
                .required_alignment_bits
                .assign_max(requested_alignment);
            layout.field_alignment_bits.assign_max(requested_alignment);
            Ok(Type {
                layout,
                annotations: ty.annotations.clone(),
                variant: TypeVariant::Enum(v.clone()),
            })
        }
    }
}

fn compute_record_layout(
    target: Target,
    ty: RecordKind,
    annotations: &[Annotation],
    u: &[RecordField<()>],
) -> Result<Type<TypeLayout>> {
    RecordLayoutBuilder::new(target, ty, annotations)?.compute(u)
}

pub(crate) struct RecordLayoutBuilder<'a> {
    target: Target,
    // The annotations of this type.
    annotations: &'a [Annotation],
    // The required alignment of the type.
    required_alignment_bits: u64,
    // The alignment of pointers that point to an object of this type. This is greater to or equal
    // to the required alignment. Once all fields have been laid out, the size of the record will be
    // rounded up to this value.
    pointer_alignment_bits: u64,
    // The alignment of this type when it is used as a record field. This is greater to or equal to
    // the pointer alignment.
    field_alignment_bits: u64,
    // The size of the record.
    size_bits: u64,
    // The minimum value of all __attribute__((packed)) and #pragma pack(N) at the type level.
    max_field_alignment_bits: Option<u64>,
    // The fields in this record.
    record_fields: Vec<RecordField<TypeLayout>>,
    // The kind of this record. Struct or Union.
    kind: RecordKind,
    // Set to `Some` if and only if the previous field was a non-zero-sized bitfield.
    // This is used even in unions. In particular, the order of fields in unions is significant.
    ongoing_bitfield: Option<OngoingBitfield>,
    // Set to `true` if and only if the record contains at least on non-bitfield field.
    contains_non_bitfield: bool,
}

struct OngoingBitfield {
    // The size of the storage unit of the previous bitfield. This is the size of the underlying
    // type, e.g., `int`.
    ty_size_bits: u64,
    // The number of bits that remain unused in the storage unit. This can be 0 if all of the bits
    // have been used.
    unused_size_bits: u64,
}

impl<'a> RecordLayoutBuilder<'a> {
    pub(crate) fn new(
        target: Target,
        kind: RecordKind,
        annotations: &'a [Annotation],
    ) -> Result<Self> {
        // __attribute__((packed)) behaves like #pragma pack(1) in clang. See test case 0056.
        let pack_value = match is_attr_packed(annotations) {
            true => Some(BITS_PER_BYTE),
            false => pragma_pack_value(annotations),
        };
        // The effect of #pragma pack(N) depends on the target.
        //
        // x86: By default, there is no maximum field alignment. N={1,2,4} set the maximum field
        //      alignment to that value. All other N activate the default.
        // x64: By default, there is no maximum field alignment. N={1,2,4,8} set the maximum field
        //      alignment to that value. All other N activate the default.
        // arm: By default, the maximum field alignment is 8. N={1,2,4,8,16} set the maximum field
        //      alignment to that value. All other N activate the default.
        // arm64: By default, the maximum field alignment is 8. N={1,2,4,8} set the maximum field
        //        alignment to that value. N=16 disables the maximum field alignment. All other N
        //        activate the default.
        //
        // See test case 0020.
        use Target::*;
        let max_field_alignment_bits = match (pack_value, target) {
            (Some(8), _) | (Some(16), _) | (Some(32), _) => pack_value,
            (Some(64), I586PcWindowsMsvc)
            | (Some(64), I686PcWindowsMsvc)
            | (Some(64), I686UnknownWindows) => None,
            (Some(64), _) => pack_value,
            (Some(128), Thumbv7aPcWindowsMsvc) => pack_value,
            (Some(128), _) => None,
            (_, Thumbv7aPcWindowsMsvc) | (_, Aarch64PcWindowsMsvc) => Some(64),
            _ => None,
        };
        // The required alignment can be increased by adding a __declspec(align)
        // annotation. See test case 0023.
        let required_alignment_bits =
            annotation_alignment(target, annotations).unwrap_or(BITS_PER_BYTE);
        Ok(Self {
            target,
            annotations,
            required_alignment_bits,
            // pointer and field alignment are at least as strict as the required
            // alignment
            pointer_alignment_bits: required_alignment_bits,
            field_alignment_bits: required_alignment_bits,
            size_bits: 0,
            max_field_alignment_bits,
            record_fields: vec![],
            kind,
            ongoing_bitfield: None,
            contains_non_bitfield: false,
        })
    }

    fn compute(mut self, fields: &[RecordField<()>]) -> Result<Type<TypeLayout>> {
        for f in fields {
            self.layout_field(f)?;
        }
        if self.size_bits == 0 {
            // As an extension, MSVC allows records that only contain zero-sized bitfields and empty
            // arrays. Such records would be zero-sized but this case is handled here separately to
            // ensure that there are no zero-sized records.
            self.handle_zero_sized_record();
        }
        // The size is always a multiple of the pointer alignment.
        self.size_bits = align_to(self.size_bits, self.pointer_alignment_bits)?;
        Ok(Type {
            layout: TypeLayout {
                size_bits: self.size_bits,
                field_alignment_bits: self.field_alignment_bits,
                pointer_alignment_bits: self.pointer_alignment_bits,
                required_alignment_bits: self.required_alignment_bits,
            },
            annotations: self.annotations.to_vec(),
            variant: TypeVariant::Record(Record {
                kind: self.kind,
                fields: self.record_fields,
            }),
        })
    }

    fn handle_zero_sized_record(&mut self) {
        match self.kind {
            RecordKind::Union => {
                // MSVC does not allow unions without fields.
                // If all fields in a union have size 0, the size of the union is set to
                // - its field alignment if it contains at least one non-bitfield
                // - 4 bytes if it contains only bitfields
                // See test case 0025.
                if self.contains_non_bitfield {
                    self.size_bits = self.field_alignment_bits;
                } else {
                    self.size_bits = 4 * BITS_PER_BYTE;
                }
            }
            RecordKind::Struct => {
                // If all fields in a struct have size 0, its size is set to its required alignment
                // but at least to 4 bytes. See test case 0026.
                self.size_bits = self.required_alignment_bits.max(4 * BITS_PER_BYTE);
                self.pointer_alignment_bits.assign_min(self.size_bits);
            }
        }
    }

    fn layout_field(&mut self, field: &RecordField<()>) -> Result<()> {
        // The offset and the size of the field is based on the layout of the underlying type.
        let field_ty = compute_layout(self.target, &field.ty)?;
        let (ty_size_bits, field_alignment_bits) = {
            let layout = field_ty.layout;
            // The required alignment of the field is the maximum of the required alignment of the
            // underlying type and the __declspec(align) annotation on the field itself.
            // See test case 0028.
            let required_alignment_bits = annotation_alignment(self.target, &field.annotations)
                .max2(layout.required_alignment_bits);
            // The required alignment of a record is the maximum of the required alignments of its
            // fields except that the required alignment of bitfields is ignored.
            // See test case 0029.
            if field.bit_width.is_none() {
                self.required_alignment_bits
                    .assign_max(required_alignment_bits);
            }
            // The offset of the field is based on the field alignment of the underlying type.
            // See test case 0027.
            let mut field_alignment_bits = layout.field_alignment_bits;
            // The effect of the field alignment of the underlying type is limited by #pragma pack.
            // See test case 0030.
            field_alignment_bits.assign_min(self.max_field_alignment_bits);
            if is_attr_packed(&field.annotations) {
                // __attribute__((packed)) on a field is a clang extension. It behaves as if #pragma
                // pack(1) had been applied only to this field. See test case 0057.
                field_alignment_bits = BITS_PER_BYTE;
            }
            // The required alignment of the field takes precedence over #pragma pack.
            // See test cases 0031 and 0058.
            field_alignment_bits.assign_max(required_alignment_bits);
            (layout.size_bits, field_alignment_bits)
        };
        // These functions return `None` if and only if the field is unnamed.
        let layout = match field.bit_width {
            Some(n) => self.layout_bit_field(ty_size_bits, field_alignment_bits, field.named, n),
            None => self.layout_regular_field(ty_size_bits, field_alignment_bits),
        }?;
        self.record_fields.push(RecordField {
            layout,
            annotations: field.annotations.clone(),
            named: field.named,
            bit_width: field.bit_width,
            ty: field_ty,
        });
        Ok(())
    }

    fn layout_regular_field(
        &mut self,
        size_bits: u64,
        field_alignment_bits: u64,
    ) -> Result<Option<FieldLayout>> {
        self.contains_non_bitfield = true;
        self.ongoing_bitfield = None;
        // The alignment of the field affects both the pointer alignment and the field
        // alignment of the record. See test case 0032.
        self.pointer_alignment_bits.assign_max(field_alignment_bits);
        self.field_alignment_bits.assign_max(field_alignment_bits);
        let offset_bits = match self.kind {
            // A struct field starts at the next offset in the struct that is properly
            // aligned with respect to the start of the struct. See test case 0033.
            RecordKind::Struct => align_to(self.size_bits, field_alignment_bits)?,
            // A union field always starts at offset 0.
            RecordKind::Union => 0,
        };
        // Set the size of the record to the maximum of the current size and the end of
        // the field. See test case 0034.
        self.size_bits.assign_max(size_add(offset_bits, size_bits)?);
        Ok(Some(FieldLayout {
            offset_bits,
            size_bits,
        }))
    }

    fn layout_bit_field(
        &mut self,
        ty_size_bits: u64,
        field_alignment_bits: u64,
        named: bool,
        width: u64,
    ) -> Result<Option<FieldLayout>> {
        macro_rules! ok {
            ($offset:expr) => {
                Ok(match named {
                    true => Some(FieldLayout {
                        offset_bits: $offset,
                        size_bits: width,
                    }),
                    false => None,
                })
            };
        }
        if width == 0 {
            // A zero-sized bit-field that does not follow a non-zero-sized bit-field does not affect
            // the overall layout of the record. Even in a union where the order would otherwise
            // not matter. See test case 0035.
            if self.ongoing_bitfield.is_none() {
                return Ok(None);
            }
            self.ongoing_bitfield = None;
        } else {
            if width > ty_size_bits {
                return Err(err(ErrorType::OversizedBitfield));
            }
            // If there is an ongoing bit-field in a struct whose underlying type has the same size and
            // if there is enough space left to place this bit-field, then this bit-field is placed in
            // the ongoing bit-field and the overall layout of the struct is not affected by this
            // bit-field. See test case 0037.
            if self.kind == RecordKind::Struct {
                if let Some(ref mut p) = &mut self.ongoing_bitfield {
                    if p.ty_size_bits == ty_size_bits && p.unused_size_bits >= width {
                        let offset_bits = self.size_bits - p.unused_size_bits;
                        p.unused_size_bits -= width;
                        return ok!(offset_bits);
                    }
                }
            }
            // Otherwise this field is part of a new ongoing bit-field.
            self.ongoing_bitfield = Some(OngoingBitfield {
                ty_size_bits,
                unused_size_bits: ty_size_bits - width,
            });
        }
        let offset_bits = match self.kind {
            RecordKind::Struct => {
                // This is the one place in the layout of a record where the pointer alignment might
                // get assigned a smaller value than the field alignment. This can only happen if
                // the field or the type of the field has a required alignment. Otherwise the value
                // of field_alignment_bits is already bound by max_field_alignment_bits.
                // See test case 0038.
                self.pointer_alignment_bits
                    .assign_max(field_alignment_bits.min2(self.max_field_alignment_bits));
                self.field_alignment_bits.assign_max(field_alignment_bits);
                let offset_bits = align_to(self.size_bits, field_alignment_bits)?;
                self.size_bits = match width {
                    // A zero-width bitfield only increases the size of the struct to the
                    // offset a non-zero-width bitfield with the same alignment would
                    // start. See test case 0039.
                    0 => offset_bits,
                    // A non-zero-width bitfield always increases the size by the full
                    // size of the underlying type. Even if we are in a packed context.
                    // See test case 0040.
                    _ => size_add(offset_bits, ty_size_bits)?,
                };
                offset_bits
            }
            RecordKind::Union => {
                // Bit-fields do not affect the alignment of a union. See test case 0041.
                self.size_bits.assign_max(ty_size_bits);
                0
            }
        };
        ok!(offset_bits)
    }
}