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interpret: refactor projection handling code

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Moves our projection handling code into a common file, and avoids the use of a
general mplace-based fallback function by have more specialized implementations.

mplace_index (and the other slice-related functions) could be more efficient by
copy-pasting the body of operand_index. Or we could do some trait magic to share
the code between them. But for now this is probably fine.
This commit is contained in:
Ralf Jung 2022-07-04 08:48:05 -04:00
parent f893495e3d
commit ab225ade1e
7 changed files with 531 additions and 421 deletions
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393
compiler/rustc_const_eval/src/interpret/projection.rs Normal file
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//! This file implements "place projections"; basically a symmetric API for 3 types: MPlaceTy, OpTy, PlaceTy.
//!
//! OpTy and PlaceTy genrally work by "let's see if we are actually an MPlaceTy, and do something custom if not".
//! For PlaceTy, the custom thing is basically always to call `force_allocation` and then use the MPlaceTy logic anyway.
//! For OpTy, the custom thing on field pojections has to be pretty clever (since `Operand::Immediate` can have fields),
//! but for array/slice operations it only has to worry about `Operand::Uninit`. That makes the value part trivial,
//! but we still need to do bounds checking and adjust the layout. To not duplicate that with MPlaceTy, we actually
//! implement the logic on OpTy, and MPlaceTy calls that.
use std::hash::Hash;
use rustc_middle::mir;
use rustc_middle::ty;
use rustc_middle::ty::layout::LayoutOf;
use rustc_target::abi::{self, Abi, VariantIdx};
use super::{
ImmTy, Immediate, InterpCx, InterpResult, MPlaceTy, Machine, MemPlaceMeta, OpTy, PlaceTy,
Provenance, Scalar,
};
// FIXME: Working around https://github.com/rust-lang/rust/issues/54385
impl<'mir, 'tcx: 'mir, Tag, M> InterpCx<'mir, 'tcx, M>
where
Tag: Provenance + Eq + Hash + 'static,
M: Machine<'mir, 'tcx, PointerTag = Tag>,
{
//# Field access
/// Offset a pointer to project to a field of a struct/union. Unlike `place_field`, this is
/// always possible without allocating, so it can take `&self`. Also return the field's layout.
/// This supports both struct and array fields.
///
/// This also works for arrays, but then the `usize` index type is restricting.
/// For indexing into arrays, use `mplace_index`.
pub fn mplace_field(
&self,
base: &MPlaceTy<'tcx, M::PointerTag>,
field: usize,
) -> InterpResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
let offset = base.layout.fields.offset(field);
let field_layout = base.layout.field(self, field);
// Offset may need adjustment for unsized fields.
let (meta, offset) = if field_layout.is_unsized() {
// Re-use parent metadata to determine dynamic field layout.
// With custom DSTS, this *will* execute user-defined code, but the same
// happens at run-time so that's okay.
match self.size_and_align_of(&base.meta, &field_layout)? {
Some((_, align)) => (base.meta, offset.align_to(align)),
None => {
// For unsized types with an extern type tail we perform no adjustments.
// NOTE: keep this in sync with `PlaceRef::project_field` in the codegen backend.
assert!(matches!(base.meta, MemPlaceMeta::None));
(base.meta, offset)
}
}
} else {
// base.meta could be present; we might be accessing a sized field of an unsized
// struct.
(MemPlaceMeta::None, offset)
};
// We do not look at `base.layout.align` nor `field_layout.align`, unlike
// codegen -- mostly to see if we can get away with that
base.offset(offset, meta, field_layout, self)
}
/// Gets the place of a field inside the place, and also the field's type.
/// Just a convenience function, but used quite a bit.
/// This is the only projection that might have a side-effect: We cannot project
/// into the field of a local `ScalarPair`, we have to first allocate it.
pub fn place_field(
&mut self,
base: &PlaceTy<'tcx, M::PointerTag>,
field: usize,
) -> InterpResult<'tcx, PlaceTy<'tcx, M::PointerTag>> {
// FIXME: We could try to be smarter and avoid allocation for fields that span the
// entire place.
let base = self.force_allocation(base)?;
Ok(self.mplace_field(&base, field)?.into())
}
pub fn operand_field(
&self,
base: &OpTy<'tcx, M::PointerTag>,
field: usize,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
let base = match base.try_as_mplace() {
Ok(ref mplace) => {
// We can reuse the mplace field computation logic for indirect operands.
let field = self.mplace_field(mplace, field)?;
return Ok(field.into());
}
Err(value) => value,
};
let field_layout = base.layout.field(self, field);
let offset = base.layout.fields.offset(field);
// This makes several assumptions about what layouts we will encounter; we match what
// codegen does as good as we can (see `extract_field` in `rustc_codegen_ssa/src/mir/operand.rs`).
let field_val: Immediate<_> = match (*base, base.layout.abi) {
// the field contains no information, can be left uninit
_ if field_layout.is_zst() => Immediate::Uninit,
// the field covers the entire type
_ if field_layout.size == base.layout.size => {
assert!(match (base.layout.abi, field_layout.abi) {
(Abi::Scalar(..), Abi::Scalar(..)) => true,
(Abi::ScalarPair(..), Abi::ScalarPair(..)) => true,
_ => false,
});
assert!(offset.bytes() == 0);
*base
}
// extract fields from types with `ScalarPair` ABI
(Immediate::ScalarPair(a_val, b_val), Abi::ScalarPair(a, b)) => {
assert!(matches!(field_layout.abi, Abi::Scalar(..)));
Immediate::from(if offset.bytes() == 0 {
debug_assert_eq!(field_layout.size, a.size(self));
a_val
} else {
debug_assert_eq!(offset, a.size(self).align_to(b.align(self).abi));
debug_assert_eq!(field_layout.size, b.size(self));
b_val
})
}
_ => span_bug!(
self.cur_span(),
"invalid field access on immediate {}, layout {:#?}",
base,
base.layout
),
};
Ok(ImmTy::from_immediate(field_val, field_layout).into())
}
//# Downcasting
pub fn mplace_downcast(
&self,
base: &MPlaceTy<'tcx, M::PointerTag>,
variant: VariantIdx,
) -> InterpResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
// Downcasts only change the layout.
// (In particular, no check about whether this is even the active variant -- that's by design,
// see https://github.com/rust-lang/rust/issues/93688#issuecomment-1032929496.)
assert!(!base.meta.has_meta());
let mut base = *base;
base.layout = base.layout.for_variant(self, variant);
Ok(base)
}
pub fn place_downcast(
&self,
base: &PlaceTy<'tcx, M::PointerTag>,
variant: VariantIdx,
) -> InterpResult<'tcx, PlaceTy<'tcx, M::PointerTag>> {
// Downcast just changes the layout
let mut base = *base;
base.layout = base.layout.for_variant(self, variant);
Ok(base)
}
pub fn operand_downcast(
&self,
base: &OpTy<'tcx, M::PointerTag>,
variant: VariantIdx,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
// Downcast just changes the layout
let mut base = *base;
base.layout = base.layout.for_variant(self, variant);
Ok(base)
}
//# Slice indexing
#[inline(always)]
pub fn operand_index(
&self,
base: &OpTy<'tcx, M::PointerTag>,
index: u64,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
// Not using the layout method because we want to compute on u64
match base.layout.fields {
abi::FieldsShape::Array { stride, count: _ } => {
// `count` is nonsense for slices, use the dynamic length instead.
let len = base.len(self)?;
if index >= len {
// This can only be reached in ConstProp and non-rustc-MIR.
throw_ub!(BoundsCheckFailed { len, index });
}
let offset = stride * index; // `Size` multiplication
// All fields have the same layout.
let field_layout = base.layout.field(self, 0);
assert!(!field_layout.is_unsized());
base.offset(offset, MemPlaceMeta::None, field_layout, self)
}
_ => span_bug!(
self.cur_span(),
"`mplace_index` called on non-array type {:?}",
base.layout.ty
),
}
}
// Iterates over all fields of an array. Much more efficient than doing the
// same by repeatedly calling `operand_index`.
pub fn operand_array_fields<'a>(
&self,
base: &'a OpTy<'tcx, Tag>,
) -> InterpResult<'tcx, impl Iterator<Item = InterpResult<'tcx, OpTy<'tcx, Tag>>> + 'a> {
let len = base.len(self)?; // also asserts that we have a type where this makes sense
let abi::FieldsShape::Array { stride, .. } = base.layout.fields else {
span_bug!(self.cur_span(), "operand_array_fields: expected an array layout");
};
let layout = base.layout.field(self, 0);
let dl = &self.tcx.data_layout;
// `Size` multiplication
Ok((0..len).map(move |i| base.offset(stride * i, MemPlaceMeta::None, layout, dl)))
}
/// Index into an array.
pub fn mplace_index(
&self,
base: &MPlaceTy<'tcx, M::PointerTag>,
index: u64,
) -> InterpResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
Ok(self.operand_index(&base.into(), index)?.assert_mem_place())
}
pub fn place_index(
&mut self,
base: &PlaceTy<'tcx, M::PointerTag>,
index: u64,
) -> InterpResult<'tcx, PlaceTy<'tcx, M::PointerTag>> {
// There's not a lot we can do here, since we cannot have a place to a part of a local. If
// we are accessing the only element of a 1-element array, it's still the entire local...
// that doesn't seem worth it.
let base = self.force_allocation(base)?;
Ok(self.mplace_index(&base, index)?.into())
}
//# ConstantIndex support
fn operand_constant_index(
&self,
base: &OpTy<'tcx, M::PointerTag>,
offset: u64,
min_length: u64,
from_end: bool,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
let n = base.len(self)?;
if n < min_length {
// This can only be reached in ConstProp and non-rustc-MIR.
throw_ub!(BoundsCheckFailed { len: min_length, index: n });
}
let index = if from_end {
assert!(0 < offset && offset <= min_length);
n.checked_sub(offset).unwrap()
} else {
assert!(offset < min_length);
offset
};
self.operand_index(base, index)
}
fn place_constant_index(
&mut self,
base: &PlaceTy<'tcx, M::PointerTag>,
offset: u64,
min_length: u64,
from_end: bool,
) -> InterpResult<'tcx, PlaceTy<'tcx, M::PointerTag>> {
let base = self.force_allocation(base)?;
Ok(self
.operand_constant_index(&base.into(), offset, min_length, from_end)?
.assert_mem_place()
.into())
}
//# Subslicing
fn operand_subslice(
&self,
base: &OpTy<'tcx, M::PointerTag>,
from: u64,
to: u64,
from_end: bool,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
let len = base.len(self)?; // also asserts that we have a type where this makes sense
let actual_to = if from_end {
if from.checked_add(to).map_or(true, |to| to > len) {
// This can only be reached in ConstProp and non-rustc-MIR.
throw_ub!(BoundsCheckFailed { len: len, index: from.saturating_add(to) });
}
len.checked_sub(to).unwrap()
} else {
to
};
// Not using layout method because that works with usize, and does not work with slices
// (that have count 0 in their layout).
let from_offset = match base.layout.fields {
abi::FieldsShape::Array { stride, .. } => stride * from, // `Size` multiplication is checked
_ => {
span_bug!(self.cur_span(), "unexpected layout of index access: {:#?}", base.layout)
}
};
// Compute meta and new layout
let inner_len = actual_to.checked_sub(from).unwrap();
let (meta, ty) = match base.layout.ty.kind() {
// It is not nice to match on the type, but that seems to be the only way to
// implement this.
ty::Array(inner, _) => (MemPlaceMeta::None, self.tcx.mk_array(*inner, inner_len)),
ty::Slice(..) => {
let len = Scalar::from_machine_usize(inner_len, self);
(MemPlaceMeta::Meta(len), base.layout.ty)
}
_ => {
span_bug!(self.cur_span(), "cannot subslice non-array type: `{:?}`", base.layout.ty)
}
};
let layout = self.layout_of(ty)?;
base.offset(from_offset, meta, layout, self)
}
pub fn place_subslice(
&mut self,
base: &PlaceTy<'tcx, M::PointerTag>,
from: u64,
to: u64,
from_end: bool,
) -> InterpResult<'tcx, PlaceTy<'tcx, M::PointerTag>> {
let base = self.force_allocation(base)?;
Ok(self.operand_subslice(&base.into(), from, to, from_end)?.assert_mem_place().into())
}
//# Applying a general projection
/// Projects into a place.
#[instrument(skip(self), level = "trace")]
pub fn place_projection(
&mut self,
base: &PlaceTy<'tcx, M::PointerTag>,
proj_elem: mir::PlaceElem<'tcx>,
) -> InterpResult<'tcx, PlaceTy<'tcx, M::PointerTag>> {
use rustc_middle::mir::ProjectionElem::*;
Ok(match proj_elem {
Field(field, _) => self.place_field(base, field.index())?,
Downcast(_, variant) => self.place_downcast(base, variant)?,
Deref => self.deref_operand(&self.place_to_op(base)?)?.into(),
Index(local) => {
let layout = self.layout_of(self.tcx.types.usize)?;
let n = self.local_to_op(self.frame(), local, Some(layout))?;
let n = self.read_scalar(&n)?.to_machine_usize(self)?;
self.place_index(base, n)?
}
ConstantIndex { offset, min_length, from_end } => {
self.place_constant_index(base, offset, min_length, from_end)?
}
Subslice { from, to, from_end } => self.place_subslice(base, from, to, from_end)?,
})
}
#[instrument(skip(self), level = "trace")]
pub fn operand_projection(
&self,
base: &OpTy<'tcx, M::PointerTag>,
proj_elem: mir::PlaceElem<'tcx>,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
use rustc_middle::mir::ProjectionElem::*;
Ok(match proj_elem {
Field(field, _) => self.operand_field(base, field.index())?,
Downcast(_, variant) => self.operand_downcast(base, variant)?,
Deref => self.deref_operand(base)?.into(),
Index(local) => {
let layout = self.layout_of(self.tcx.types.usize)?;
let n = self.local_to_op(self.frame(), local, Some(layout))?;
let n = self.read_scalar(&n)?.to_machine_usize(self)?;
self.operand_index(base, n)?
}
ConstantIndex { offset, min_length, from_end } => {
self.operand_constant_index(base, offset, min_length, from_end)?
}
Subslice { from, to, from_end } => self.operand_subslice(base, from, to, from_end)?,
})
}
}