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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.
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@RalfJung
RalfJung committed Jul 12, 2022
1 parent f893495 commit ab225ad
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Showing 7 changed files with 531 additions and 421 deletions.
1 change: 1 addition & 0 deletions compiler/rustc_const_eval/src/interpret/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@ mod memory;
mod operand;
mod operator;
mod place;
mod projection;
mod step;
mod terminator;
mod traits;
Expand Down
197 changes: 72 additions & 125 deletions compiler/rustc_const_eval/src/interpret/operand.rs
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Original file line number Diff line number Diff line change
@@ -1,7 +1,6 @@
//! Functions concerning immediate values and operands, and reading from operands.
//! All high-level functions to read from memory work on operands as sources.
use std::convert::TryFrom;
use std::fmt::Write;

use rustc_hir::def::Namespace;
Expand All @@ -15,7 +14,7 @@ use rustc_target::abi::{VariantIdx, Variants};

use super::{
alloc_range, from_known_layout, mir_assign_valid_types, AllocId, ConstValue, Frame, GlobalId,
InterpCx, InterpResult, MPlaceTy, Machine, MemPlace, Place, PlaceTy, Pointer,
InterpCx, InterpResult, MPlaceTy, Machine, MemPlace, MemPlaceMeta, Place, PlaceTy, Pointer,
PointerArithmetic, Provenance, Scalar, ScalarMaybeUninit,
};

Expand Down Expand Up @@ -253,6 +252,11 @@ impl<'tcx, Tag: Provenance> ImmTy<'tcx, Tag> {
ImmTy { imm, layout }
}

#[inline]
pub fn uninit(layout: TyAndLayout<'tcx>) -> Self {
ImmTy { imm: Immediate::Uninit, layout }
}

#[inline]
pub fn try_from_uint(i: impl Into<u128>, layout: TyAndLayout<'tcx>) -> Option<Self> {
Some(Self::from_scalar(Scalar::try_from_uint(i, layout.size)?, layout))
Expand Down Expand Up @@ -280,6 +284,41 @@ impl<'tcx, Tag: Provenance> ImmTy<'tcx, Tag> {
}
}

impl<'tcx, Tag: Provenance> OpTy<'tcx, Tag> {
pub fn len(&self, cx: &impl HasDataLayout) -> InterpResult<'tcx, u64> {
if self.layout.is_unsized() {
// There are no unsized immediates.
self.assert_mem_place().len(cx)
} else {
match self.layout.fields {
abi::FieldsShape::Array { count, .. } => Ok(count),
_ => bug!("len not supported on sized type {:?}", self.layout.ty),
}
}
}

pub fn offset(
&self,
offset: Size,
meta: MemPlaceMeta<Tag>,
layout: TyAndLayout<'tcx>,
cx: &impl HasDataLayout,
) -> InterpResult<'tcx, Self> {
match self.try_as_mplace() {
Ok(mplace) => Ok(mplace.offset(offset, meta, layout, cx)?.into()),
Err(imm) => {
assert!(
matches!(*imm, Immediate::Uninit),
"Scalar/ScalarPair cannot be offset into"
);
assert!(!meta.has_meta()); // no place to store metadata here
// Every part of an uninit is uninit.
Ok(ImmTy::uninit(layout).into())
}
}
}
}

impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// Try reading an immediate in memory; this is interesting particularly for `ScalarPair`.
/// Returns `None` if the layout does not permit loading this as a value.
Expand All @@ -296,11 +335,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
}

let Some(alloc) = self.get_place_alloc(mplace)? else {
return Ok(Some(ImmTy {
// zero-sized type can be left uninit
imm: Immediate::Uninit,
layout: mplace.layout,
}));
// zero-sized type can be left uninit
return Ok(Some(ImmTy::uninit(mplace.layout)));
};

// It may seem like all types with `Scalar` or `ScalarPair` ABI are fair game at this point.
Expand Down Expand Up @@ -367,6 +403,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// This flag exists only for validity checking.
///
/// This is an internal function that should not usually be used; call `read_immediate` instead.
/// ConstProp needs it, though.
pub fn read_immediate_raw(
&self,
src: &OpTy<'tcx, M::PointerTag>,
Expand Down Expand Up @@ -421,123 +458,28 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
Ok(str)
}

/// Projection functions
pub fn operand_field(
&self,
op: &OpTy<'tcx, M::PointerTag>,
field: usize,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
let base = match op.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(OpTy { op: Operand::Immediate(field_val), layout: field_layout, align: None })
}

pub fn operand_index(
&self,
op: &OpTy<'tcx, M::PointerTag>,
index: u64,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
if let Ok(index) = usize::try_from(index) {
// We can just treat this as a field.
self.operand_field(op, index)
} else {
// Indexing into a big array. This must be an mplace.
let mplace = op.assert_mem_place();
Ok(self.mplace_index(&mplace, index)?.into())
}
}

pub fn operand_downcast(
&self,
op: &OpTy<'tcx, M::PointerTag>,
variant: VariantIdx,
) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
Ok(match op.try_as_mplace() {
Ok(ref mplace) => self.mplace_downcast(mplace, variant)?.into(),
Err(..) => {
// 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.)
let layout = op.layout.for_variant(self, variant);
OpTy { layout, ..*op }
}
})
}

#[instrument(skip(self), level = "debug")]
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(),
Subslice { .. } | ConstantIndex { .. } | Index(_) => {
// The rest should only occur as mplace, we do not use Immediates for types
// allowing such operations. This matches place_projection forcing an allocation.
let mplace = base.assert_mem_place();
self.mplace_projection(&mplace, proj_elem)?.into()
}
})
}

/// Converts a repr(simd) operand into an operand where `place_index` accesses the SIMD elements.
/// Also returns the number of elements.
///
/// Can (but does not always) trigger UB if `op` is uninitialized.
pub fn operand_to_simd(
&self,
base: &OpTy<'tcx, M::PointerTag>,
op: &OpTy<'tcx, M::PointerTag>,
) -> InterpResult<'tcx, (MPlaceTy<'tcx, M::PointerTag>, u64)> {
// Basically we just transmute this place into an array following simd_size_and_type.
// This only works in memory, but repr(simd) types should never be immediates anyway.
assert!(base.layout.ty.is_simd());
self.mplace_to_simd(&base.assert_mem_place())
assert!(op.layout.ty.is_simd());
match op.try_as_mplace() {
Ok(mplace) => self.mplace_to_simd(&mplace),
Err(imm) => match *imm {
Immediate::Uninit => {
throw_ub!(InvalidUninitBytes(None))
}
Immediate::Scalar(..) | Immediate::ScalarPair(..) => {
bug!("arrays/slices can never have Scalar/ScalarPair layout")
}
},
}
}

/// Read from a local. Will not actually access the local if reading from a ZST.
Expand Down Expand Up @@ -598,14 +540,19 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {

trace!("eval_place_to_op: got {:?}", *op);
// Sanity-check the type we ended up with.
debug_assert!(mir_assign_valid_types(
*self.tcx,
self.param_env,
self.layout_of(self.subst_from_current_frame_and_normalize_erasing_regions(
place.ty(&self.frame().body.local_decls, *self.tcx).ty
)?)?,
op.layout,
));
debug_assert!(
mir_assign_valid_types(
*self.tcx,
self.param_env,
self.layout_of(self.subst_from_current_frame_and_normalize_erasing_regions(
place.ty(&self.frame().body.local_decls, *self.tcx).ty
)?)?,
op.layout,
),
"eval_place of a MIR place with type {:?} produced an interpreter operand with type {:?}",
place.ty(&self.frame().body.local_decls, *self.tcx).ty,
op.layout.ty,
);
Ok(op)
}

Expand Down
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