also make ConstValue::Slice use an AllocId

Author: RalfJung

compiler/rustc_codegen_cranelift/src/constant.rs

@@ -76,7 +76,7 @@ pub(crate) fn codegen_tls_ref<'tcx>(
 pub(crate) fn eval_mir_constant<'tcx>(
     fx: &FunctionCx<'_, '_, 'tcx>,
     constant: &Constant<'tcx>,
-) -> Option<(ConstValue<'tcx>, Ty<'tcx>)> {
+) -> Option<(ConstValue, Ty<'tcx>)> {
     let constant_kind = fx.monomorphize(constant.literal);
     let uv = match constant_kind {
         ConstantKind::Ty(const_) => match const_.kind() {
@@ -127,7 +127,7 @@ pub(crate) fn codegen_constant_operand<'tcx>(
 
 pub(crate) fn codegen_const_value<'tcx>(
     fx: &mut FunctionCx<'_, '_, 'tcx>,
-    const_val: ConstValue<'tcx>,
+    const_val: ConstValue,
     ty: Ty<'tcx>,
 ) -> CValue<'tcx> {
     let layout = fx.layout_of(ty);
@@ -231,7 +231,9 @@ pub(crate) fn codegen_const_value<'tcx>(
                 layout,
             )
         }
-        ConstValue::Slice { data, start, end } => {
+        ConstValue::Slice { alloc_id: Some(alloc_id), start, end } => {
+            let data = fx.tcx.global_alloc(alloc_id).unwrap_memory();
+            // FIXME: avoid creating multiple allocations for the same AllocId?
             let ptr = pointer_for_allocation(fx, data)
                 .offset_i64(fx, i64::try_from(start).unwrap())
                 .get_addr(fx);
@@ -241,6 +243,15 @@ pub(crate) fn codegen_const_value<'tcx>(
                 .iconst(fx.pointer_type, i64::try_from(end.checked_sub(start).unwrap()).unwrap());
             CValue::by_val_pair(ptr, len, layout)
         }
+        ConstValue::Slice { alloc_id: None, start, end } => {
+            assert_eq!(start, end);
+            // Empty slice at `start`.
+            // FIXME: `start` could in principle be big enough to overflow into the negative.
+            // How should the clif value be constructed then?
+            let addr = fx.bcx.ins().iconst(fx.pointer_type, i64::try_from(start).unwrap());
+            let len = fx.bcx.ins().iconst(fx.pointer_type, 0);
+            CValue::by_val_pair(addr, len, layout)
+        }
     }
 }
 
@@ -481,7 +492,7 @@ fn define_all_allocs(tcx: TyCtxt<'_>, module: &mut dyn Module, cx: &mut Constant
 pub(crate) fn mir_operand_get_const_val<'tcx>(
     fx: &FunctionCx<'_, '_, 'tcx>,
     operand: &Operand<'tcx>,
-) -> Option<ConstValue<'tcx>> {
+) -> Option<ConstValue> {
     match operand {
         Operand::Constant(const_) => Some(eval_mir_constant(fx, const_).unwrap().0),
         // FIXME(rust-lang/rust#85105): Casts like `IMM8 as u32` result in the const being stored

compiler/rustc_codegen_ssa/src/common.rs

@@ -194,7 +194,7 @@ pub fn shift_mask_val<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 pub fn asm_const_to_str<'tcx>(
     tcx: TyCtxt<'tcx>,
     sp: Span,
-    const_value: ConstValue<'tcx>,
+    const_value: ConstValue,
     ty_and_layout: TyAndLayout<'tcx>,
 ) -> String {
     let ConstValue::Scalar(scalar) = const_value else {

compiler/rustc_codegen_ssa/src/mir/constant.rs

@@ -23,7 +23,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     pub fn eval_mir_constant(
         &self,
         constant: &mir::Constant<'tcx>,
-    ) -> Result<ConstValue<'tcx>, ErrorHandled> {
+    ) -> Result<ConstValue, ErrorHandled> {
         let ct = self.monomorphize(constant.literal);
         let uv = match ct {
             mir::ConstantKind::Ty(ct) => match ct.kind() {

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -86,7 +86,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
 
     pub fn from_const<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
         bx: &mut Bx,
-        val: ConstValue<'tcx>,
+        val: ConstValue,
         ty: Ty<'tcx>,
     ) -> Self {
         let layout = bx.layout_of(ty);
@@ -100,12 +100,12 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 OperandValue::Immediate(llval)
             }
             ConstValue::ZeroSized => return OperandRef::zero_sized(layout),
-            ConstValue::Slice { data, start, end } => {
+            ConstValue::Slice { alloc_id, start, end } => {
                 let Abi::ScalarPair(a_scalar, _) = layout.abi else {
                     bug!("from_const: invalid ScalarPair layout: {:#?}", layout);
                 };
-                let a = Scalar::from_pointer(
-                    Pointer::new(bx.tcx().create_memory_alloc(data), Size::from_bytes(start)),
+                let a = Scalar::from_maybe_pointer(
+                    Pointer::new(alloc_id, Size::from_bytes(start)),
                     &bx.tcx(),
                 );
                 let a_llval = bx.scalar_to_backend(

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -18,9 +18,9 @@ use super::{CanAccessStatics, CompileTimeEvalContext, CompileTimeInterpreter};
 use crate::errors;
 use crate::interpret::eval_nullary_intrinsic;
 use crate::interpret::{
-    intern_const_alloc_recursive, Allocation, ConstAlloc, ConstValue, CtfeValidationMode, GlobalId,
-    Immediate, InternKind, InterpCx, InterpError, InterpResult, MPlaceTy, MemoryKind, OpTy,
-    RefTracking, StackPopCleanup,
+    intern_const_alloc_recursive, ConstAlloc, ConstValue, CtfeValidationMode, GlobalId, Immediate,
+    InternKind, InterpCx, InterpError, InterpResult, MPlaceTy, MemoryKind, OpTy, RefTracking,
+    StackPopCleanup,
 };
 
 // Returns a pointer to where the result lives
@@ -111,7 +111,7 @@ pub(super) fn mk_eval_cx<'mir, 'tcx>(
 pub(super) fn op_to_const<'tcx>(
     ecx: &CompileTimeEvalContext<'_, 'tcx>,
     op: &OpTy<'tcx>,
-) -> ConstValue<'tcx> {
+) -> ConstValue {
     // We do not have value optimizations for everything.
     // Only scalars and slices, since they are very common.
     // Note that further down we turn scalars of uninitialized bits back to `ByRef`. These can result
@@ -170,21 +170,11 @@ pub(super) fn op_to_const<'tcx>(
             Immediate::ScalarPair(a, b) => {
                 debug!("ScalarPair(a: {:?}, b: {:?})", a, b);
                 // We know `offset` is relative to the allocation, so we can use `into_parts`.
-                let (data, start) = match a.to_pointer(ecx).unwrap().into_parts() {
-                    (Some(alloc_id), offset) => {
-                        (ecx.tcx.global_alloc(alloc_id).unwrap_memory(), offset.bytes())
-                    }
-                    (None, _offset) => (
-                        ecx.tcx.mk_const_alloc(Allocation::from_bytes_byte_aligned_immutable(
-                            b"" as &[u8],
-                        )),
-                        0,
-                    ),
-                };
+                let (alloc_id, start) = a.to_pointer(ecx).unwrap().into_parts();
                 let len = b.to_target_usize(ecx).unwrap();
-                let start = start.try_into().unwrap();
+                let start = start.bytes().try_into().unwrap();
                 let len: usize = len.try_into().unwrap();
-                ConstValue::Slice { data, start, end: start + len }
+                ConstValue::Slice { alloc_id, start, end: start + len }
             }
             Immediate::Uninit => to_const_value(&op.assert_mem_place()),
         },
@@ -196,7 +186,7 @@ pub(crate) fn turn_into_const_value<'tcx>(
     tcx: TyCtxt<'tcx>,
     constant: ConstAlloc<'tcx>,
     key: ty::ParamEnvAnd<'tcx, GlobalId<'tcx>>,
-) -> ConstValue<'tcx> {
+) -> ConstValue {
     let cid = key.value;
     let def_id = cid.instance.def.def_id();
     let is_static = tcx.is_static(def_id);

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -22,7 +22,7 @@ pub(crate) use valtrees::{const_to_valtree_inner, valtree_to_const_value};
 pub(crate) fn const_caller_location(
     tcx: TyCtxt<'_>,
     (file, line, col): (Symbol, u32, u32),
-) -> ConstValue<'_> {
+) -> ConstValue {
     trace!("const_caller_location: {}:{}:{}", file, line, col);
     let mut ecx = mk_eval_cx(tcx, DUMMY_SP, ty::ParamEnv::reveal_all(), CanAccessStatics::No);
 
@@ -87,7 +87,7 @@ pub(crate) fn eval_to_valtree<'tcx>(
 #[instrument(skip(tcx), level = "debug")]
 pub(crate) fn try_destructure_mir_constant_for_diagnostics<'tcx>(
     tcx: TyCtxt<'tcx>,
-    val: ConstValue<'tcx>,
+    val: ConstValue,
     ty: Ty<'tcx>,
 ) -> Option<mir::DestructuredConstant<'tcx>> {
     let param_env = ty::ParamEnv::reveal_all();

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -207,7 +207,7 @@ pub fn valtree_to_const_value<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env_ty: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
     valtree: ty::ValTree<'tcx>,
-) -> ConstValue<'tcx> {
+) -> ConstValue {
     // Basic idea: We directly construct `Scalar` values from trivial `ValTree`s
     // (those for constants with type bool, int, uint, float or char).
     // For all other types we create an `MPlace` and fill that by walking

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -5,9 +5,7 @@
 use rustc_hir::def_id::DefId;
 use rustc_middle::mir::{
     self,
-    interpret::{
-        Allocation, ConstAllocation, ConstValue, GlobalId, InterpResult, PointerArithmetic, Scalar,
-    },
+    interpret::{Allocation, ConstValue, GlobalId, InterpResult, PointerArithmetic, Scalar},
     BinOp, NonDivergingIntrinsic,
 };
 use rustc_middle::ty;
@@ -43,28 +41,24 @@ fn numeric_intrinsic<Prov>(name: Symbol, bits: u128, kind: Primitive) -> Scalar<
     Scalar::from_uint(bits_out, size)
 }
 
-/// Directly returns an `Allocation` containing an absolute path representation of the given type.
-pub(crate) fn alloc_type_name<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> ConstAllocation<'tcx> {
-    let path = crate::util::type_name(tcx, ty);
-    let alloc = Allocation::from_bytes_byte_aligned_immutable(path.into_bytes());
-    tcx.mk_const_alloc(alloc)
-}
-
 /// The logic for all nullary intrinsics is implemented here. These intrinsics don't get evaluated
 /// inside an `InterpCx` and instead have their value computed directly from rustc internal info.
 pub(crate) fn eval_nullary_intrinsic<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     def_id: DefId,
     args: GenericArgsRef<'tcx>,
-) -> InterpResult<'tcx, ConstValue<'tcx>> {
+) -> InterpResult<'tcx, ConstValue> {
     let tp_ty = args.type_at(0);
     let name = tcx.item_name(def_id);
     Ok(match name {
         sym::type_name => {
             ensure_monomorphic_enough(tcx, tp_ty)?;
-            let alloc = alloc_type_name(tcx, tp_ty);
-            ConstValue::Slice { data: alloc, start: 0, end: alloc.inner().len() }
+            let path = crate::util::type_name(tcx, tp_ty);
+            let alloc = Allocation::from_bytes_byte_aligned_immutable(path.into_bytes());
+            let alloc = tcx.mk_const_alloc(alloc);
+            let alloc_id = tcx.create_memory_alloc(alloc);
+            ConstValue::Slice { alloc_id: Some(alloc_id), start: 0, end: alloc.inner().len() }
         }
         sym::needs_drop => {
             ensure_monomorphic_enough(tcx, tp_ty)?;

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -743,7 +743,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
     pub(crate) fn const_val_to_op(
         &self,
-        val_val: ConstValue<'tcx>,
+        val_val: ConstValue,
         ty: Ty<'tcx>,
         layout: Option<TyAndLayout<'tcx>>,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::Provenance>> {
@@ -764,11 +764,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
             ConstValue::Scalar(x) => Operand::Immediate(adjust_scalar(x)?.into()),
             ConstValue::ZeroSized => Operand::Immediate(Immediate::Uninit),
-            ConstValue::Slice { data, start, end } => {
+            ConstValue::Slice { alloc_id: Some(alloc_id), start, end } => {
                 // We rely on mutability being set correctly in `data` to prevent writes
                 // where none should happen.
                 let ptr = Pointer::new(
-                    self.tcx.create_memory_alloc(data),
+                    alloc_id,
                     Size::from_bytes(start), // offset: `start`
                 );
                 Operand::Immediate(Immediate::new_slice(
@@ -777,6 +777,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     self,
                 ))
             }
+            ConstValue::Slice { alloc_id: None, start, end } => {
+                assert_eq!(start, end);
+                let ptr = Pointer::from_addr_invalid(start.try_into().unwrap());
+                Operand::Immediate(Immediate::new_slice(
+                    Scalar::from_maybe_pointer(ptr, &*self.tcx),
+                    0, // len: `end - start`
+                    self,
+                ))
+            }
         };
         Ok(OpTy { op, layout, align: Some(layout.align.abi) })
     }

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -70,7 +70,7 @@ impl Into<ErrorGuaranteed> for ReportedErrorInfo {
 TrivialTypeTraversalAndLiftImpls! { ErrorHandled }
 
 pub type EvalToAllocationRawResult<'tcx> = Result<ConstAlloc<'tcx>, ErrorHandled>;
-pub type EvalToConstValueResult<'tcx> = Result<ConstValue<'tcx>, ErrorHandled>;
+pub type EvalToConstValueResult<'tcx> = Result<ConstValue, ErrorHandled>;
 pub type EvalToValTreeResult<'tcx> = Result<Option<ValTree<'tcx>>, ErrorHandled>;
 
 pub fn struct_error<'tcx>(

compiler/rustc_middle/src/mir/interpret/mod.rs

@@ -149,7 +149,7 @@ pub use self::error::{
     UnsupportedOpInfo, ValidationErrorInfo, ValidationErrorKind,
 };
 
-pub use self::value::{get_slice_bytes, ConstAlloc, ConstValue, Scalar};
+pub use self::value::{ConstAlloc, ConstValue, Scalar};
 
 pub use self::allocation::{
     alloc_range, AllocBytes, AllocError, AllocRange, AllocResult, Allocation, ConstAllocation,

compiler/rustc_middle/src/mir/interpret/value.rs

@@ -11,10 +11,7 @@ use rustc_target::abi::{HasDataLayout, Size};
 
 use crate::ty::{ParamEnv, ScalarInt, Ty, TyCtxt};
 
-use super::{
-    AllocId, AllocRange, ConstAllocation, InterpResult, Pointer, PointerArithmetic, Provenance,
-    ScalarSizeMismatch,
-};
+use super::{AllocId, InterpResult, Pointer, PointerArithmetic, Provenance, ScalarSizeMismatch};
 
 /// Represents the result of const evaluation via the `eval_to_allocation` query.
 #[derive(Copy, Clone, HashStable, TyEncodable, TyDecodable, Debug, Hash, Eq, PartialEq)]
@@ -28,8 +25,8 @@ pub struct ConstAlloc<'tcx> {
 /// Represents a constant value in Rust. `Scalar` and `Slice` are optimizations for
 /// array length computations, enum discriminants and the pattern matching logic.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, TyEncodable, TyDecodable, Hash)]
-#[derive(HashStable, Lift)]
-pub enum ConstValue<'tcx> {
+#[derive(HashStable)]
+pub enum ConstValue {
     /// Used only for types with `layout::abi::Scalar` ABI.
     ///
     /// Not using the enum `Value` to encode that this must not be `Uninit`.
@@ -41,7 +38,10 @@ pub enum ConstValue<'tcx> {
     /// Used only for `&[u8]` and `&str`.
     ///
     /// This is worth the optimization since Rust has literals of that type.
-    Slice { data: ConstAllocation<'tcx>, start: usize, end: usize },
+    /// See `Indirect` for why we are using an `AllocId`.
+    /// We allow `None` for empty slices; in that case `start == end` and `start` is the absolute
+    /// address of the slice.
+    Slice { alloc_id: Option<AllocId>, start: usize, end: usize },
 
     /// A value not represented/representable by the other variants, needs to be stored in-memory.
     Indirect {
@@ -58,9 +58,11 @@ pub enum ConstValue<'tcx> {
 }
 
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-static_assert_size!(ConstValue<'_>, 32);
+static_assert_size!(ConstValue, 32);
+
+TrivialTypeTraversalAndLiftImpls! { ConstValue }
 
-impl<'tcx> ConstValue<'tcx> {
+impl<'tcx> ConstValue {
     #[inline]
     pub fn try_to_scalar(&self) -> Option<Scalar<AllocId>> {
         match *self {
@@ -511,18 +513,3 @@ impl<'tcx, Prov: Provenance> Scalar<Prov> {
         Ok(Double::from_bits(self.to_u64()?.into()))
     }
 }
-
-/// Gets the bytes of a constant slice value.
-pub fn get_slice_bytes<'tcx>(cx: &impl HasDataLayout, val: ConstValue<'tcx>) -> &'tcx [u8] {
-    if let ConstValue::Slice { data, start, end } = val {
-        let len = end - start;
-        data.inner()
-            .get_bytes_strip_provenance(
-                cx,
-                AllocRange { start: Size::from_bytes(start), size: Size::from_bytes(len) },
-            )
-            .unwrap_or_else(|err| bug!("const slice is invalid: {:?}", err))
-    } else {
-        bug!("expected const slice, but found another const value");
-    }
-}