use AllocId instead of Allocation in ConstValue::ByRef

Author: RalfJung

compiler/rustc_codegen_cranelift/src/constant.rs

@@ -200,11 +200,15 @@ pub(crate) fn codegen_const_value<'tcx>(
                 CValue::by_val(val, layout)
             }
         },
-        ConstValue::ByRef { alloc, offset } => CValue::by_ref(
-            pointer_for_allocation(fx, alloc)
-                .offset_i64(fx, i64::try_from(offset.bytes()).unwrap()),
-            layout,
-        ),
+        ConstValue::ByRef { alloc_id, offset } => {
+            let alloc = fx.tcx.global_alloc(alloc_id).unwrap_memory();
+            // FIXME: avoid creating multiple allocations for the same AllocId?
+            CValue::by_ref(
+                pointer_for_allocation(fx, alloc)
+                    .offset_i64(fx, i64::try_from(offset.bytes()).unwrap()),
+                layout,
+            )
+        }
         ConstValue::Slice { data, start, end } => {
             let ptr = pointer_for_allocation(fx, data)
                 .offset_i64(fx, i64::try_from(start).unwrap())

compiler/rustc_codegen_cranelift/src/intrinsics/simd.rs

@@ -172,7 +172,8 @@ pub(super) fn codegen_simd_intrinsic_call<'tcx>(
                     .expect("simd_shuffle idx not const");
 
                 let idx_bytes = match idx_const {
-                    ConstValue::ByRef { alloc, offset } => {
+                    ConstValue::ByRef { alloc_id, offset } => {
+                        let alloc = fx.tcx.global_alloc(alloc_id).unwrap_memory();
                         let size = Size::from_bytes(
                             4 * ret_lane_count, /* size_of([u32; ret_lane_count]) */
                         );

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -116,7 +116,9 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 let b_llval = bx.const_usize((end - start) as u64);
                 OperandValue::Pair(a_llval, b_llval)
             }
-            ConstValue::ByRef { alloc, offset } => {
+            ConstValue::ByRef { alloc_id, offset } => {
+                let alloc = bx.tcx().global_alloc(alloc_id).unwrap_memory();
+                // FIXME: should we attempt to avoid building the same AllocId multiple times?
                 return Self::from_const_alloc(bx, layout, alloc, offset);
             }
         };

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -148,10 +148,7 @@ pub(super) fn op_to_const<'tcx>(
     let to_const_value = |mplace: &MPlaceTy<'_>| {
         debug!("to_const_value(mplace: {:?})", mplace);
         match mplace.ptr().into_parts() {
-            (Some(alloc_id), offset) => {
-                let alloc = ecx.tcx.global_alloc(alloc_id).unwrap_memory();
-                ConstValue::ByRef { alloc, offset }
-            }
+            (Some(alloc_id), offset) => ConstValue::ByRef { alloc_id, offset },
             (None, offset) => {
                 assert!(mplace.layout.is_zst());
                 assert_eq!(

compiler/rustc_const_eval/src/interpret/intern.rs

@@ -24,8 +24,8 @@ use rustc_middle::ty::{self, layout::TyAndLayout, Ty};
 use rustc_ast::Mutability;
 
 use super::{
-    AllocId, Allocation, ConstAllocation, InterpCx, MPlaceTy, Machine, MemoryKind, PlaceTy,
-    Projectable, ValueVisitor,
+    AllocId, Allocation, InterpCx, MPlaceTy, Machine, MemoryKind, PlaceTy, Projectable,
+    ValueVisitor,
 };
 use crate::const_eval;
 use crate::errors::{DanglingPtrInFinal, UnsupportedUntypedPointer};
@@ -455,19 +455,23 @@ impl<'mir, 'tcx: 'mir, M: super::intern::CompileTimeMachine<'mir, 'tcx, !>>
 {
     /// A helper function that allocates memory for the layout given and gives you access to mutate
     /// it. Once your own mutation code is done, the backing `Allocation` is removed from the
-    /// current `Memory` and returned.
+    /// current `Memory` and interned as read-only into the global memory.
     pub fn intern_with_temp_alloc(
         &mut self,
         layout: TyAndLayout<'tcx>,
         f: impl FnOnce(
             &mut InterpCx<'mir, 'tcx, M>,
             &PlaceTy<'tcx, M::Provenance>,
         ) -> InterpResult<'tcx, ()>,
-    ) -> InterpResult<'tcx, ConstAllocation<'tcx>> {
+    ) -> InterpResult<'tcx, AllocId> {
+        // `allocate` picks a fresh AllocId that we will associate with its data below.
         let dest = self.allocate(layout, MemoryKind::Stack)?;
         f(self, &dest.clone().into())?;
         let mut alloc = self.memory.alloc_map.remove(&dest.ptr().provenance.unwrap()).unwrap().1;
         alloc.mutability = Mutability::Not;
-        Ok(self.tcx.mk_const_alloc(alloc))
+        let alloc = self.tcx.mk_const_alloc(alloc);
+        let alloc_id = dest.ptr().provenance.unwrap(); // this was just allocated, it must have provenance
+        self.tcx.set_alloc_id_memory(alloc_id, alloc);
+        Ok(alloc_id)
     }
 }

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -756,11 +756,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         };
         let layout = from_known_layout(self.tcx, self.param_env, layout, || self.layout_of(ty))?;
         let op = match val_val {
-            ConstValue::ByRef { alloc, offset } => {
-                let id = self.tcx.create_memory_alloc(alloc);
+            ConstValue::ByRef { alloc_id, offset } => {
                 // We rely on mutability being set correctly in that allocation to prevent writes
                 // where none should happen.
-                let ptr = self.global_base_pointer(Pointer::new(id, offset))?;
+                let ptr = self.global_base_pointer(Pointer::new(alloc_id, offset))?;
                 Operand::Indirect(MemPlace::from_ptr(ptr.into()))
             }
             ConstValue::Scalar(x) => Operand::Immediate(adjust_scalar(x)?.into()),

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

@@ -43,9 +43,13 @@ pub enum ConstValue<'tcx> {
 
     /// A value not represented/representable by `Scalar` or `Slice`
     ByRef {
-        /// The backing memory of the value, may contain more memory than needed for just the value
-        /// in order to share `ConstAllocation`s between values
-        alloc: ConstAllocation<'tcx>,
+        /// The backing memory of the value. May contain more memory than needed for just the value
+        /// if this points into some other larger ConstValue.
+        ///
+        /// We use an `AllocId` here instead of a `ConstAllocation<'tcx>` to make sure that when a
+        /// raw constant (which is basically just an `AllocId`) is turned into a `ConstValue` and
+        /// back, we can preserve the original `AllocId`.
+        alloc_id: AllocId,
         /// Offset into `alloc`
         offset: Size,
     },

compiler/rustc_middle/src/mir/mod.rs

@@ -2914,8 +2914,9 @@ fn pretty_print_const_value<'tcx>(
                     _ => {}
                 }
             }
-            (ConstValue::ByRef { alloc, offset }, ty::Array(t, n)) if *t == u8_type => {
+            (ConstValue::ByRef { alloc_id, offset }, ty::Array(t, n)) if *t == u8_type => {
                 let n = n.try_to_target_usize(tcx).unwrap();
+                let alloc = tcx.global_alloc(alloc_id).unwrap_memory();
                 // cast is ok because we already checked for pointer size (32 or 64 bit) above
                 let range = AllocRange { start: offset, size: Size::from_bytes(n) };
                 let byte_str = alloc.inner().get_bytes_strip_provenance(&tcx, range).unwrap();

compiler/rustc_middle/src/mir/pretty.rs

@@ -701,15 +701,15 @@ pub fn write_allocations<'tcx>(
     fn alloc_ids_from_const_val(val: ConstValue<'_>) -> impl Iterator<Item = AllocId> + '_ {
         match val {
             ConstValue::Scalar(interpret::Scalar::Ptr(ptr, _)) => {
-                Either::Left(Either::Left(std::iter::once(ptr.provenance)))
+                Either::Left(std::iter::once(ptr.provenance))
             }
-            ConstValue::Scalar(interpret::Scalar::Int { .. }) => {
-                Either::Left(Either::Right(std::iter::empty()))
-            }
-            ConstValue::ZeroSized => Either::Left(Either::Right(std::iter::empty())),
-            ConstValue::ByRef { alloc, .. } | ConstValue::Slice { data: alloc, .. } => {
-                Either::Right(alloc_ids_from_alloc(alloc))
+            ConstValue::Scalar(interpret::Scalar::Int { .. }) => Either::Right(std::iter::empty()),
+            ConstValue::ZeroSized => Either::Right(std::iter::empty()),
+            ConstValue::Slice { .. } => {
+                // `u8`/`str` slices, shouldn't contain pointers that we want to print.
+                Either::Right(std::iter::empty())
             }
+            ConstValue::ByRef { alloc_id, .. } => Either::Left(std::iter::once(alloc_id)),
         }
     }
     struct CollectAllocIds(BTreeSet<AllocId>);

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -510,6 +510,7 @@ TrivialTypeTraversalAndLiftImpls! {
     ::rustc_span::symbol::Ident,
     ::rustc_errors::ErrorGuaranteed,
     interpret::Scalar,
+    interpret::AllocId,
     rustc_target::abi::Size,
     ty::BoundVar,
 }