Automatically prefer integer addresses for zst MPlace

Author: oli-obk

src/librustc_mir/const_eval/eval_queries.rs

@@ -115,7 +115,7 @@ pub(super) fn op_to_const<'tcx>(
         // by-val is if we are in const_field, i.e., if this is (a field of) something that we
         // "tried to make immediate" before. We wouldn't do that for non-slice scalar pairs or
         // structs containing such.
-        op.try_as_mplace()
+        op.try_as_mplace(ecx)
     };
     let val = match immediate {
         Ok(mplace) => {
@@ -132,7 +132,7 @@ pub(super) fn op_to_const<'tcx>(
                 // `Immediate` is when we are called from `const_field`, and that `Immediate`
                 // comes from a constant so it can happen have `Undef`, because the indirect
                 // memory that was read had undefined bytes.
-                let mplace = op.assert_mem_place();
+                let mplace = op.assert_mem_place(ecx);
                 let ptr = mplace.ptr.assert_ptr();
                 let alloc = ecx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
                 ConstValue::ByRef { alloc, offset: ptr.offset }

src/librustc_mir/interpret/operand.rs

@@ -267,7 +267,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         &self,
         op: OpTy<'tcx, M::PointerTag>,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
-        match op.try_as_mplace() {
+        match op.try_as_mplace(self) {
             Ok(mplace) => Ok(self.force_mplace_ptr(mplace)?.into()),
             Err(imm) => Ok(imm.into()), // Nothing to cast/force
         }
@@ -335,7 +335,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         &self,
         src: OpTy<'tcx, M::PointerTag>,
     ) -> InterpResult<'tcx, Result<ImmTy<'tcx, M::PointerTag>, MPlaceTy<'tcx, M::PointerTag>>> {
-        Ok(match src.try_as_mplace() {
+        Ok(match src.try_as_mplace(self) {
             Ok(mplace) => {
                 if let Some(val) = self.try_read_immediate_from_mplace(mplace)? {
                     Ok(val)
@@ -383,7 +383,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         op: OpTy<'tcx, M::PointerTag>,
         field: u64,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
-        let base = match op.try_as_mplace() {
+        let base = match op.try_as_mplace(self) {
             Ok(mplace) => {
                 // The easy case
                 let field = self.mplace_field(mplace, field)?;
@@ -420,7 +420,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         variant: VariantIdx,
     ) -> InterpResult<'tcx, OpTy<'tcx, M::PointerTag>> {
         // Downcasts only change the layout
-        Ok(match op.try_as_mplace() {
+        Ok(match op.try_as_mplace(self) {
             Ok(mplace) => self.mplace_downcast(mplace, variant)?.into(),
             Err(..) => {
                 let layout = op.layout.for_variant(self, variant);
@@ -439,30 +439,10 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Field(field, _) => self.operand_field(base, field.index() as u64)?,
             Downcast(_, variant) => self.operand_downcast(base, variant)?,
             Deref => self.deref_operand(base)?.into(),
-            ConstantIndex { .. } | Index(_) if base.layout.is_zst() => {
-                OpTy {
-                    op: Operand::Immediate(Scalar::zst().into()),
-                    // the actual index doesn't matter, so we just pick a convenient one like 0
-                    layout: base.layout.field(self, 0)?,
-                }
-            }
-            Subslice { from, to, from_end } if base.layout.is_zst() => {
-                let elem_ty = if let ty::Array(elem_ty, _) = base.layout.ty.kind {
-                    elem_ty
-                } else {
-                    bug!("slices shouldn't be zero-sized");
-                };
-                assert!(!from_end, "arrays shouldn't be subsliced from the end");
-
-                OpTy {
-                    op: Operand::Immediate(Scalar::zst().into()),
-                    layout: self.layout_of(self.tcx.mk_array(elem_ty, (to - from) as u64))?,
-                }
-            }
             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();
+                let mplace = base.assert_mem_place(self);
                 self.mplace_projection(mplace, proj_elem)?.into()
             }
         })

src/librustc_mir/interpret/place.rs

@@ -200,16 +200,17 @@ impl<'tcx, Tag> MPlaceTy<'tcx, Tag> {
 // These are defined here because they produce a place.
 impl<'tcx, Tag: ::std::fmt::Debug + Copy> OpTy<'tcx, Tag> {
     #[inline(always)]
-    pub fn try_as_mplace(self) -> Result<MPlaceTy<'tcx, Tag>, ImmTy<'tcx, Tag>> {
+    pub fn try_as_mplace(self, cx: &impl HasDataLayout) -> Result<MPlaceTy<'tcx, Tag>, ImmTy<'tcx, Tag>> {
         match *self {
             Operand::Indirect(mplace) => Ok(MPlaceTy { mplace, layout: self.layout }),
+            Operand::Immediate(_) if self.layout.is_zst() => Ok(MPlaceTy::dangling(self.layout, cx)),
             Operand::Immediate(imm) => Err(ImmTy { imm, layout: self.layout }),
         }
     }
 
     #[inline(always)]
-    pub fn assert_mem_place(self) -> MPlaceTy<'tcx, Tag> {
-        self.try_as_mplace().unwrap()
+    pub fn assert_mem_place(self, cx: &impl HasDataLayout) -> MPlaceTy<'tcx, Tag> {
+        self.try_as_mplace(cx).unwrap()
     }
 }
 
@@ -305,7 +306,7 @@ where
     /// On success, returns `None` for zero-sized accesses (where nothing else is
     /// left to do) and a `Pointer` to use for the actual access otherwise.
     #[inline]
-    pub fn check_mplace_access(
+    pub(super) fn check_mplace_access(
         &self,
         place: MPlaceTy<'tcx, M::PointerTag>,
         size: Option<Size>,
@@ -338,7 +339,7 @@ where
 
     /// Force `place.ptr` to a `Pointer`.
     /// Can be helpful to avoid lots of `force_ptr` calls later, if this place is used a lot.
-    pub fn force_mplace_ptr(
+    pub(super) fn force_mplace_ptr(
         &self,
         mut place: MPlaceTy<'tcx, M::PointerTag>,
     ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
@@ -415,7 +416,7 @@ where
 
     // Iterates over all fields of an array. Much more efficient than doing the
     // same by repeatedly calling `mplace_array`.
-    pub fn mplace_array_fields(
+    pub(super) fn mplace_array_fields(
         &self,
         base: MPlaceTy<'tcx, Tag>,
     ) -> InterpResult<'tcx, impl Iterator<Item = InterpResult<'tcx, MPlaceTy<'tcx, Tag>>> + 'tcx>
@@ -430,7 +431,7 @@ where
         Ok((0..len).map(move |i| base.offset(i * stride, None, layout, dl)))
     }
 
-    pub fn mplace_subslice(
+    fn mplace_subslice(
         &self,
         base: MPlaceTy<'tcx, M::PointerTag>,
         from: u64,
@@ -471,7 +472,7 @@ where
         base.offset(from_offset, meta, layout, self)
     }
 
-    pub fn mplace_downcast(
+    pub(super) fn mplace_downcast(
         &self,
         base: MPlaceTy<'tcx, M::PointerTag>,
         variant: VariantIdx,
@@ -482,7 +483,7 @@ where
     }
 
     /// Project into an mplace
-    pub fn mplace_projection(
+    pub(super) fn mplace_projection(
         &self,
         base: MPlaceTy<'tcx, M::PointerTag>,
         proj_elem: &mir::PlaceElem<'tcx>,

src/librustc_mir/interpret/terminator.rs

@@ -378,7 +378,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     }
                     None => {
                         // Unsized self.
-                        args[0].assert_mem_place()
+                        args[0].assert_mem_place(self)
                     }
                 };
                 // Find and consult vtable

src/librustc_mir/interpret/validity.rs

@@ -571,12 +571,9 @@ impl<'rt, 'mir, 'tcx, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
     ) -> InterpResult<'tcx> {
         match op.layout.ty.kind {
             ty::Str => {
-                let mplace = op.assert_mem_place(); // strings are never immediate
-                try_validation!(
-                    self.ecx.read_str(mplace),
-                    "uninitialized or non-UTF-8 data in str",
-                    self.path
-                );
+                let mplace = op.assert_mem_place(self.ecx); // strings are never immediate
+                try_validation!(self.ecx.read_str(mplace),
+                    "uninitialized or non-UTF-8 data in str", self.path);
             }
             ty::Array(tys, ..) | ty::Slice(tys)
                 if {
@@ -604,7 +601,7 @@ impl<'rt, 'mir, 'tcx, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
                     return Ok(());
                 }
                 // non-ZST array cannot be immediate, slices are never immediate
-                let mplace = op.assert_mem_place();
+                let mplace = op.assert_mem_place(self.ecx);
                 // This is the length of the array/slice.
                 let len = mplace.len(self.ecx)?;
                 // zero length slices have nothing to be checked

src/librustc_mir/interpret/visitor.rs

@@ -223,7 +223,7 @@ macro_rules! make_value_visitor {
                 match v.layout().ty.kind {
                     ty::Dynamic(..) => {
                         // immediate trait objects are not a thing
-                        let dest = v.to_op(self.ecx())?.assert_mem_place();
+                        let dest = v.to_op(self.ecx())?.assert_mem_place(self.ecx());
                         let inner = self.ecx().unpack_dyn_trait(dest)?.1;
                         trace!("walk_value: dyn object layout: {:#?}", inner.layout);
                         // recurse with the inner type
@@ -292,13 +292,7 @@ macro_rules! make_value_visitor {
                     },
                     layout::FieldPlacement::Array { .. } => {
                         // Let's get an mplace first.
-                        let mplace = if v.layout().is_zst() {
-                            // it's a ZST, the memory content cannot matter
-                            MPlaceTy::dangling(v.layout(), self.ecx())
-                        } else {
-                            // non-ZST array/slice/str cannot be immediate
-                            v.to_op(self.ecx())?.assert_mem_place()
-                        };
+                        let mplace = v.to_op(self.ecx())?.assert_mem_place(self.ecx());
                         // Now we can go over all the fields.
                         let iter = self.ecx().mplace_array_fields(mplace)?
                             .map(|f| f.and_then(|f| {

src/librustc_mir/transform/const_prop.rs

@@ -707,7 +707,8 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
                 ScalarMaybeUndef::Scalar(r),
             )) => l.is_bits() && r.is_bits(),
             interpret::Operand::Indirect(_) if mir_opt_level >= 2 => {
-                intern_const_alloc_recursive(&mut self.ecx, None, op.assert_mem_place())
+                let mplace = op.assert_mem_place(&self.ecx);
+                intern_const_alloc_recursive(&mut self.ecx, None, mplace)
                     .expect("failed to intern alloc");
                 true
             }