fix Miri discriminant load/store when overflows are involved

Author: RalfJung

src/librustc_mir/interpret/operand.rs

@@ -609,15 +609,20 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     ) -> InterpResult<'tcx, (u128, VariantIdx)> {
         trace!("read_discriminant_value {:#?}", rval.layout);
 
-        let (discr_kind, discr_index) = match rval.layout.variants {
+        let (discr_layout, discr_kind, discr_index) = match rval.layout.variants {
             layout::Variants::Single { index } => {
                 let discr_val = rval.layout.ty.discriminant_for_variant(*self.tcx, index).map_or(
                     index.as_u32() as u128,
                     |discr| discr.val);
                 return Ok((discr_val, index));
             }
-            layout::Variants::Multiple { ref discr_kind, discr_index, .. } =>
-                (discr_kind, discr_index),
+            layout::Variants::Multiple {
+                discr: ref discr_layout,
+                ref discr_kind,
+                discr_index,
+                ..
+            } =>
+                (discr_layout, discr_kind, discr_index),
         };
 
         // read raw discriminant value
@@ -634,7 +639,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     .map_err(|_| err_unsup!(InvalidDiscriminant(raw_discr.erase_tag())))?;
                 let real_discr = if discr_val.layout.ty.is_signed() {
                     // going from layout tag type to typeck discriminant type
-                    // requires first sign extending with the layout discriminant
+                    // requires first sign extending with the discriminant layout
                     let sexted = sign_extend(bits_discr, discr_val.layout.size) as i128;
                     // and then zeroing with the typeck discriminant type
                     let discr_ty = rval.layout.ty
@@ -682,8 +687,31 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         (dataful_variant.as_u32() as u128, dataful_variant)
                     },
                     Ok(raw_discr) => {
-                        let adjusted_discr = raw_discr.wrapping_sub(niche_start)
-                            .wrapping_add(variants_start);
+                        // FIXME: WTF, some discriminants don't have integer type.
+                        use layout::Primitive;
+                        let discr_layout = self.layout_of(match discr_layout.value {
+                            Primitive::Int(int, signed) => int.to_ty(*self.tcx, signed),
+                            Primitive::Pointer => self.tcx.types.usize,
+                            Primitive::Float(..) => bug!("there are no float discriminants"),
+                        })?;
+                        let discr_val = ImmTy::from_uint(raw_discr, discr_layout);
+                        // We need to use machine arithmetic.
+                        let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
+                        let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);
+                        let adjusted_discr = self.binary_op(
+                            mir::BinOp::Sub,
+                            discr_val,
+                            niche_start_val,
+                        )?;
+                        let adjusted_discr = self.binary_op(
+                            mir::BinOp::Add,
+                            adjusted_discr,
+                            variants_start_val,
+                        )?;
+                        let adjusted_discr = adjusted_discr
+                            .to_scalar()?
+                            .assert_bits(discr_val.layout.size);
+                        // Check if this is in the range that indicates an actual discriminant.
                         if variants_start <= adjusted_discr && adjusted_discr <= variants_end {
                             let index = adjusted_discr as usize;
                             assert_eq!(index as u128, adjusted_discr);

src/librustc_mir/interpret/place.rs

@@ -8,7 +8,9 @@ use std::hash::Hash;
 use rustc::mir;
 use rustc::mir::interpret::truncate;
 use rustc::ty::{self, Ty};
-use rustc::ty::layout::{self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx};
+use rustc::ty::layout::{
+    self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, IntegerExt
+};
 use rustc::ty::TypeFoldable;
 
 use super::{
@@ -1027,7 +1029,7 @@ where
             }
             layout::Variants::Multiple {
                 discr_kind: layout::DiscriminantKind::Tag,
-                ref discr,
+                discr: ref discr_layout,
                 discr_index,
                 ..
             } => {
@@ -1038,7 +1040,7 @@ where
                 // raw discriminants for enums are isize or bigger during
                 // their computation, but the in-memory tag is the smallest possible
                 // representation
-                let size = discr.value.size(self);
+                let size = discr_layout.value.size(self);
                 let discr_val = truncate(discr_val, size);
 
                 let discr_dest = self.place_field(dest, discr_index as u64)?;
@@ -1050,22 +1052,40 @@ where
                     ref niche_variants,
                     niche_start,
                 },
+                discr: ref discr_layout,
                 discr_index,
                 ..
             } => {
                 assert!(
                     variant_index.as_usize() < dest.layout.ty.ty_adt_def().unwrap().variants.len(),
                 );
                 if variant_index != dataful_variant {
-                    let niche_dest =
-                        self.place_field(dest, discr_index as u64)?;
-                    let niche_value = variant_index.as_u32() - niche_variants.start().as_u32();
-                    let niche_value = (niche_value as u128)
-                        .wrapping_add(niche_start);
-                    self.write_scalar(
-                        Scalar::from_uint(niche_value, niche_dest.layout.size),
-                        niche_dest
+                    // FIXME: WTF, some discriminants don't have integer type.
+                    use layout::Primitive;
+                    let discr_layout = self.layout_of(match discr_layout.value {
+                        Primitive::Int(int, signed) => int.to_ty(*self.tcx, signed),
+                        Primitive::Pointer => self.tcx.types.usize,
+                        Primitive::Float(..) => bug!("there are no float discriminants"),
+                    })?;
+
+                    // We need to use machine arithmetic.
+                    let variants_start = niche_variants.start().as_u32();
+                    let variants_start_val = ImmTy::from_uint(variants_start, discr_layout);
+                    let niche_start_val = ImmTy::from_uint(niche_start, discr_layout);
+                    let variant_index_val = ImmTy::from_uint(variant_index.as_u32(), discr_layout);
+                    let niche_val = self.binary_op(
+                        mir::BinOp::Sub,
+                        variant_index_val,
+                        variants_start_val,
+                    )?;
+                    let niche_val = self.binary_op(
+                        mir::BinOp::Add,
+                        niche_val,
+                        niche_start_val,
                     )?;
+                    // Write result.
+                    let niche_dest = self.place_field(dest, discr_index as u64)?;
+                    self.write_immediate(*niche_val, niche_dest)?;
                 }
             }
         }