rustc_trans: move const & lvalue access helpers from adt.

6 files changed, 251 insertions, 329 deletions

diff --git a/src/librustc_trans/adt.rs b/src/librustc_trans/adt.rs
index b06f8e4e671..cdf66a0835d 100644
--- a/src/librustc_trans/adt.rs
+++ b/src/librustc_trans/adt.rs
@@ -41,52 +41,15 @@
 //!   used unboxed and any field can have pointers (including mutable)
 //!   taken to it, implementing them for Rust seems difficult.
 
-use std;
-
-use llvm::{ValueRef, True, IntEQ, IntNE};
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{self, LayoutTyper};
-use common::*;
-use builder::Builder;
-use base;
+
+use context::CrateContext;
 use machine;
 use monomorphize;
 use type_::Type;
 use type_of;
 
-use mir::lvalue::Alignment;
-
-/// Given an enum, struct, closure, or tuple, extracts fields.
-/// Treats closures as a struct with one variant.
-/// `empty_if_no_variants` is a switch to deal with empty enums.
-/// If true, `variant_index` is disregarded and an empty Vec returned in this case.
-pub fn compute_fields<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>, t: Ty<'tcx>,
-                                variant_index: usize,
-                                empty_if_no_variants: bool) -> Vec<Ty<'tcx>> {
-    match t.sty {
-        ty::TyAdt(ref def, _) if def.variants.len() == 0 && empty_if_no_variants => {
-            Vec::default()
-        },
-        ty::TyAdt(ref def, ref substs) => {
-            def.variants[variant_index].fields.iter().map(|f| {
-                monomorphize::field_ty(cx.tcx(), substs, f)
-            }).collect::<Vec<_>>()
-        },
-        ty::TyTuple(fields, _) => fields.to_vec(),
-        ty::TyClosure(def_id, substs) => {
-            if variant_index > 0 { bug!("{} is a closure, which only has one variant", t);}
-            substs.upvar_tys(def_id, cx.tcx()).collect()
-        },
-        ty::TyGenerator(def_id, substs, _) => {
-            if variant_index > 0 { bug!("{} is a generator, which only has one variant", t);}
-            substs.field_tys(def_id, cx.tcx()).map(|t| {
-                cx.tcx().fully_normalize_associated_types_in(&t)
-            }).collect()
-        },
-        _ => bug!("{} is not a type that can have fields.", t)
-    }
-}
-
 /// LLVM-level types are a little complicated.
 ///
 /// C-like enums need to be actual ints, not wrapped in a struct,
@@ -119,8 +82,8 @@ pub fn finish_type_of<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
                     (nndiscr, nonnull, nonnull.packed),
                 _ => unreachable!()
             };
-            let fields = compute_fields(cx, t, nonnull_variant_index as usize, true);
-            llty.set_struct_body(&struct_llfields(cx, &fields, nonnull_variant),
+            llty.set_struct_body(&struct_llfields(cx, t, nonnull_variant_index as usize,
+                                                  nonnull_variant, None),
                                  packed)
         },
         _ => bug!("This function cannot handle {} with layout {:#?}", t, l)
@@ -148,10 +111,9 @@ fn generic_type_of<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
             }
         }
         layout::StructWrappedNullablePointer { nndiscr, ref nonnull, .. } => {
-            let fields = compute_fields(cx, t, nndiscr as usize, false);
             match name {
                 None => {
-                    Type::struct_(cx, &struct_llfields(cx, &fields, nonnull),
+                    Type::struct_(cx, &struct_llfields(cx, t, nndiscr as usize, nonnull, None),
                                   nonnull.packed)
                 }
                 Some(name) => {
@@ -160,17 +122,12 @@ fn generic_type_of<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
             }
         }
         layout::Univariant { ref variant, .. } => {
-            // Note that this case also handles empty enums.
-            // Thus the true as the final parameter here.
-            let fields = compute_fields(cx, t, 0, true);
             match name {
                 None => {
-                    let fields = struct_llfields(cx, &fields, &variant);
-                    Type::struct_(cx, &fields, variant.packed)
+                    Type::struct_(cx, &struct_llfields(cx, t, 0, &variant, None),
+                                  variant.packed)
                 }
                 Some(name) => {
-                    // Hypothesis: named_struct's can never need a
-                    // drop flag. (... needs validation.)
                     Type::named_struct(cx, name)
                 }
             }
@@ -205,7 +162,7 @@ fn generic_type_of<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
             let size = size.bytes();
             let align = align.abi();
             let primitive_align = primitive_align.abi();
-            assert!(align <= std::u32::MAX as u64);
+            assert!(align <= ::std::u32::MAX as u64);
             let discr_ty = Type::from_integer(cx, discr);
             let discr_size = discr.size().bytes();
             let padded_discr_size = roundup(discr_size, align as u32);
@@ -246,35 +203,63 @@ fn union_fill(cx: &CrateContext, size: u64, align: u64) -> Type {
     }
 }
 
-
-// Double index to account for padding (FieldPath already uses `Struct::memory_index`)
-fn struct_llfields_path(discrfield: &layout::FieldPath) -> Vec<usize> {
-    discrfield.iter().map(|&i| (i as usize) << 1).collect::<Vec<_>>()
-}
-
-
 // Lookup `Struct::memory_index` and double it to account for padding
 pub fn struct_llfields_index(variant: &layout::Struct, index: usize) -> usize {
     (variant.memory_index[index] as usize) << 1
 }
 
-
-pub fn struct_llfields<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>, field_tys: &Vec<Ty<'tcx>>,
-                             variant: &layout::Struct) -> Vec<Type> {
+pub fn struct_llfields<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>,
+                                 t: Ty<'tcx>,
+                                 variant_index: usize,
+                                 variant: &layout::Struct,
+                                 discr: Option<Ty<'tcx>>) -> Vec<Type> {
+    let field_count = match t.sty {
+        ty::TyAdt(ref def, _) if def.variants.len() == 0 => return vec![],
+        ty::TyAdt(ref def, _) => {
+            discr.is_some() as usize + def.variants[variant_index].fields.len()
+        },
+        ty::TyTuple(fields, _) => fields.len(),
+        ty::TyClosure(def_id, substs) => {
+            if variant_index > 0 { bug!("{} is a closure, which only has one variant", t);}
+            substs.upvar_tys(def_id, cx.tcx()).count()
+        },
+        ty::TyGenerator(def_id, substs, _) => {
+            if variant_index > 0 { bug!("{} is a generator, which only has one variant", t);}
+            substs.field_tys(def_id, cx.tcx()).count()
+        },
+        _ => bug!("{} is not a type that can have fields.", t)
+    };
     debug!("struct_llfields: variant: {:?}", variant);
     let mut first_field = true;
     let mut min_offset = 0;
-    let mut result: Vec<Type> = Vec::with_capacity(field_tys.len() * 2);
+    let mut result: Vec<Type> = Vec::with_capacity(field_count * 2);
     let field_iter = variant.field_index_by_increasing_offset().map(|i| {
-        (i, field_tys[i as usize], variant.offsets[i as usize].bytes()) });
+        (i, match t.sty {
+            ty::TyAdt(..) if i == 0 && discr.is_some() => discr.unwrap(),
+            ty::TyAdt(ref def, ref substs) => {
+                monomorphize::field_ty(cx.tcx(), substs,
+                    &def.variants[variant_index].fields[i as usize - discr.is_some() as usize])
+            },
+            ty::TyTuple(fields, _) => fields[i as usize],
+            ty::TyClosure(def_id, substs) => {
+                substs.upvar_tys(def_id, cx.tcx()).nth(i).unwrap()
+            },
+            ty::TyGenerator(def_id, substs, _) => {
+                let ty = substs.field_tys(def_id, cx.tcx()).nth(i).unwrap();
+                cx.tcx().normalize_associated_type(&ty)
+            },
+            _ => bug!()
+        }, variant.offsets[i as usize].bytes())
+    });
     for (index, ty, target_offset) in field_iter {
+        assert!(target_offset >= min_offset);
+        let padding_bytes = target_offset - min_offset;
         if first_field {
             debug!("struct_llfields: {} ty: {} min_offset: {} target_offset: {}",
                 index, ty, min_offset, target_offset);
+            assert_eq!(padding_bytes, 0);
             first_field = false;
         } else {
-            assert!(target_offset >= min_offset);
-            let padding_bytes = if variant.packed { 0 } else { target_offset - min_offset };
             result.push(Type::array(&Type::i8(cx), padding_bytes));
             debug!("struct_llfields: {} ty: {} pad_bytes: {} min_offset: {} target_offset: {}",
                 index, ty, padding_bytes, min_offset, target_offset);
@@ -282,10 +267,18 @@ pub fn struct_llfields<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>, field_tys: &Vec<Ty
         let llty = type_of::in_memory_type_of(cx, ty);
         result.push(llty);
         let layout = cx.layout_of(ty);
+        if variant.packed {
+            assert_eq!(padding_bytes, 0);
+        } else {
+            let field_align = layout.align(cx);
+            assert!(field_align.abi() <= variant.align.abi(),
+                    "non-packed type has field with larger align ({}): {:#?}",
+                    field_align.abi(), variant);
+        }
         let target_size = layout.size(&cx.tcx().data_layout).bytes();
         min_offset = target_offset + target_size;
     }
-    if variant.sized && !field_tys.is_empty() {
+    if variant.sized && field_count > 0 {
         if variant.stride().bytes() < min_offset {
             bug!("variant: {:?} stride: {} min_offset: {}", variant, variant.stride().bytes(),
             min_offset);
@@ -294,7 +287,7 @@ pub fn struct_llfields<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>, field_tys: &Vec<Ty
         debug!("struct_llfields: pad_bytes: {} min_offset: {} min_size: {} stride: {}\n",
                padding_bytes, min_offset, variant.min_size.bytes(), variant.stride().bytes());
         result.push(Type::array(&Type::i8(cx), padding_bytes));
-        assert!(result.len() == (field_tys.len() * 2));
+        assert!(result.len() == (field_count * 2));
     } else {
         debug!("struct_llfields: min_offset: {} min_size: {} stride: {}\n",
                min_offset, variant.min_size.bytes(), variant.stride().bytes());
@@ -310,138 +303,6 @@ pub fn is_discr_signed<'tcx>(l: &layout::Layout) -> bool {
     }
 }
 
-/// Obtain the actual discriminant of a value.
-pub fn trans_get_discr<'a, 'tcx>(
-    bcx: &Builder<'a, 'tcx>,
-    t: Ty<'tcx>,
-    scrutinee: ValueRef,
-    alignment: Alignment,
-    cast_to: Option<Type>,
-    range_assert: bool
-) -> ValueRef {
-    debug!("trans_get_discr t: {:?}", t);
-    let l = bcx.ccx.layout_of(t);
-
-    let val = match *l {
-        layout::CEnum { discr, min, max, .. } => {
-            load_discr(bcx, discr, scrutinee, alignment, min, max, range_assert)
-        }
-        layout::General { discr, ref variants, .. } => {
-            let ptr = bcx.struct_gep(scrutinee, 0);
-            load_discr(bcx, discr, ptr, alignment,
-                       0, variants.len() as u64 - 1,
-                       range_assert)
-        }
-        layout::Univariant { .. } | layout::UntaggedUnion { .. } => C_u8(bcx.ccx, 0),
-        layout::RawNullablePointer { nndiscr, .. } => {
-            let cmp = if nndiscr == 0 { IntEQ } else { IntNE };
-            let discr = bcx.load(scrutinee, alignment.to_align());
-            bcx.icmp(cmp, discr, C_null(val_ty(discr)))
-        }
-        layout::StructWrappedNullablePointer { nndiscr, ref discrfield, .. } => {
-            struct_wrapped_nullable_bitdiscr(bcx, nndiscr, discrfield, scrutinee, alignment)
-        },
-        _ => bug!("{} is not an enum", t)
-    };
-    match cast_to {
-        None => val,
-        Some(llty) => bcx.intcast(val, llty, is_discr_signed(&l))
-    }
-}
-
-fn struct_wrapped_nullable_bitdiscr(
-    bcx: &Builder,
-    nndiscr: u64,
-    discrfield: &layout::FieldPath,
-    scrutinee: ValueRef,
-    alignment: Alignment,
-) -> ValueRef {
-    let path = struct_llfields_path(discrfield);
-    let llptrptr = bcx.gepi(scrutinee, &path);
-    let llptr = bcx.load(llptrptr, alignment.to_align());
-    let cmp = if nndiscr == 0 { IntEQ } else { IntNE };
-    bcx.icmp(cmp, llptr, C_null(val_ty(llptr)))
-}
-
-/// Helper for cases where the discriminant is simply loaded.
-fn load_discr(bcx: &Builder, ity: layout::Integer, ptr: ValueRef,
-              alignment: Alignment, min: u64, max: u64,
-              range_assert: bool)
-    -> ValueRef {
-    let llty = Type::from_integer(bcx.ccx, ity);
-    assert_eq!(val_ty(ptr), llty.ptr_to());
-    let bits = ity.size().bits();
-    assert!(bits <= 64);
-    let bits = bits as usize;
-    let mask = !0u64 >> (64 - bits);
-    // For a (max) discr of -1, max will be `-1 as usize`, which overflows.
-    // However, that is fine here (it would still represent the full range),
-    if max.wrapping_add(1) & mask == min & mask || !range_assert {
-        // i.e., if the range is everything.  The lo==hi case would be
-        // rejected by the LLVM verifier (it would mean either an
-        // empty set, which is impossible, or the entire range of the
-        // type, which is pointless).
-        bcx.load(ptr, alignment.to_align())
-    } else {
-        // llvm::ConstantRange can deal with ranges that wrap around,
-        // so an overflow on (max + 1) is fine.
-        bcx.load_range_assert(ptr, min, max.wrapping_add(1), /* signed: */ True,
-                              alignment.to_align())
-    }
-}
-
-/// Set the discriminant for a new value of the given case of the given
-/// representation.
-pub fn trans_set_discr<'a, 'tcx>(bcx: &Builder<'a, 'tcx>, t: Ty<'tcx>, val: ValueRef, to: u64) {
-    let l = bcx.ccx.layout_of(t);
-    match *l {
-        layout::CEnum{ discr, min, max, .. } => {
-            assert_discr_in_range(min, max, to);
-            bcx.store(C_int(Type::from_integer(bcx.ccx, discr), to as i64),
-                  val, None);
-        }
-        layout::General{ discr, .. } => {
-            bcx.store(C_int(Type::from_integer(bcx.ccx, discr), to as i64),
-                  bcx.struct_gep(val, 0), None);
-        }
-        layout::Univariant { .. }
-        | layout::UntaggedUnion { .. }
-        | layout::Vector { .. } => {
-            assert_eq!(to, 0);
-        }
-        layout::RawNullablePointer { nndiscr, .. } => {
-            if to != nndiscr {
-                let llptrty = val_ty(val).element_type();
-                bcx.store(C_null(llptrty), val, None);
-            }
-        }
-        layout::StructWrappedNullablePointer { nndiscr, ref discrfield, ref nonnull, .. } => {
-            if to != nndiscr {
-                if target_sets_discr_via_memset(bcx) {
-                    // Issue #34427: As workaround for LLVM bug on
-                    // ARM, use memset of 0 on whole struct rather
-                    // than storing null to single target field.
-                    let llptr = bcx.pointercast(val, Type::i8(bcx.ccx).ptr_to());
-                    let fill_byte = C_u8(bcx.ccx, 0);
-                    let size = C_usize(bcx.ccx, nonnull.stride().bytes());
-                    let align = C_i32(bcx.ccx, nonnull.align.abi() as i32);
-                    base::call_memset(bcx, llptr, fill_byte, size, align, false);
-                } else {
-                    let path = struct_llfields_path(discrfield);
-                    let llptrptr = bcx.gepi(val, &path);
-                    let llptrty = val_ty(llptrptr).element_type();
-                    bcx.store(C_null(llptrty), llptrptr, None);
-                }
-            }
-        }
-        _ => bug!("Cannot handle {} represented as {:#?}", t, l)
-    }
-}
-
-fn target_sets_discr_via_memset<'a, 'tcx>(bcx: &Builder<'a, 'tcx>) -> bool {
-    bcx.sess().target.target.arch == "arm" || bcx.sess().target.target.arch == "aarch64"
-}
-
 pub fn assert_discr_in_range<D: PartialOrd>(min: D, max: D, discr: D) {
     if min <= max {
         assert!(min <= discr && discr <= max)
@@ -453,45 +314,3 @@ pub fn assert_discr_in_range<D: PartialOrd>(min: D, max: D, discr: D) {
 // FIXME this utility routine should be somewhere more general
 #[inline]
 fn roundup(x: u64, a: u32) -> u64 { let a = a as u64; ((x + (a - 1)) / a) * a }
-
-/// Extract a field of a constant value, as appropriate for its
-/// representation.
-///
-/// (Not to be confused with `common::const_get_elt`, which operates on
-/// raw LLVM-level structs and arrays.)
-pub fn const_get_field<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, t: Ty<'tcx>,
-                       val: ValueRef,
-                       ix: usize) -> ValueRef {
-    let l = ccx.layout_of(t);
-    match *l {
-        layout::CEnum { .. } => bug!("element access in C-like enum const"),
-        layout::Univariant { ref variant, .. } => {
-            const_struct_field(val, variant.memory_index[ix] as usize)
-        }
-        layout::Vector { .. } => const_struct_field(val, ix),
-        layout::UntaggedUnion { .. } => const_struct_field(val, 0),
-        _ => bug!("{} does not have fields.", t)
-    }
-}
-
-/// Extract field of struct-like const, skipping our alignment padding.
-fn const_struct_field(val: ValueRef, ix: usize) -> ValueRef {
-    // Get the ix-th non-undef element of the struct.
-    let mut real_ix = 0; // actual position in the struct
-    let mut ix = ix; // logical index relative to real_ix
-    let mut field;
-    loop {
-        loop {
-            field = const_get_elt(val, &[real_ix]);
-            if !is_undef(field) {
-                break;
-            }
-            real_ix = real_ix + 1;
-        }
-        if ix == 0 {
-            return field;
-        }
-        ix = ix - 1;
-        real_ix = real_ix + 1;
-    }
-}
diff --git a/src/librustc_trans/intrinsic.rs b/src/librustc_trans/intrinsic.rs
index 2f1a95038ea..daeb0dd680f 100644
--- a/src/librustc_trans/intrinsic.rs
+++ b/src/librustc_trans/intrinsic.rs
@@ -15,7 +15,6 @@ use libc;
 use llvm;
 use llvm::{ValueRef};
 use abi::{Abi, FnType};
-use adt;
 use mir::lvalue::{LvalueRef, Alignment};
 use base::*;
 use common::*;
@@ -379,10 +378,10 @@ pub fn trans_intrinsic_call<'a, 'tcx>(bcx: &Builder<'a, 'tcx>,
 
         "discriminant_value" => {
             let val_ty = substs.type_at(0);
+            let adt_val = LvalueRef::new_sized_ty(llargs[0], val_ty, Alignment::AbiAligned);
             match val_ty.sty {
                 ty::TyAdt(adt, ..) if adt.is_enum() => {
-                    adt::trans_get_discr(bcx, val_ty, llargs[0], Alignment::AbiAligned,
-                                         Some(llret_ty), true)
+                    adt_val.trans_get_discr(bcx, ret_ty)
                 }
                 _ => C_null(llret_ty)
             }
diff --git a/src/librustc_trans/mir/constant.rs b/src/librustc_trans/mir/constant.rs
index 6573e507bd3..67fdc1e640a 100644
--- a/src/librustc_trans/mir/constant.rs
+++ b/src/librustc_trans/mir/constant.rs
@@ -462,8 +462,32 @@ impl<'a, 'tcx> MirConstContext<'a, 'tcx> {
                         }
                     }
                     mir::ProjectionElem::Field(ref field, _) => {
-                        let llprojected = adt::const_get_field(self.ccx, tr_base.ty, base.llval,
-                                                               field.index());
+                        // Extract field of struct-like const, skipping our alignment padding.
+                        let mut ix = field.index();
+                        let layout = self.ccx.layout_of(tr_base.ty);
+                        if let layout::Univariant { ref variant, .. } = *layout {
+                            ix = variant.memory_index[ix] as usize;
+                        }
+
+                        // Get the ix-th non-undef element of the struct.
+                        let mut real_ix = 0; // actual position in the struct
+                        let mut ix = ix; // logical index relative to real_ix
+                        let mut llprojected;
+                        loop {
+                            loop {
+                                llprojected = const_get_elt(base.llval, &[real_ix]);
+                                if !is_undef(llprojected) {
+                                    break;
+                                }
+                                real_ix = real_ix + 1;
+                            }
+                            if ix == 0 {
+                                break;
+                            }
+                            ix = ix - 1;
+                            real_ix = real_ix + 1;
+                        }
+
                         let llextra = if !has_metadata {
                             ptr::null_mut()
                         } else {
diff --git a/src/librustc_trans/mir/lvalue.rs b/src/librustc_trans/mir/lvalue.rs
index d939acaccd9..5faaef6ebff 100644
--- a/src/librustc_trans/mir/lvalue.rs
+++ b/src/librustc_trans/mir/lvalue.rs
@@ -8,15 +8,16 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use llvm::ValueRef;
+use llvm::{self, ValueRef};
 use rustc::ty::{self, Ty, TypeFoldable};
 use rustc::ty::layout::{self, LayoutTyper};
 use rustc::mir;
 use rustc::mir::tcx::LvalueTy;
 use rustc_data_structures::indexed_vec::Idx;
 use adt;
+use base;
 use builder::Builder;
-use common::{self, CrateContext, C_usize};
+use common::{self, CrateContext, C_usize, C_u8, C_i32, C_int, C_null, val_ty};
 use consts;
 use machine;
 use type_of;
@@ -70,6 +71,10 @@ impl Alignment {
     }
 }
 
+fn target_sets_discr_via_memset<'a, 'tcx>(bcx: &Builder<'a, 'tcx>) -> bool {
+    bcx.sess().target.target.arch == "arm" || bcx.sess().target.target.arch == "aarch64"
+}
+
 #[derive(Copy, Clone, Debug)]
 pub struct LvalueRef<'tcx> {
     /// Pointer to the contents of the lvalue
@@ -121,23 +126,56 @@ impl<'a, 'tcx> LvalueRef<'tcx> {
         !self.llextra.is_null()
     }
 
-    fn struct_field_ptr(
-        self,
-        bcx: &Builder<'a, 'tcx>,
-        st: &layout::Struct,
-        fields: &Vec<Ty<'tcx>>,
-        ix: usize,
-        needs_cast: bool
-    ) -> (ValueRef, Alignment) {
-        let fty = fields[ix];
+    /// Access a field, at a point when the value's case is known.
+    pub fn trans_field_ptr(self, bcx: &Builder<'a, 'tcx>, ix: usize) -> (ValueRef, Alignment) {
         let ccx = bcx.ccx;
+        let mut l = ccx.layout_of(self.ty.to_ty(bcx.tcx()));
+        match self.ty {
+            LvalueTy::Ty { .. } => {}
+            LvalueTy::Downcast { variant_index, .. } => {
+                l = l.for_variant(variant_index)
+            }
+        }
+        let fty = l.field(ccx, ix).ty;
+        let mut ix = ix;
+        let st = match *l {
+            layout::Vector { .. } => {
+                return (bcx.struct_gep(self.llval, ix), self.alignment);
+            }
+            layout::UntaggedUnion { ref variants } => {
+                let ty = type_of::in_memory_type_of(ccx, fty);
+                return (bcx.pointercast(self.llval, ty.ptr_to()),
+                    self.alignment | Alignment::from_packed(variants.packed));
+            }
+            layout::RawNullablePointer { nndiscr, .. } |
+            layout::StructWrappedNullablePointer { nndiscr,  .. }
+                if l.variant_index.unwrap() as u64 != nndiscr => {
+                // The unit-like case might have a nonzero number of unit-like fields.
+                // (e.d., Result of Either with (), as one side.)
+                let ty = type_of::type_of(ccx, fty);
+                assert_eq!(machine::llsize_of_alloc(ccx, ty), 0);
+                return (bcx.pointercast(self.llval, ty.ptr_to()), Alignment::Packed);
+            }
+            layout::RawNullablePointer { .. } => {
+                let ty = type_of::type_of(ccx, fty);
+                return (bcx.pointercast(self.llval, ty.ptr_to()), self.alignment);
+            }
+            layout::Univariant { ref variant, .. } => variant,
+            layout::StructWrappedNullablePointer { ref nonnull, .. } => nonnull,
+            layout::General { ref variants, .. } => {
+                ix += 1;
+                &variants[l.variant_index.unwrap()]
+            }
+            _ => bug!("element access in type without elements: {} represented as {:#?}", l.ty, l)
+        };
 
         let alignment = self.alignment | Alignment::from_packed(st.packed);
 
-        let llfields = adt::struct_llfields(ccx, fields, st);
-        let ptr_val = if needs_cast {
-            let real_ty = Type::struct_(ccx, &llfields[..], st.packed);
-            bcx.pointercast(self.llval, real_ty.ptr_to())
+        let ptr_val = if let layout::General { discr, .. } = *l {
+            let variant_ty = Type::struct_(ccx,
+                &adt::struct_llfields(ccx, l.ty, l.variant_index.unwrap(), st,
+                                      Some(discr.to_ty(&bcx.tcx(), false))), st.packed);
+            bcx.pointercast(self.llval, variant_ty.ptr_to())
         } else {
             self.llval
         };
@@ -147,7 +185,7 @@ impl<'a, 'tcx> LvalueRef<'tcx> {
         //   * Packed struct - There is no alignment padding
         //   * Field is sized - pointer is properly aligned already
         if st.offsets[ix] == layout::Size::from_bytes(0) || st.packed ||
-            bcx.ccx.shared().type_is_sized(fty)
+            ccx.shared().type_is_sized(fty)
         {
             return (bcx.struct_gep(
                     ptr_val, adt::struct_llfields_index(st, ix)), alignment);
@@ -189,7 +227,7 @@ impl<'a, 'tcx> LvalueRef<'tcx> {
 
 
         let offset = st.offsets[ix].bytes();
-        let unaligned_offset = C_usize(bcx.ccx, offset);
+        let unaligned_offset = C_usize(ccx, offset);
 
         // Get the alignment of the field
         let (_, align) = glue::size_and_align_of_dst(bcx, fty, meta);
@@ -200,77 +238,130 @@ impl<'a, 'tcx> LvalueRef<'tcx> {
         //   (unaligned offset + (align - 1)) & -align
 
         // Calculate offset
-        let align_sub_1 = bcx.sub(align, C_usize(bcx.ccx, 1));
+        let align_sub_1 = bcx.sub(align, C_usize(ccx, 1u64));
         let offset = bcx.and(bcx.add(unaligned_offset, align_sub_1),
         bcx.neg(align));
 
         debug!("struct_field_ptr: DST field offset: {:?}", Value(offset));
 
         // Cast and adjust pointer
-        let byte_ptr = bcx.pointercast(ptr_val, Type::i8p(bcx.ccx));
+        let byte_ptr = bcx.pointercast(ptr_val, Type::i8p(ccx));
         let byte_ptr = bcx.gep(byte_ptr, &[offset]);
 
         // Finally, cast back to the type expected
-        let ll_fty = type_of::in_memory_type_of(bcx.ccx, fty);
+        let ll_fty = type_of::in_memory_type_of(ccx, fty);
         debug!("struct_field_ptr: Field type is {:?}", ll_fty);
         (bcx.pointercast(byte_ptr, ll_fty.ptr_to()), alignment)
     }
 
-    /// Access a field, at a point when the value's case is known.
-    pub fn trans_field_ptr(self, bcx: &Builder<'a, 'tcx>, ix: usize) -> (ValueRef, Alignment) {
-        let discr = match self.ty {
-            LvalueTy::Ty { .. } => 0,
-            LvalueTy::Downcast { variant_index, .. } => variant_index,
-        };
-        let t = self.ty.to_ty(bcx.tcx());
-        let l = bcx.ccx.layout_of(t);
-        // Note: if this ever needs to generate conditionals (e.g., if we
-        // decide to do some kind of cdr-coding-like non-unique repr
-        // someday), it will need to return a possibly-new bcx as well.
-        match *l {
-            layout::Univariant { ref variant, .. } => {
-                assert_eq!(discr, 0);
-                self.struct_field_ptr(bcx, &variant,
-                    &adt::compute_fields(bcx.ccx, t, 0, false), ix, false)
+    // Double index to account for padding (FieldPath already uses `Struct::memory_index`)
+    fn gepi_struct_llfields_path(self, bcx: &Builder, discrfield: &layout::FieldPath) -> ValueRef {
+        let path = discrfield.iter().map(|&i| (i as usize) << 1).collect::<Vec<_>>();
+        bcx.gepi(self.llval, &path)
+    }
+
+    /// Helper for cases where the discriminant is simply loaded.
+    fn load_discr(self, bcx: &Builder, ity: layout::Integer, ptr: ValueRef,
+                  min: u64, max: u64) -> ValueRef {
+        let llty = Type::from_integer(bcx.ccx, ity);
+        assert_eq!(val_ty(ptr), llty.ptr_to());
+        let bits = ity.size().bits();
+        assert!(bits <= 64);
+        let bits = bits as usize;
+        let mask = !0u64 >> (64 - bits);
+        // For a (max) discr of -1, max will be `-1 as usize`, which overflows.
+        // However, that is fine here (it would still represent the full range),
+        if max.wrapping_add(1) & mask == min & mask {
+            // i.e., if the range is everything.  The lo==hi case would be
+            // rejected by the LLVM verifier (it would mean either an
+            // empty set, which is impossible, or the entire range of the
+            // type, which is pointless).
+            bcx.load(ptr, self.alignment.to_align())
+        } else {
+            // llvm::ConstantRange can deal with ranges that wrap around,
+            // so an overflow on (max + 1) is fine.
+            bcx.load_range_assert(ptr, min, max.wrapping_add(1), /* signed: */ llvm::True,
+                                  self.alignment.to_align())
+        }
+    }
+
+    /// Obtain the actual discriminant of a value.
+    pub fn trans_get_discr(self, bcx: &Builder<'a, 'tcx>, cast_to: Ty<'tcx>) -> ValueRef {
+        let l = bcx.ccx.layout_of(self.ty.to_ty(bcx.tcx()));
+
+        let val = match *l {
+            layout::CEnum { discr, min, max, .. } => {
+                self.load_discr(bcx, discr, self.llval, min, max)
             }
-            layout::Vector { count, .. } => {
-                assert_eq!(discr, 0);
-                assert!((ix as u64) < count);
-                (bcx.struct_gep(self.llval, ix), self.alignment)
+            layout::General { discr, ref variants, .. } => {
+                let ptr = bcx.struct_gep(self.llval, 0);
+                self.load_discr(bcx, discr, ptr, 0, variants.len() as u64 - 1)
             }
-            layout::General { discr: d, ref variants, .. } => {
-                let mut fields = adt::compute_fields(bcx.ccx, t, discr, false);
-                fields.insert(0, d.to_ty(&bcx.tcx(), false));
-                self.struct_field_ptr(bcx, &variants[discr], &fields, ix + 1, true)
+            layout::Univariant { .. } | layout::UntaggedUnion { .. } => C_u8(bcx.ccx, 0),
+            layout::RawNullablePointer { nndiscr, .. } => {
+                let cmp = if nndiscr == 0 { llvm::IntEQ } else { llvm::IntNE };
+                let discr = bcx.load(self.llval, self.alignment.to_align());
+                bcx.icmp(cmp, discr, C_null(val_ty(discr)))
             }
-            layout::UntaggedUnion { ref variants } => {
-                let fields = adt::compute_fields(bcx.ccx, t, 0, false);
-                let ty = type_of::in_memory_type_of(bcx.ccx, fields[ix]);
-                (bcx.pointercast(self.llval, ty.ptr_to()),
-                 self.alignment | Alignment::from_packed(variants.packed))
+            layout::StructWrappedNullablePointer { nndiscr, ref discrfield, .. } => {
+                let llptrptr = self.gepi_struct_llfields_path(bcx, discrfield);
+                let llptr = bcx.load(llptrptr, self.alignment.to_align());
+                let cmp = if nndiscr == 0 { llvm::IntEQ } else { llvm::IntNE };
+                bcx.icmp(cmp, llptr, C_null(val_ty(llptr)))
+            },
+            _ => bug!("{} is not an enum", l.ty)
+        };
+        let cast_to = type_of::immediate_type_of(bcx.ccx, cast_to);
+        bcx.intcast(val, cast_to, adt::is_discr_signed(&l))
+    }
+
+    /// Set the discriminant for a new value of the given case of the given
+    /// representation.
+    pub fn trans_set_discr(&self, bcx: &Builder<'a, 'tcx>, variant_index: usize) {
+        let l = bcx.ccx.layout_of(self.ty.to_ty(bcx.tcx()));
+        let to = l.ty.ty_adt_def().unwrap()
+            .discriminant_for_variant(bcx.tcx(), variant_index)
+            .to_u128_unchecked() as u64;
+        match *l {
+            layout::CEnum { discr, min, max, .. } => {
+                adt::assert_discr_in_range(min, max, to);
+                bcx.store(C_int(Type::from_integer(bcx.ccx, discr), to as i64),
+                    self.llval, self.alignment.to_align());
             }
-            layout::RawNullablePointer { nndiscr, .. } |
-            layout::StructWrappedNullablePointer { nndiscr,  .. } if discr as u64 != nndiscr => {
-                let nullfields = adt::compute_fields(bcx.ccx, t, (1-nndiscr) as usize, false);
-                // The unit-like case might have a nonzero number of unit-like fields.
-                // (e.d., Result of Either with (), as one side.)
-                let ty = type_of::type_of(bcx.ccx, nullfields[ix]);
-                assert_eq!(machine::llsize_of_alloc(bcx.ccx, ty), 0);
-                (bcx.pointercast(self.llval, ty.ptr_to()), Alignment::Packed)
+            layout::General { discr, .. } => {
+                bcx.store(C_int(Type::from_integer(bcx.ccx, discr), to as i64),
+                    bcx.struct_gep(self.llval, 0), self.alignment.to_align());
+            }
+            layout::Univariant { .. }
+            | layout::UntaggedUnion { .. }
+            | layout::Vector { .. } => {
+                assert_eq!(to, 0);
             }
             layout::RawNullablePointer { nndiscr, .. } => {
-                let nnty = adt::compute_fields(bcx.ccx, t, nndiscr as usize, false)[0];
-                assert_eq!(ix, 0);
-                assert_eq!(discr as u64, nndiscr);
-                let ty = type_of::type_of(bcx.ccx, nnty);
-                (bcx.pointercast(self.llval, ty.ptr_to()), self.alignment)
+                if to != nndiscr {
+                    let llptrty = val_ty(self.llval).element_type();
+                    bcx.store(C_null(llptrty), self.llval, self.alignment.to_align());
+                }
             }
-            layout::StructWrappedNullablePointer { ref nonnull, nndiscr, .. } => {
-                assert_eq!(discr as u64, nndiscr);
-                self.struct_field_ptr(bcx, &nonnull,
-                     &adt::compute_fields(bcx.ccx, t, discr, false), ix, false)
+            layout::StructWrappedNullablePointer { nndiscr, ref discrfield, ref nonnull, .. } => {
+                if to != nndiscr {
+                    if target_sets_discr_via_memset(bcx) {
+                        // Issue #34427: As workaround for LLVM bug on
+                        // ARM, use memset of 0 on whole struct rather
+                        // than storing null to single target field.
+                        let llptr = bcx.pointercast(self.llval, Type::i8(bcx.ccx).ptr_to());
+                        let fill_byte = C_u8(bcx.ccx, 0);
+                        let size = C_usize(bcx.ccx, nonnull.stride().bytes());
+                        let align = C_i32(bcx.ccx, nonnull.align.abi() as i32);
+                        base::call_memset(bcx, llptr, fill_byte, size, align, false);
+                    } else {
+                        let llptrptr = self.gepi_struct_llfields_path(bcx, discrfield);
+                        let llptrty = val_ty(llptrptr).element_type();
+                        bcx.store(C_null(llptrty), llptrptr, self.alignment.to_align());
+                    }
+                }
             }
-            _ => bug!("element access in type without elements: {} represented as {:#?}", t, l)
+            _ => bug!("Cannot handle {} represented as {:#?}", l.ty, l)
         }
     }
 
diff --git a/src/librustc_trans/mir/rvalue.rs b/src/librustc_trans/mir/rvalue.rs
index 7e187a85867..bc263fd60a2 100644
--- a/src/librustc_trans/mir/rvalue.rs
+++ b/src/librustc_trans/mir/rvalue.rs
@@ -139,10 +139,7 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
             mir::Rvalue::Aggregate(ref kind, ref operands) => {
                 match **kind {
                     mir::AggregateKind::Adt(adt_def, variant_index, substs, active_field_index) => {
-                        let discr = adt_def.discriminant_for_variant(bcx.tcx(), variant_index)
-                           .to_u128_unchecked() as u64;
-                        let dest_ty = dest.ty.to_ty(bcx.tcx());
-                        adt::trans_set_discr(&bcx, dest_ty, dest.llval, discr);
+                        dest.trans_set_discr(&bcx, variant_index);
                         for (i, operand) in operands.iter().enumerate() {
                             let op = self.trans_operand(&bcx, operand);
                             // Do not generate stores and GEPis for zero-sized fields.
@@ -451,12 +448,9 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
             }
 
             mir::Rvalue::Discriminant(ref lvalue) => {
-                let discr_lvalue = self.trans_lvalue(&bcx, lvalue);
-                let enum_ty = discr_lvalue.ty.to_ty(bcx.tcx());
                 let discr_ty = rvalue.ty(&*self.mir, bcx.tcx());
-                let discr_type = type_of::immediate_type_of(bcx.ccx, discr_ty);
-                let discr = adt::trans_get_discr(&bcx, enum_ty, discr_lvalue.llval,
-                                                  discr_lvalue.alignment, Some(discr_type), true);
+                let discr =  self.trans_lvalue(&bcx, lvalue)
+                    .trans_get_discr(&bcx, discr_ty);
                 (bcx, OperandRef {
                     val: OperandValue::Immediate(discr),
                     ty: discr_ty
diff --git a/src/librustc_trans/mir/statement.rs b/src/librustc_trans/mir/statement.rs
index bbf661ae9a7..6e9b1f36c2c 100644
--- a/src/librustc_trans/mir/statement.rs
+++ b/src/librustc_trans/mir/statement.rs
@@ -17,7 +17,6 @@ use builder::Builder;
 
 use super::MirContext;
 use super::LocalRef;
-use super::super::adt;
 
 impl<'a, 'tcx> MirContext<'a, 'tcx> {
     pub fn trans_statement(&mut self,
@@ -59,12 +58,8 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
                 }
             }
             mir::StatementKind::SetDiscriminant{ref lvalue, variant_index} => {
-                let ty = self.monomorphized_lvalue_ty(lvalue);
-                let lvalue_transed = self.trans_lvalue(&bcx, lvalue);
-                adt::trans_set_discr(&bcx,
-                    ty,
-                    lvalue_transed.llval,
-                    variant_index as u64);
+                self.trans_lvalue(&bcx, lvalue)
+                    .trans_set_discr(&bcx, variant_index);
                 bcx
             }
             mir::StatementKind::StorageLive(local) => {