src/librustc_trans/mir/constant.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use llvm::{self, ValueRef};
  12 use rustc::middle::const_val::{ConstEvalErr, ConstVal, ErrKind};
  13 use rustc_const_math::ConstInt::*;
  14 use rustc_const_math::ConstFloat::*;
  15 use rustc_const_math::{ConstInt, ConstMathErr};
  16 use rustc::hir::def_id::DefId;
  17 use rustc::infer::TransNormalize;
  18 use rustc::mir;
  19 use rustc::mir::tcx::LvalueTy;
  20 use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
  21 use rustc::ty::layout::{self, LayoutTyper};
  22 use rustc::ty::cast::{CastTy, IntTy};
  23 use rustc::ty::subst::{Kind, Substs, Subst};
  24 use rustc_data_structures::indexed_vec::{Idx, IndexVec};
  25 use {adt, base, machine};
  26 use abi::{self, Abi};
  27 use callee;
  28 use builder::Builder;
  29 use common::{self, CrateContext, const_get_elt, val_ty};
  30 use common::{C_array, C_bool, C_bytes, C_floating_f64, C_integral, C_big_integral};
  31 use common::{C_null, C_struct, C_str_slice, C_undef, C_uint, C_vector, is_undef};
  32 use common::const_to_opt_u128;
  33 use consts;
  34 use monomorphize;
  35 use type_of;
  36 use type_::Type;
  37 use value::Value;
  38
  39 use syntax_pos::Span;
  40
  41 use std::fmt;
  42 use std::ptr;
  43
  44 use super::lvalue::Alignment;
  45 use super::operand::{OperandRef, OperandValue};
  46 use super::MirContext;
  47
  48 /// A sized constant rvalue.
  49 /// The LLVM type might not be the same for a single Rust type,
  50 /// e.g. each enum variant would have its own LLVM struct type.
  51 #[derive(Copy, Clone)]
  52 pub struct Const<'tcx> {
  53     pub llval: ValueRef,
  54     pub ty: Ty<'tcx>
  55 }
  56
  57 impl<'tcx> Const<'tcx> {
  58     pub fn new(llval: ValueRef, ty: Ty<'tcx>) -> Const<'tcx> {
  59         Const {
  60             llval: llval,
  61             ty: ty
  62         }
  63     }
  64
  65     pub fn from_constint<'a>(ccx: &CrateContext<'a, 'tcx>, ci: &ConstInt)
  66     -> Const<'tcx> {
  67         let tcx = ccx.tcx();
  68         let (llval, ty) = match *ci {
  69             I8(v) => (C_integral(Type::i8(ccx), v as u64, true), tcx.types.i8),
  70             I16(v) => (C_integral(Type::i16(ccx), v as u64, true), tcx.types.i16),
  71             I32(v) => (C_integral(Type::i32(ccx), v as u64, true), tcx.types.i32),
  72             I64(v) => (C_integral(Type::i64(ccx), v as u64, true), tcx.types.i64),
  73             I128(v) => (C_big_integral(Type::i128(ccx), v as u128), tcx.types.i128),
  74             Isize(v) => {
  75                 let i = v.as_i64(ccx.tcx().sess.target.int_type);
  76                 (C_integral(Type::int(ccx), i as u64, true), tcx.types.isize)
  77             },
  78             U8(v) => (C_integral(Type::i8(ccx), v as u64, false), tcx.types.u8),
  79             U16(v) => (C_integral(Type::i16(ccx), v as u64, false), tcx.types.u16),
  80             U32(v) => (C_integral(Type::i32(ccx), v as u64, false), tcx.types.u32),
  81             U64(v) => (C_integral(Type::i64(ccx), v, false), tcx.types.u64),
  82             U128(v) => (C_big_integral(Type::i128(ccx), v), tcx.types.u128),
  83             Usize(v) => {
  84                 let u = v.as_u64(ccx.tcx().sess.target.uint_type);
  85                 (C_integral(Type::int(ccx), u, false), tcx.types.usize)
  86             },
  87         };
  88         Const { llval: llval, ty: ty }
  89     }
  90
  91     /// Translate ConstVal into a LLVM constant value.
  92     pub fn from_constval<'a>(ccx: &CrateContext<'a, 'tcx>,
  93                              cv: ConstVal,
  94                              ty: Ty<'tcx>)
  95                              -> Const<'tcx> {
  96         let llty = type_of::type_of(ccx, ty);
  97         let val = match cv {
  98             ConstVal::Float(F32(v)) => C_floating_f64(v as f64, llty),
  99             ConstVal::Float(F64(v)) => C_floating_f64(v, llty),
 100             ConstVal::Bool(v) => C_bool(ccx, v),
 101             ConstVal::Integral(ref i) => return Const::from_constint(ccx, i),
 102             ConstVal::Str(ref v) => C_str_slice(ccx, v.clone()),
 103             ConstVal::ByteStr(ref v) => consts::addr_of(ccx, C_bytes(ccx, v), 1, "byte_str"),
 104             ConstVal::Char(c) => C_integral(Type::char(ccx), c as u64, false),
 105             ConstVal::Function(..) => C_null(type_of::type_of(ccx, ty)),
 106             ConstVal::Variant(_) |
 107             ConstVal::Struct(_) | ConstVal::Tuple(_) |
 108             ConstVal::Array(..) | ConstVal::Repeat(..) => {
 109                 bug!("MIR must not use `{:?}` (aggregates are expanded to MIR rvalues)", cv)
 110             }
 111         };
 112
 113         assert!(!ty.has_erasable_regions());
 114
 115         Const::new(val, ty)
 116     }
 117
 118     fn get_pair(&self) -> (ValueRef, ValueRef) {
 119         (const_get_elt(self.llval, &[0]),
 120          const_get_elt(self.llval, &[1]))
 121     }
 122
 123     fn get_fat_ptr(&self) -> (ValueRef, ValueRef) {
 124         assert_eq!(abi::FAT_PTR_ADDR, 0);
 125         assert_eq!(abi::FAT_PTR_EXTRA, 1);
 126         self.get_pair()
 127     }
 128
 129     fn as_lvalue(&self) -> ConstLvalue<'tcx> {
 130         ConstLvalue {
 131             base: Base::Value(self.llval),
 132             llextra: ptr::null_mut(),
 133             ty: self.ty
 134         }
 135     }
 136
 137     pub fn to_operand<'a>(&self, ccx: &CrateContext<'a, 'tcx>) -> OperandRef<'tcx> {
 138         let llty = type_of::immediate_type_of(ccx, self.ty);
 139         let llvalty = val_ty(self.llval);
 140
 141         let val = if llty == llvalty && common::type_is_imm_pair(ccx, self.ty) {
 142             let (a, b) = self.get_pair();
 143             OperandValue::Pair(a, b)
 144         } else if llty == llvalty && common::type_is_immediate(ccx, self.ty) {
 145             // If the types match, we can use the value directly.
 146             OperandValue::Immediate(self.llval)
 147         } else {
 148             // Otherwise, or if the value is not immediate, we create
 149             // a constant LLVM global and cast its address if necessary.
 150             let align = ccx.align_of(self.ty);
 151             let ptr = consts::addr_of(ccx, self.llval, align, "const");
 152             OperandValue::Ref(consts::ptrcast(ptr, llty.ptr_to()), Alignment::AbiAligned)
 153         };
 154
 155         OperandRef {
 156             val: val,
 157             ty: self.ty
 158         }
 159     }
 160 }
 161
 162 impl<'tcx> fmt::Debug for Const<'tcx> {
 163     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 164         write!(f, "Const({:?}: {:?})", Value(self.llval), self.ty)
 165     }
 166 }
 167
 168 #[derive(Copy, Clone)]
 169 enum Base {
 170     /// A constant value without an unique address.
 171     Value(ValueRef),
 172
 173     /// String literal base pointer (cast from array).
 174     Str(ValueRef),
 175
 176     /// The address of a static.
 177     Static(ValueRef)
 178 }
 179
 180 /// An lvalue as seen from a constant.
 181 #[derive(Copy, Clone)]
 182 struct ConstLvalue<'tcx> {
 183     base: Base,
 184     llextra: ValueRef,
 185     ty: Ty<'tcx>
 186 }
 187
 188 impl<'tcx> ConstLvalue<'tcx> {
 189     fn to_const(&self, span: Span) -> Const<'tcx> {
 190         match self.base {
 191             Base::Value(val) => Const::new(val, self.ty),
 192             Base::Str(ptr) => {
 193                 span_bug!(span, "loading from `str` ({:?}) in constant",
 194                           Value(ptr))
 195             }
 196             Base::Static(val) => {
 197                 span_bug!(span, "loading from `static` ({:?}) in constant",
 198                           Value(val))
 199             }
 200         }
 201     }
 202
 203     pub fn len<'a>(&self, ccx: &CrateContext<'a, 'tcx>) -> ValueRef {
 204         match self.ty.sty {
 205             ty::TyArray(_, n) => C_uint(ccx, n),
 206             ty::TySlice(_) | ty::TyStr => {
 207                 assert!(self.llextra != ptr::null_mut());
 208                 self.llextra
 209             }
 210             _ => bug!("unexpected type `{}` in ConstLvalue::len", self.ty)
 211         }
 212     }
 213 }
 214
 215 /// Machinery for translating a constant's MIR to LLVM values.
 216 /// FIXME(eddyb) use miri and lower its allocations to LLVM.
 217 struct MirConstContext<'a, 'tcx: 'a> {
 218     ccx: &'a CrateContext<'a, 'tcx>,
 219     mir: &'a mir::Mir<'tcx>,
 220
 221     /// Type parameters for const fn and associated constants.
 222     substs: &'tcx Substs<'tcx>,
 223
 224     /// Values of locals in a constant or const fn.
 225     locals: IndexVec<mir::Local, Option<Result<Const<'tcx>, ConstEvalErr<'tcx>>>>
 226 }
 227
 228 fn add_err<'tcx, U, V>(failure: &mut Result<U, ConstEvalErr<'tcx>>,
 229                        value: &Result<V, ConstEvalErr<'tcx>>)
 230 {
 231     if let &Err(ref err) = value {
 232         if failure.is_ok() {
 233             *failure = Err(err.clone());
 234         }
 235     }
 236 }
 237
 238 impl<'a, 'tcx> MirConstContext<'a, 'tcx> {
 239     fn new(ccx: &'a CrateContext<'a, 'tcx>,
 240            mir: &'a mir::Mir<'tcx>,
 241            substs: &'tcx Substs<'tcx>,
 242            args: IndexVec<mir::Local, Result<Const<'tcx>, ConstEvalErr<'tcx>>>)
 243            -> MirConstContext<'a, 'tcx> {
 244         let mut context = MirConstContext {
 245             ccx: ccx,
 246             mir: mir,
 247             substs: substs,
 248             locals: (0..mir.local_decls.len()).map(|_| None).collect(),
 249         };
 250         for (i, arg) in args.into_iter().enumerate() {
 251             // Locals after local 0 are the function arguments
 252             let index = mir::Local::new(i + 1);
 253             context.locals[index] = Some(arg);
 254         }
 255         context
 256     }
 257
 258     fn trans_def(ccx: &'a CrateContext<'a, 'tcx>,
 259                  def_id: DefId,
 260                  substs: &'tcx Substs<'tcx>,
 261                  args: IndexVec<mir::Local, Result<Const<'tcx>, ConstEvalErr<'tcx>>>)
 262                  -> Result<Const<'tcx>, ConstEvalErr<'tcx>> {
 263         let instance = monomorphize::resolve(ccx.shared(), def_id, substs);
 264         let mir = ccx.tcx().instance_mir(instance.def);
 265         MirConstContext::new(ccx, &mir, instance.substs, args).trans()
 266     }
 267
 268     fn monomorphize<T>(&self, value: &T) -> T
 269         where T: TransNormalize<'tcx>
 270     {
 271         self.ccx.tcx().trans_apply_param_substs(self.substs, value)
 272     }
 273
 274     fn trans(&mut self) -> Result<Const<'tcx>, ConstEvalErr<'tcx>> {
 275         let tcx = self.ccx.tcx();
 276         let mut bb = mir::START_BLOCK;
 277
 278         // Make sure to evaluate all statemenets to
 279         // report as many errors as we possibly can.
 280         let mut failure = Ok(());
 281
 282         loop {
 283             let data = &self.mir[bb];
 284             for statement in &data.statements {
 285                 let span = statement.source_info.span;
 286                 match statement.kind {
 287                     mir::StatementKind::Assign(ref dest, ref rvalue) => {
 288                         let ty = dest.ty(self.mir, tcx);
 289                         let ty = self.monomorphize(&ty).to_ty(tcx);
 290                         let value = self.const_rvalue(rvalue, ty, span);
 291                         add_err(&mut failure, &value);
 292                         self.store(dest, value, span);
 293                     }
 294                     mir::StatementKind::StorageLive(_) |
 295                     mir::StatementKind::StorageDead(_) |
 296                     mir::StatementKind::EndRegion(_) |
 297                     mir::StatementKind::Nop => {}
 298                     mir::StatementKind::InlineAsm { .. } |
 299                     mir::StatementKind::SetDiscriminant{ .. } => {
 300                         span_bug!(span, "{:?} should not appear in constants?", statement.kind);
 301                     }
 302                 }
 303             }
 304
 305             let terminator = data.terminator();
 306             let span = terminator.source_info.span;
 307             bb = match terminator.kind {
 308                 mir::TerminatorKind::Drop { target, .. } | // No dropping.
 309                 mir::TerminatorKind::Goto { target } => target,
 310                 mir::TerminatorKind::Return => {
 311                     failure?;
 312                     return self.locals[mir::RETURN_POINTER].clone().unwrap_or_else(|| {
 313                         span_bug!(span, "no returned value in constant");
 314                     });
 315                 }
 316
 317                 mir::TerminatorKind::Assert { ref cond, expected, ref msg, target, .. } => {
 318                     let cond = self.const_operand(cond, span)?;
 319                     let cond_bool = common::const_to_uint(cond.llval) != 0;
 320                     if cond_bool != expected {
 321                         let err = match *msg {
 322                             mir::AssertMessage::BoundsCheck { ref len, ref index } => {
 323                                 let len = self.const_operand(len, span)?;
 324                                 let index = self.const_operand(index, span)?;
 325                                 ErrKind::IndexOutOfBounds {
 326                                     len: common::const_to_uint(len.llval),
 327                                     index: common::const_to_uint(index.llval)
 328                                 }
 329                             }
 330                             mir::AssertMessage::Math(ref err) => {
 331                                 ErrKind::Math(err.clone())
 332                             }
 333                         };
 334
 335                         let err = ConstEvalErr { span: span, kind: err };
 336                         err.report(tcx, span, "expression");
 337                         failure = Err(err);
 338                     }
 339                     target
 340                 }
 341
 342                 mir::TerminatorKind::Call { ref func, ref args, ref destination, .. } => {
 343                     let fn_ty = func.ty(self.mir, tcx);
 344                     let fn_ty = self.monomorphize(&fn_ty);
 345                     let (def_id, substs) = match fn_ty.sty {
 346                         ty::TyFnDef(def_id, substs) => (def_id, substs),
 347                         _ => span_bug!(span, "calling {:?} (of type {}) in constant",
 348                                        func, fn_ty)
 349                     };
 350
 351                     let mut arg_vals = IndexVec::with_capacity(args.len());
 352                     for arg in args {
 353                         let arg_val = self.const_operand(arg, span);
 354                         add_err(&mut failure, &arg_val);
 355                         arg_vals.push(arg_val);
 356                     }
 357                     if let Some((ref dest, target)) = *destination {
 358                         let result = if fn_ty.fn_sig(tcx).abi() == Abi::RustIntrinsic {
 359                             match &tcx.item_name(def_id).as_str()[..] {
 360                                 "size_of" => {
 361                                     let llval = C_uint(self.ccx,
 362                                         self.ccx.size_of(substs.type_at(0)));
 363                                     Ok(Const::new(llval, tcx.types.usize))
 364                                 }
 365                                 "min_align_of" => {
 366                                     let llval = C_uint(self.ccx,
 367                                         self.ccx.align_of(substs.type_at(0)));
 368                                     Ok(Const::new(llval, tcx.types.usize))
 369                                 }
 370                                 _ => span_bug!(span, "{:?} in constant", terminator.kind)
 371                             }
 372                         } else {
 373                             MirConstContext::trans_def(self.ccx, def_id, substs, arg_vals)
 374                         };
 375                         add_err(&mut failure, &result);
 376                         self.store(dest, result, span);
 377                         target
 378                     } else {
 379                         span_bug!(span, "diverging {:?} in constant", terminator.kind);
 380                     }
 381                 }
 382                 _ => span_bug!(span, "{:?} in constant", terminator.kind)
 383             };
 384         }
 385     }
 386
 387     fn store(&mut self,
 388              dest: &mir::Lvalue<'tcx>,
 389              value: Result<Const<'tcx>, ConstEvalErr<'tcx>>,
 390              span: Span) {
 391         if let mir::Lvalue::Local(index) = *dest {
 392             self.locals[index] = Some(value);
 393         } else {
 394             span_bug!(span, "assignment to {:?} in constant", dest);
 395         }
 396     }
 397
 398     fn const_lvalue(&self, lvalue: &mir::Lvalue<'tcx>, span: Span)
 399                     -> Result<ConstLvalue<'tcx>, ConstEvalErr<'tcx>> {
 400         let tcx = self.ccx.tcx();
 401
 402         if let mir::Lvalue::Local(index) = *lvalue {
 403             return self.locals[index].clone().unwrap_or_else(|| {
 404                 span_bug!(span, "{:?} not initialized", lvalue)
 405             }).map(|v| v.as_lvalue());
 406         }
 407
 408         let lvalue = match *lvalue {
 409             mir::Lvalue::Local(_)  => bug!(), // handled above
 410             mir::Lvalue::Static(box mir::Static { def_id, ty }) => {
 411                 ConstLvalue {
 412                     base: Base::Static(consts::get_static(self.ccx, def_id)),
 413                     llextra: ptr::null_mut(),
 414                     ty: self.monomorphize(&ty),
 415                 }
 416             }
 417             mir::Lvalue::Projection(ref projection) => {
 418                 let tr_base = self.const_lvalue(&projection.base, span)?;
 419                 let projected_ty = LvalueTy::Ty { ty: tr_base.ty }
 420                     .projection_ty(tcx, &projection.elem);
 421                 let base = tr_base.to_const(span);
 422                 let projected_ty = self.monomorphize(&projected_ty).to_ty(tcx);
 423                 let is_sized = self.ccx.shared().type_is_sized(projected_ty);
 424
 425                 let (projected, llextra) = match projection.elem {
 426                     mir::ProjectionElem::Deref => {
 427                         let (base, extra) = if is_sized {
 428                             (base.llval, ptr::null_mut())
 429                         } else {
 430                             base.get_fat_ptr()
 431                         };
 432                         if self.ccx.statics().borrow().contains_key(&base) {
 433                             (Base::Static(base), extra)
 434                         } else if let ty::TyStr = projected_ty.sty {
 435                             (Base::Str(base), extra)
 436                         } else {
 437                             let v = base;
 438                             let v = self.ccx.const_unsized().borrow().get(&v).map_or(v, |&v| v);
 439                             let mut val = unsafe { llvm::LLVMGetInitializer(v) };
 440                             if val.is_null() {
 441                                 span_bug!(span, "dereference of non-constant pointer `{:?}`",
 442                                           Value(base));
 443                             }
 444                             if projected_ty.is_bool() {
 445                                 let i1_type = Type::i1(self.ccx);
 446                                 if val_ty(val) != i1_type {
 447                                     unsafe {
 448                                         val = llvm::LLVMConstTrunc(val, i1_type.to_ref());
 449                                     }
 450                                 }
 451                             }
 452                             (Base::Value(val), extra)
 453                         }
 454                     }
 455                     mir::ProjectionElem::Field(ref field, _) => {
 456                         let llprojected = adt::const_get_field(self.ccx, tr_base.ty, base.llval,
 457                                                                field.index());
 458                         let llextra = if is_sized {
 459                             ptr::null_mut()
 460                         } else {
 461                             tr_base.llextra
 462                         };
 463                         (Base::Value(llprojected), llextra)
 464                     }
 465                     mir::ProjectionElem::Index(ref index) => {
 466                         let llindex = self.const_operand(index, span)?.llval;
 467
 468                         let iv = if let Some(iv) = common::const_to_opt_u128(llindex, false) {
 469                             iv
 470                         } else {
 471                             span_bug!(span, "index is not an integer-constant expression")
 472                         };
 473
 474                         // Produce an undef instead of a LLVM assertion on OOB.
 475                         let len = common::const_to_uint(tr_base.len(self.ccx));
 476                         let llelem = if iv < len as u128 {
 477                             const_get_elt(base.llval, &[iv as u32])
 478                         } else {
 479                             C_undef(type_of::type_of(self.ccx, projected_ty))
 480                         };
 481
 482                         (Base::Value(llelem), ptr::null_mut())
 483                     }
 484                     _ => span_bug!(span, "{:?} in constant", projection.elem)
 485                 };
 486                 ConstLvalue {
 487                     base: projected,
 488                     llextra: llextra,
 489                     ty: projected_ty
 490                 }
 491             }
 492         };
 493         Ok(lvalue)
 494     }
 495
 496     fn const_operand(&self, operand: &mir::Operand<'tcx>, span: Span)
 497                      -> Result<Const<'tcx>, ConstEvalErr<'tcx>> {
 498         debug!("const_operand({:?} @ {:?})", operand, span);
 499         let result = match *operand {
 500             mir::Operand::Consume(ref lvalue) => {
 501                 Ok(self.const_lvalue(lvalue, span)?.to_const(span))
 502             }
 503
 504             mir::Operand::Constant(ref constant) => {
 505                 let ty = self.monomorphize(&constant.ty);
 506                 match constant.literal.clone() {
 507                     mir::Literal::Item { def_id, substs } => {
 508                         let substs = self.monomorphize(&substs);
 509                         MirConstContext::trans_def(self.ccx, def_id, substs, IndexVec::new())
 510                     }
 511                     mir::Literal::Promoted { index } => {
 512                         let mir = &self.mir.promoted[index];
 513                         MirConstContext::new(self.ccx, mir, self.substs, IndexVec::new()).trans()
 514                     }
 515                     mir::Literal::Value { value } => {
 516                         Ok(Const::from_constval(self.ccx, value, ty))
 517                     }
 518                 }
 519             }
 520         };
 521         debug!("const_operand({:?} @ {:?}) = {:?}", operand, span,
 522                result.as_ref().ok());
 523         result
 524     }
 525
 526     fn const_array(&self, array_ty: Ty<'tcx>, fields: &[ValueRef])
 527                    -> Const<'tcx>
 528     {
 529         let elem_ty = array_ty.builtin_index().unwrap_or_else(|| {
 530             bug!("bad array type {:?}", array_ty)
 531         });
 532         let llunitty = type_of::type_of(self.ccx, elem_ty);
 533         // If the array contains enums, an LLVM array won't work.
 534         let val = if fields.iter().all(|&f| val_ty(f) == llunitty) {
 535             C_array(llunitty, fields)
 536         } else {
 537             C_struct(self.ccx, fields, false)
 538         };
 539         Const::new(val, array_ty)
 540     }
 541
 542     fn const_rvalue(&self, rvalue: &mir::Rvalue<'tcx>,
 543                     dest_ty: Ty<'tcx>, span: Span)
 544                     -> Result<Const<'tcx>, ConstEvalErr<'tcx>> {
 545         let tcx = self.ccx.tcx();
 546         debug!("const_rvalue({:?}: {:?} @ {:?})", rvalue, dest_ty, span);
 547         let val = match *rvalue {
 548             mir::Rvalue::Use(ref operand) => self.const_operand(operand, span)?,
 549
 550             mir::Rvalue::Repeat(ref elem, ref count) => {
 551                 let elem = self.const_operand(elem, span)?;
 552                 let size = count.as_u64(tcx.sess.target.uint_type);
 553                 let fields = vec![elem.llval; size as usize];
 554                 self.const_array(dest_ty, &fields)
 555             }
 556
 557             mir::Rvalue::Aggregate(ref kind, ref operands) => {
 558                 // Make sure to evaluate all operands to
 559                 // report as many errors as we possibly can.
 560                 let mut fields = Vec::with_capacity(operands.len());
 561                 let mut failure = Ok(());
 562                 for operand in operands {
 563                     match self.const_operand(operand, span) {
 564                         Ok(val) => fields.push(val.llval),
 565                         Err(err) => if failure.is_ok() { failure = Err(err); }
 566                     }
 567                 }
 568                 failure?;
 569
 570                 match **kind {
 571                     mir::AggregateKind::Array(_) => {
 572                         self.const_array(dest_ty, &fields)
 573                     }
 574                     mir::AggregateKind::Adt(..) |
 575                     mir::AggregateKind::Closure(..) |
 576                     mir::AggregateKind::Tuple => {
 577                         Const::new(trans_const(self.ccx, dest_ty, kind, &fields), dest_ty)
 578                     }
 579                 }
 580             }
 581
 582             mir::Rvalue::Cast(ref kind, ref source, cast_ty) => {
 583                 let operand = self.const_operand(source, span)?;
 584                 let cast_ty = self.monomorphize(&cast_ty);
 585
 586                 let val = match *kind {
 587                     mir::CastKind::ReifyFnPointer => {
 588                         match operand.ty.sty {
 589                             ty::TyFnDef(def_id, substs) => {
 590                                 callee::resolve_and_get_fn(self.ccx, def_id, substs)
 591                             }
 592                             _ => {
 593                                 span_bug!(span, "{} cannot be reified to a fn ptr",
 594                                           operand.ty)
 595                             }
 596                         }
 597                     }
 598                     mir::CastKind::ClosureFnPointer => {
 599                         match operand.ty.sty {
 600                             ty::TyClosure(def_id, substs) => {
 601                                 // Get the def_id for FnOnce::call_once
 602                                 let fn_once = tcx.lang_items.fn_once_trait().unwrap();
 603                                 let call_once = tcx
 604                                     .global_tcx().associated_items(fn_once)
 605                                     .find(|it| it.kind == ty::AssociatedKind::Method)
 606                                     .unwrap().def_id;
 607                                 // Now create its substs [Closure, Tuple]
 608                                 let input = tcx.fn_sig(def_id)
 609                                     .subst(tcx, substs.substs).input(0);
 610                                 let input = tcx.erase_late_bound_regions_and_normalize(&input);
 611                                 let substs = tcx.mk_substs([operand.ty, input]
 612                                     .iter().cloned().map(Kind::from));
 613                                 callee::resolve_and_get_fn(self.ccx, call_once, substs)
 614                             }
 615                             _ => {
 616                                 bug!("{} cannot be cast to a fn ptr", operand.ty)
 617                             }
 618                         }
 619                     }
 620                     mir::CastKind::UnsafeFnPointer => {
 621                         // this is a no-op at the LLVM level
 622                         operand.llval
 623                     }
 624                     mir::CastKind::Unsize => {
 625                         // unsize targets other than to a fat pointer currently
 626                         // can't be in constants.
 627                         assert!(common::type_is_fat_ptr(self.ccx, cast_ty));
 628
 629                         let pointee_ty = operand.ty.builtin_deref(true, ty::NoPreference)
 630                             .expect("consts: unsizing got non-pointer type").ty;
 631                         let (base, old_info) = if !self.ccx.shared().type_is_sized(pointee_ty) {
 632                             // Normally, the source is a thin pointer and we are
 633                             // adding extra info to make a fat pointer. The exception
 634                             // is when we are upcasting an existing object fat pointer
 635                             // to use a different vtable. In that case, we want to
 636                             // load out the original data pointer so we can repackage
 637                             // it.
 638                             let (base, extra) = operand.get_fat_ptr();
 639                             (base, Some(extra))
 640                         } else {
 641                             (operand.llval, None)
 642                         };
 643
 644                         let unsized_ty = cast_ty.builtin_deref(true, ty::NoPreference)
 645                             .expect("consts: unsizing got non-pointer target type").ty;
 646                         let ptr_ty = type_of::in_memory_type_of(self.ccx, unsized_ty).ptr_to();
 647                         let base = consts::ptrcast(base, ptr_ty);
 648                         let info = base::unsized_info(self.ccx, pointee_ty,
 649                                                       unsized_ty, old_info);
 650
 651                         if old_info.is_none() {
 652                             let prev_const = self.ccx.const_unsized().borrow_mut()
 653                                                      .insert(base, operand.llval);
 654                             assert!(prev_const.is_none() || prev_const == Some(operand.llval));
 655                         }
 656                         assert_eq!(abi::FAT_PTR_ADDR, 0);
 657                         assert_eq!(abi::FAT_PTR_EXTRA, 1);
 658                         C_struct(self.ccx, &[base, info], false)
 659                     }
 660                     mir::CastKind::Misc if common::type_is_immediate(self.ccx, operand.ty) => {
 661                         debug_assert!(common::type_is_immediate(self.ccx, cast_ty));
 662                         let r_t_in = CastTy::from_ty(operand.ty).expect("bad input type for cast");
 663                         let r_t_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
 664                         let ll_t_out = type_of::immediate_type_of(self.ccx, cast_ty);
 665                         let llval = operand.llval;
 666                         let signed = if let CastTy::Int(IntTy::CEnum) = r_t_in {
 667                             let l = self.ccx.layout_of(operand.ty);
 668                             adt::is_discr_signed(&l)
 669                         } else {
 670                             operand.ty.is_signed()
 671                         };
 672
 673                         unsafe {
 674                             match (r_t_in, r_t_out) {
 675                                 (CastTy::Int(_), CastTy::Int(_)) => {
 676                                     let s = signed as llvm::Bool;
 677                                     llvm::LLVMConstIntCast(llval, ll_t_out.to_ref(), s)
 678                                 }
 679                                 (CastTy::Int(_), CastTy::Float) => {
 680                                     if signed {
 681                                         llvm::LLVMConstSIToFP(llval, ll_t_out.to_ref())
 682                                     } else {
 683                                         llvm::LLVMConstUIToFP(llval, ll_t_out.to_ref())
 684                                     }
 685                                 }
 686                                 (CastTy::Float, CastTy::Float) => {
 687                                     llvm::LLVMConstFPCast(llval, ll_t_out.to_ref())
 688                                 }
 689                                 (CastTy::Float, CastTy::Int(IntTy::I)) => {
 690                                     llvm::LLVMConstFPToSI(llval, ll_t_out.to_ref())
 691                                 }
 692                                 (CastTy::Float, CastTy::Int(_)) => {
 693                                     llvm::LLVMConstFPToUI(llval, ll_t_out.to_ref())
 694                                 }
 695                                 (CastTy::Ptr(_), CastTy::Ptr(_)) |
 696                                 (CastTy::FnPtr, CastTy::Ptr(_)) |
 697                                 (CastTy::RPtr(_), CastTy::Ptr(_)) => {
 698                                     consts::ptrcast(llval, ll_t_out)
 699                                 }
 700                                 (CastTy::Int(_), CastTy::Ptr(_)) => {
 701                                     llvm::LLVMConstIntToPtr(llval, ll_t_out.to_ref())
 702                                 }
 703                                 (CastTy::Ptr(_), CastTy::Int(_)) |
 704                                 (CastTy::FnPtr, CastTy::Int(_)) => {
 705                                     llvm::LLVMConstPtrToInt(llval, ll_t_out.to_ref())
 706                                 }
 707                                 _ => bug!("unsupported cast: {:?} to {:?}", operand.ty, cast_ty)
 708                             }
 709                         }
 710                     }
 711                     mir::CastKind::Misc => { // Casts from a fat-ptr.
 712                         let ll_cast_ty = type_of::immediate_type_of(self.ccx, cast_ty);
 713                         let ll_from_ty = type_of::immediate_type_of(self.ccx, operand.ty);
 714                         if common::type_is_fat_ptr(self.ccx, operand.ty) {
 715                             let (data_ptr, meta_ptr) = operand.get_fat_ptr();
 716                             if common::type_is_fat_ptr(self.ccx, cast_ty) {
 717                                 let ll_cft = ll_cast_ty.field_types();
 718                                 let ll_fft = ll_from_ty.field_types();
 719                                 let data_cast = consts::ptrcast(data_ptr, ll_cft[0]);
 720                                 assert_eq!(ll_cft[1].kind(), ll_fft[1].kind());
 721                                 C_struct(self.ccx, &[data_cast, meta_ptr], false)
 722                             } else { // cast to thin-ptr
 723                                 // Cast of fat-ptr to thin-ptr is an extraction of data-ptr and
 724                                 // pointer-cast of that pointer to desired pointer type.
 725                                 consts::ptrcast(data_ptr, ll_cast_ty)
 726                             }
 727                         } else {
 728                             bug!("Unexpected non-fat-pointer operand")
 729                         }
 730                     }
 731                 };
 732                 Const::new(val, cast_ty)
 733             }
 734
 735             mir::Rvalue::Ref(_, bk, ref lvalue) => {
 736                 let tr_lvalue = self.const_lvalue(lvalue, span)?;
 737
 738                 let ty = tr_lvalue.ty;
 739                 let ref_ty = tcx.mk_ref(tcx.types.re_erased,
 740                     ty::TypeAndMut { ty: ty, mutbl: bk.to_mutbl_lossy() });
 741
 742                 let base = match tr_lvalue.base {
 743                     Base::Value(llval) => {
 744                         // FIXME: may be wrong for &*(&simd_vec as &fmt::Debug)
 745                         let align = if self.ccx.shared().type_is_sized(ty) {
 746                             self.ccx.align_of(ty)
 747                         } else {
 748                             self.ccx.tcx().data_layout.pointer_align.abi() as machine::llalign
 749                         };
 750                         if bk == mir::BorrowKind::Mut {
 751                             consts::addr_of_mut(self.ccx, llval, align, "ref_mut")
 752                         } else {
 753                             consts::addr_of(self.ccx, llval, align, "ref")
 754                         }
 755                     }
 756                     Base::Str(llval) |
 757                     Base::Static(llval) => llval
 758                 };
 759
 760                 let ptr = if self.ccx.shared().type_is_sized(ty) {
 761                     base
 762                 } else {
 763                     C_struct(self.ccx, &[base, tr_lvalue.llextra], false)
 764                 };
 765                 Const::new(ptr, ref_ty)
 766             }
 767
 768             mir::Rvalue::Len(ref lvalue) => {
 769                 let tr_lvalue = self.const_lvalue(lvalue, span)?;
 770                 Const::new(tr_lvalue.len(self.ccx), tcx.types.usize)
 771             }
 772
 773             mir::Rvalue::BinaryOp(op, ref lhs, ref rhs) => {
 774                 let lhs = self.const_operand(lhs, span)?;
 775                 let rhs = self.const_operand(rhs, span)?;
 776                 let ty = lhs.ty;
 777                 let binop_ty = op.ty(tcx, lhs.ty, rhs.ty);
 778                 let (lhs, rhs) = (lhs.llval, rhs.llval);
 779                 Const::new(const_scalar_binop(op, lhs, rhs, ty), binop_ty)
 780             }
 781
 782             mir::Rvalue::CheckedBinaryOp(op, ref lhs, ref rhs) => {
 783                 let lhs = self.const_operand(lhs, span)?;
 784                 let rhs = self.const_operand(rhs, span)?;
 785                 let ty = lhs.ty;
 786                 let val_ty = op.ty(tcx, lhs.ty, rhs.ty);
 787                 let binop_ty = tcx.intern_tup(&[val_ty, tcx.types.bool], false);
 788                 let (lhs, rhs) = (lhs.llval, rhs.llval);
 789                 assert!(!ty.is_fp());
 790
 791                 match const_scalar_checked_binop(tcx, op, lhs, rhs, ty) {
 792                     Some((llval, of)) => {
 793                         let llof = C_bool(self.ccx, of);
 794                         Const::new(C_struct(self.ccx, &[llval, llof], false), binop_ty)
 795                     }
 796                     None => {
 797                         span_bug!(span, "{:?} got non-integer operands: {:?} and {:?}",
 798                                   rvalue, Value(lhs), Value(rhs));
 799                     }
 800                 }
 801             }
 802
 803             mir::Rvalue::UnaryOp(op, ref operand) => {
 804                 let operand = self.const_operand(operand, span)?;
 805                 let lloperand = operand.llval;
 806                 let llval = match op {
 807                     mir::UnOp::Not => {
 808                         unsafe {
 809                             llvm::LLVMConstNot(lloperand)
 810                         }
 811                     }
 812                     mir::UnOp::Neg => {
 813                         let is_float = operand.ty.is_fp();
 814                         unsafe {
 815                             if is_float {
 816                                 llvm::LLVMConstFNeg(lloperand)
 817                             } else {
 818                                 llvm::LLVMConstNeg(lloperand)
 819                             }
 820                         }
 821                     }
 822                 };
 823                 Const::new(llval, operand.ty)
 824             }
 825
 826             mir::Rvalue::NullaryOp(mir::NullOp::SizeOf, ty) => {
 827                 assert!(self.ccx.shared().type_is_sized(ty));
 828                 let llval = C_uint(self.ccx, self.ccx.size_of(ty));
 829                 Const::new(llval, tcx.types.usize)
 830             }
 831
 832             _ => span_bug!(span, "{:?} in constant", rvalue)
 833         };
 834
 835         debug!("const_rvalue({:?}: {:?} @ {:?}) = {:?}", rvalue, dest_ty, span, val);
 836
 837         Ok(val)
 838     }
 839
 840 }
 841
 842 fn to_const_int(value: ValueRef, t: Ty, tcx: TyCtxt) -> Option<ConstInt> {
 843     match t.sty {
 844         ty::TyInt(int_type) => const_to_opt_u128(value, true)
 845             .and_then(|input| ConstInt::new_signed(input as i128, int_type,
 846                                                    tcx.sess.target.int_type)),
 847         ty::TyUint(uint_type) => const_to_opt_u128(value, false)
 848             .and_then(|input| ConstInt::new_unsigned(input, uint_type,
 849                                                      tcx.sess.target.uint_type)),
 850         _ => None
 851
 852     }
 853 }
 854
 855 pub fn const_scalar_binop(op: mir::BinOp,
 856                           lhs: ValueRef,
 857                           rhs: ValueRef,
 858                           input_ty: Ty) -> ValueRef {
 859     assert!(!input_ty.is_simd());
 860     let is_float = input_ty.is_fp();
 861     let signed = input_ty.is_signed();
 862
 863     unsafe {
 864         match op {
 865             mir::BinOp::Add if is_float => llvm::LLVMConstFAdd(lhs, rhs),
 866             mir::BinOp::Add             => llvm::LLVMConstAdd(lhs, rhs),
 867
 868             mir::BinOp::Sub if is_float => llvm::LLVMConstFSub(lhs, rhs),
 869             mir::BinOp::Sub             => llvm::LLVMConstSub(lhs, rhs),
 870
 871             mir::BinOp::Mul if is_float => llvm::LLVMConstFMul(lhs, rhs),
 872             mir::BinOp::Mul             => llvm::LLVMConstMul(lhs, rhs),
 873
 874             mir::BinOp::Div if is_float => llvm::LLVMConstFDiv(lhs, rhs),
 875             mir::BinOp::Div if signed   => llvm::LLVMConstSDiv(lhs, rhs),
 876             mir::BinOp::Div             => llvm::LLVMConstUDiv(lhs, rhs),
 877
 878             mir::BinOp::Rem if is_float => llvm::LLVMConstFRem(lhs, rhs),
 879             mir::BinOp::Rem if signed   => llvm::LLVMConstSRem(lhs, rhs),
 880             mir::BinOp::Rem             => llvm::LLVMConstURem(lhs, rhs),
 881
 882             mir::BinOp::BitXor => llvm::LLVMConstXor(lhs, rhs),
 883             mir::BinOp::BitAnd => llvm::LLVMConstAnd(lhs, rhs),
 884             mir::BinOp::BitOr  => llvm::LLVMConstOr(lhs, rhs),
 885             mir::BinOp::Shl    => {
 886                 let rhs = base::cast_shift_const_rhs(op.to_hir_binop(), lhs, rhs);
 887                 llvm::LLVMConstShl(lhs, rhs)
 888             }
 889             mir::BinOp::Shr    => {
 890                 let rhs = base::cast_shift_const_rhs(op.to_hir_binop(), lhs, rhs);
 891                 if signed { llvm::LLVMConstAShr(lhs, rhs) }
 892                 else      { llvm::LLVMConstLShr(lhs, rhs) }
 893             }
 894             mir::BinOp::Eq | mir::BinOp::Ne |
 895             mir::BinOp::Lt | mir::BinOp::Le |
 896             mir::BinOp::Gt | mir::BinOp::Ge => {
 897                 if is_float {
 898                     let cmp = base::bin_op_to_fcmp_predicate(op.to_hir_binop());
 899                     llvm::LLVMConstFCmp(cmp, lhs, rhs)
 900                 } else {
 901                     let cmp = base::bin_op_to_icmp_predicate(op.to_hir_binop(),
 902                                                                 signed);
 903                     llvm::LLVMConstICmp(cmp, lhs, rhs)
 904                 }
 905             }
 906             mir::BinOp::Offset => unreachable!("BinOp::Offset in const-eval!")
 907         }
 908     }
 909 }
 910
 911 pub fn const_scalar_checked_binop<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 912                                             op: mir::BinOp,
 913                                             lllhs: ValueRef,
 914                                             llrhs: ValueRef,
 915                                             input_ty: Ty<'tcx>)
 916                                             -> Option<(ValueRef, bool)> {
 917     if let (Some(lhs), Some(rhs)) = (to_const_int(lllhs, input_ty, tcx),
 918                                      to_const_int(llrhs, input_ty, tcx)) {
 919         let result = match op {
 920             mir::BinOp::Add => lhs + rhs,
 921             mir::BinOp::Sub => lhs - rhs,
 922             mir::BinOp::Mul => lhs * rhs,
 923             mir::BinOp::Shl => lhs << rhs,
 924             mir::BinOp::Shr => lhs >> rhs,
 925             _ => {
 926                 bug!("Operator `{:?}` is not a checkable operator", op)
 927             }
 928         };
 929
 930         let of = match result {
 931             Ok(_) => false,
 932             Err(ConstMathErr::Overflow(_)) |
 933             Err(ConstMathErr::ShiftNegative) => true,
 934             Err(err) => {
 935                 bug!("Operator `{:?}` on `{:?}` and `{:?}` errored: {}",
 936                      op, lhs, rhs, err.description());
 937             }
 938         };
 939
 940         Some((const_scalar_binop(op, lllhs, llrhs, input_ty), of))
 941     } else {
 942         None
 943     }
 944 }
 945
 946 impl<'a, 'tcx> MirContext<'a, 'tcx> {
 947     pub fn trans_constant(&mut self,
 948                           bcx: &Builder<'a, 'tcx>,
 949                           constant: &mir::Constant<'tcx>)
 950                           -> Const<'tcx>
 951     {
 952         debug!("trans_constant({:?})", constant);
 953         let ty = self.monomorphize(&constant.ty);
 954         let result = match constant.literal.clone() {
 955             mir::Literal::Item { def_id, substs } => {
 956                 let substs = self.monomorphize(&substs);
 957                 MirConstContext::trans_def(bcx.ccx, def_id, substs, IndexVec::new())
 958             }
 959             mir::Literal::Promoted { index } => {
 960                 let mir = &self.mir.promoted[index];
 961                 MirConstContext::new(bcx.ccx, mir, self.param_substs, IndexVec::new()).trans()
 962             }
 963             mir::Literal::Value { value } => {
 964                 Ok(Const::from_constval(bcx.ccx, value, ty))
 965             }
 966         };
 967
 968         let result = result.unwrap_or_else(|_| {
 969             // We've errored, so we don't have to produce working code.
 970             let llty = type_of::type_of(bcx.ccx, ty);
 971             Const::new(C_undef(llty), ty)
 972         });
 973
 974         debug!("trans_constant({:?}) = {:?}", constant, result);
 975         result
 976     }
 977 }
 978
 979
 980 pub fn trans_static_initializer<'a, 'tcx>(
 981     ccx: &CrateContext<'a, 'tcx>,
 982     def_id: DefId)
 983     -> Result<ValueRef, ConstEvalErr<'tcx>>
 984 {
 985     MirConstContext::trans_def(ccx, def_id, Substs::empty(), IndexVec::new())
 986         .map(|c| c.llval)
 987 }
 988
 989 /// Construct a constant value, suitable for initializing a
 990 /// GlobalVariable, given a case and constant values for its fields.
 991 /// Note that this may have a different LLVM type (and different
 992 /// alignment!) from the representation's `type_of`, so it needs a
 993 /// pointer cast before use.
 994 ///
 995 /// The LLVM type system does not directly support unions, and only
 996 /// pointers can be bitcast, so a constant (and, by extension, the
 997 /// GlobalVariable initialized by it) will have a type that can vary
 998 /// depending on which case of an enum it is.
 999 ///
1000 /// To understand the alignment situation, consider `enum E { V64(u64),
1001 /// V32(u32, u32) }` on Windows.  The type has 8-byte alignment to
1002 /// accommodate the u64, but `V32(x, y)` would have LLVM type `{i32,
1003 /// i32, i32}`, which is 4-byte aligned.
1004 ///
1005 /// Currently the returned value has the same size as the type, but
1006 /// this could be changed in the future to avoid allocating unnecessary
1007 /// space after values of shorter-than-maximum cases.
1008 fn trans_const<'a, 'tcx>(
1009     ccx: &CrateContext<'a, 'tcx>,
1010     t: Ty<'tcx>,
1011     kind: &mir::AggregateKind,
1012     vals: &[ValueRef]
1013 ) -> ValueRef {
1014     let l = ccx.layout_of(t);
1015     let variant_index = match *kind {
1016         mir::AggregateKind::Adt(_, index, _, _) => index,
1017         _ => 0,
1018     };
1019     match *l {
1020         layout::CEnum { discr: d, min, max, .. } => {
1021             let discr = match *kind {
1022                 mir::AggregateKind::Adt(adt_def, _, _, _) => {
1023                     adt_def.discriminant_for_variant(ccx.tcx(), variant_index)
1024                            .to_u128_unchecked() as u64
1025                 },
1026                 _ => 0,
1027             };
1028             assert_eq!(vals.len(), 0);
1029             adt::assert_discr_in_range(min, max, discr);
1030             C_integral(Type::from_integer(ccx, d), discr, true)
1031         }
1032         layout::General { discr: d, ref variants, .. } => {
1033             let variant = &variants[variant_index];
1034             let lldiscr = C_integral(Type::from_integer(ccx, d), variant_index as u64, true);
1035             let mut vals_with_discr = vec![lldiscr];
1036             vals_with_discr.extend_from_slice(vals);
1037             let mut contents = build_const_struct(ccx, &variant, &vals_with_discr[..]);
1038             let needed_padding = l.size(ccx).bytes() - variant.stride().bytes();
1039             if needed_padding > 0 {
1040                 contents.push(padding(ccx, needed_padding));
1041             }
1042             C_struct(ccx, &contents[..], false)
1043         }
1044         layout::UntaggedUnion { ref variants, .. }=> {
1045             assert_eq!(variant_index, 0);
1046             let contents = build_const_union(ccx, variants, vals[0]);
1047             C_struct(ccx, &contents, variants.packed)
1048         }
1049         layout::Univariant { ref variant, .. } => {
1050             assert_eq!(variant_index, 0);
1051             let contents = build_const_struct(ccx, &variant, vals);
1052             C_struct(ccx, &contents[..], variant.packed)
1053         }
1054         layout::Vector { .. } => {
1055             C_vector(vals)
1056         }
1057         layout::RawNullablePointer { nndiscr, .. } => {
1058             if variant_index as u64 == nndiscr {
1059                 assert_eq!(vals.len(), 1);
1060                 vals[0]
1061             } else {
1062                 C_null(type_of::type_of(ccx, t))
1063             }
1064         }
1065         layout::StructWrappedNullablePointer { ref nonnull, nndiscr, .. } => {
1066             if variant_index as u64 == nndiscr {
1067                 C_struct(ccx, &build_const_struct(ccx, &nonnull, vals), false)
1068             } else {
1069                 // Always use null even if it's not the `discrfield`th
1070                 // field; see #8506.
1071                 C_null(type_of::type_of(ccx, t))
1072             }
1073         }
1074         _ => bug!("trans_const: cannot handle type {} repreented as {:#?}", t, l)
1075     }
1076 }
1077
1078 /// Building structs is a little complicated, because we might need to
1079 /// insert padding if a field's value is less aligned than its type.
1080 ///
1081 /// Continuing the example from `trans_const`, a value of type `(u32,
1082 /// E)` should have the `E` at offset 8, but if that field's
1083 /// initializer is 4-byte aligned then simply translating the tuple as
1084 /// a two-element struct will locate it at offset 4, and accesses to it
1085 /// will read the wrong memory.
1086 fn build_const_struct<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
1087                                 st: &layout::Struct,
1088                                 vals: &[ValueRef])
1089                                 -> Vec<ValueRef> {
1090     assert_eq!(vals.len(), st.offsets.len());
1091
1092     if vals.len() == 0 {
1093         return Vec::new();
1094     }
1095
1096     // offset of current value
1097     let mut offset = 0;
1098     let mut cfields = Vec::new();
1099     cfields.reserve(st.offsets.len()*2);
1100
1101     let parts = st.field_index_by_increasing_offset().map(|i| {
1102         (&vals[i], st.offsets[i].bytes())
1103     });
1104     for (&val, target_offset) in parts {
1105         if offset < target_offset {
1106             cfields.push(padding(ccx, target_offset - offset));
1107             offset = target_offset;
1108         }
1109         assert!(!is_undef(val));
1110         cfields.push(val);
1111         offset += machine::llsize_of_alloc(ccx, val_ty(val));
1112     }
1113
1114     if offset < st.stride().bytes() {
1115         cfields.push(padding(ccx, st.stride().bytes() - offset));
1116     }
1117
1118     cfields
1119 }
1120
1121 fn build_const_union<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
1122                                un: &layout::Union,
1123                                field_val: ValueRef)
1124                                -> Vec<ValueRef> {
1125     let mut cfields = vec![field_val];
1126
1127     let offset = machine::llsize_of_alloc(ccx, val_ty(field_val));
1128     let size = un.stride().bytes();
1129     if offset != size {
1130         cfields.push(padding(ccx, size - offset));
1131     }
1132
1133     cfields
1134 }
1135
1136 fn padding(ccx: &CrateContext, size: u64) -> ValueRef {
1137     C_undef(Type::array(&Type::i8(ccx), size))
1138 }