src/librustc_codegen_ssa/glue.rs

   1 //!
   2 //
   3 // Code relating to drop glue.
   4
   5 use crate::common::IntPredicate;
   6 use crate::meth;
   7 use crate::traits::*;
   8 use rustc_middle::ty::{self, Ty};
   9
  10 pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
  11     bx: &mut Bx,
  12     t: Ty<'tcx>,
  13     info: Option<Bx::Value>,
  14 ) -> (Bx::Value, Bx::Value) {
  15     let layout = bx.layout_of(t);
  16     debug!("size_and_align_of_dst(ty={}, info={:?}): layout: {:?}", t, info, layout);
  17     if !layout.is_unsized() {
  18         let size = bx.const_usize(layout.size.bytes());
  19         let align = bx.const_usize(layout.align.abi.bytes());
  20         return (size, align);
  21     }
  22     match t.kind {
  23         ty::Dynamic(..) => {
  24             // load size/align from vtable
  25             let vtable = info.unwrap();
  26             (meth::SIZE.get_usize(bx, vtable), meth::ALIGN.get_usize(bx, vtable))
  27         }
  28         ty::Slice(_) | ty::Str => {
  29             let unit = layout.field(bx, 0);
  30             // The info in this case is the length of the str, so the size is that
  31             // times the unit size.
  32             (
  33                 bx.mul(info.unwrap(), bx.const_usize(unit.size.bytes())),
  34                 bx.const_usize(unit.align.abi.bytes()),
  35             )
  36         }
  37         _ => {
  38             // First get the size of all statically known fields.
  39             // Don't use size_of because it also rounds up to alignment, which we
  40             // want to avoid, as the unsized field's alignment could be smaller.
  41             assert!(!t.is_simd());
  42             debug!("DST {} layout: {:?}", t, layout);
  43
  44             let i = layout.fields.count() - 1;
  45             let sized_size = layout.fields.offset(i).bytes();
  46             let sized_align = layout.align.abi.bytes();
  47             debug!("DST {} statically sized prefix size: {} align: {}", t, sized_size, sized_align);
  48             let sized_size = bx.const_usize(sized_size);
  49             let sized_align = bx.const_usize(sized_align);
  50
  51             // Recurse to get the size of the dynamically sized field (must be
  52             // the last field).
  53             let field_ty = layout.field(bx, i).ty;
  54             let (unsized_size, mut unsized_align) = size_and_align_of_dst(bx, field_ty, info);
  55
  56             // FIXME (#26403, #27023): We should be adding padding
  57             // to `sized_size` (to accommodate the `unsized_align`
  58             // required of the unsized field that follows) before
  59             // summing it with `sized_size`. (Note that since #26403
  60             // is unfixed, we do not yet add the necessary padding
  61             // here. But this is where the add would go.)
  62
  63             // Return the sum of sizes and max of aligns.
  64             let size = bx.add(sized_size, unsized_size);
  65
  66             // Packed types ignore the alignment of their fields.
  67             if let ty::Adt(def, _) = t.kind {
  68                 if def.repr.packed() {
  69                     unsized_align = sized_align;
  70                 }
  71             }
  72
  73             // Choose max of two known alignments (combined value must
  74             // be aligned according to more restrictive of the two).
  75             let align = match (
  76                 bx.const_to_opt_u128(sized_align, false),
  77                 bx.const_to_opt_u128(unsized_align, false),
  78             ) {
  79                 (Some(sized_align), Some(unsized_align)) => {
  80                     // If both alignments are constant, (the sized_align should always be), then
  81                     // pick the correct alignment statically.
  82                     bx.const_usize(std::cmp::max(sized_align, unsized_align) as u64)
  83                 }
  84                 _ => {
  85                     let cmp = bx.icmp(IntPredicate::IntUGT, sized_align, unsized_align);
  86                     bx.select(cmp, sized_align, unsized_align)
  87                 }
  88             };
  89
  90             // Issue #27023: must add any necessary padding to `size`
  91             // (to make it a multiple of `align`) before returning it.
  92             //
  93             // Namely, the returned size should be, in C notation:
  94             //
  95             //   `size + ((size & (align-1)) ? align : 0)`
  96             //
  97             // emulated via the semi-standard fast bit trick:
  98             //
  99             //   `(size + (align-1)) & -align`
 100             let one = bx.const_usize(1);
 101             let addend = bx.sub(align, one);
 102             let add = bx.add(size, addend);
 103             let neg = bx.neg(align);
 104             let size = bx.and(add, neg);
 105
 106             (size, align)
 107         }
 108     }
 109 }