src/discriminant.rs

   1 //! Handling of enum discriminants
   2 //!
   3 //! Adapted from <https://github.com/rust-lang/rust/blob/d760df5aea483aae041c9a241e7acacf48f75035/src/librustc_codegen_ssa/mir/place.rs>
   4
   5 use rustc_target::abi::{Int, TagEncoding, Variants};
   6
   7 use crate::prelude::*;
   8
   9 pub(crate) fn codegen_set_discriminant<'tcx>(
  10     fx: &mut FunctionCx<'_, 'tcx, impl Module>,
  11     place: CPlace<'tcx>,
  12     variant_index: VariantIdx,
  13 ) {
  14     let layout = place.layout();
  15     if layout.for_variant(fx, variant_index).abi.is_uninhabited() {
  16         return;
  17     }
  18     match layout.variants {
  19         Variants::Single { index } => {
  20             assert_eq!(index, variant_index);
  21         }
  22         Variants::Multiple {
  23             tag: _,
  24             tag_field,
  25             tag_encoding: TagEncoding::Direct,
  26             variants: _,
  27         } => {
  28             let ptr = place.place_field(fx, mir::Field::new(tag_field));
  29             let to = layout
  30                 .ty
  31                 .discriminant_for_variant(fx.tcx, variant_index)
  32                 .unwrap()
  33                 .val
  34                 .into();
  35             let discr = CValue::const_val(fx, ptr.layout(), to);
  36             ptr.write_cvalue(fx, discr);
  37         }
  38         Variants::Multiple {
  39             tag: _,
  40             tag_field,
  41             tag_encoding:
  42                 TagEncoding::Niche {
  43                     dataful_variant,
  44                     ref niche_variants,
  45                     niche_start,
  46                 },
  47             variants: _,
  48         } => {
  49             if variant_index != dataful_variant {
  50                 let niche = place.place_field(fx, mir::Field::new(tag_field));
  51                 let niche_value = variant_index.as_u32() - niche_variants.start().as_u32();
  52                 let niche_value = u128::from(niche_value).wrapping_add(niche_start);
  53                 let niche_llval = CValue::const_val(fx, niche.layout(), niche_value.into());
  54                 niche.write_cvalue(fx, niche_llval);
  55             }
  56         }
  57     }
  58 }
  59
  60 pub(crate) fn codegen_get_discriminant<'tcx>(
  61     fx: &mut FunctionCx<'_, 'tcx, impl Module>,
  62     value: CValue<'tcx>,
  63     dest_layout: TyAndLayout<'tcx>,
  64 ) -> CValue<'tcx> {
  65     let layout = value.layout();
  66
  67     if layout.abi == Abi::Uninhabited {
  68         return trap_unreachable_ret_value(
  69             fx,
  70             dest_layout,
  71             "[panic] Tried to get discriminant for uninhabited type.",
  72         );
  73     }
  74
  75     let (tag_scalar, tag_field, tag_encoding) = match &layout.variants {
  76         Variants::Single { index } => {
  77             let discr_val = layout
  78                 .ty
  79                 .discriminant_for_variant(fx.tcx, *index)
  80                 .map_or(u128::from(index.as_u32()), |discr| discr.val);
  81             return CValue::const_val(fx, dest_layout, discr_val.into());
  82         }
  83         Variants::Multiple {
  84             tag,
  85             tag_field,
  86             tag_encoding,
  87             variants: _,
  88         } => (tag, *tag_field, tag_encoding),
  89     };
  90
  91     let cast_to = fx.clif_type(dest_layout.ty).unwrap();
  92
  93     // Read the tag/niche-encoded discriminant from memory.
  94     let tag = value.value_field(fx, mir::Field::new(tag_field));
  95     let tag = tag.load_scalar(fx);
  96
  97     // Decode the discriminant (specifically if it's niche-encoded).
  98     match *tag_encoding {
  99         TagEncoding::Direct => {
 100             let signed = match tag_scalar.value {
 101                 Int(_, signed) => signed,
 102                 _ => false,
 103             };
 104             let val = clif_intcast(fx, tag, cast_to, signed);
 105             CValue::by_val(val, dest_layout)
 106         }
 107         TagEncoding::Niche {
 108             dataful_variant,
 109             ref niche_variants,
 110             niche_start,
 111         } => {
 112             // Rebase from niche values to discriminants, and check
 113             // whether the result is in range for the niche variants.
 114
 115             // We first compute the "relative discriminant" (wrt `niche_variants`),
 116             // that is, if `n = niche_variants.end() - niche_variants.start()`,
 117             // we remap `niche_start..=niche_start + n` (which may wrap around)
 118             // to (non-wrap-around) `0..=n`, to be able to check whether the
 119             // discriminant corresponds to a niche variant with one comparison.
 120             // We also can't go directly to the (variant index) discriminant
 121             // and check that it is in the range `niche_variants`, because
 122             // that might not fit in the same type, on top of needing an extra
 123             // comparison (see also the comment on `let niche_discr`).
 124             let relative_discr = if niche_start == 0 {
 125                 tag
 126             } else {
 127                 // FIXME handle niche_start > i64::MAX
 128                 fx.bcx
 129                     .ins()
 130                     .iadd_imm(tag, -i64::try_from(niche_start).unwrap())
 131             };
 132             let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32();
 133             let is_niche = {
 134                 codegen_icmp_imm(
 135                     fx,
 136                     IntCC::UnsignedLessThanOrEqual,
 137                     relative_discr,
 138                     i128::from(relative_max),
 139                 )
 140             };
 141
 142             // NOTE(eddyb) this addition needs to be performed on the final
 143             // type, in case the niche itself can't represent all variant
 144             // indices (e.g. `u8` niche with more than `256` variants,
 145             // but enough uninhabited variants so that the remaining variants
 146             // fit in the niche).
 147             // In other words, `niche_variants.end - niche_variants.start`
 148             // is representable in the niche, but `niche_variants.end`
 149             // might not be, in extreme cases.
 150             let niche_discr = {
 151                 let relative_discr = if relative_max == 0 {
 152                     // HACK(eddyb) since we have only one niche, we know which
 153                     // one it is, and we can avoid having a dynamic value here.
 154                     fx.bcx.ins().iconst(cast_to, 0)
 155                 } else {
 156                     clif_intcast(fx, relative_discr, cast_to, false)
 157                 };
 158                 fx.bcx
 159                     .ins()
 160                     .iadd_imm(relative_discr, i64::from(niche_variants.start().as_u32()))
 161             };
 162
 163             let dataful_variant = fx
 164                 .bcx
 165                 .ins()
 166                 .iconst(cast_to, i64::from(dataful_variant.as_u32()));
 167             let discr = fx.bcx.ins().select(is_niche, niche_discr, dataful_variant);
 168             CValue::by_val(discr, dest_layout)
 169         }
 170     }
 171 }