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