1 //! Handling of enum discriminants
3 //! Adapted from https://github.com/rust-lang/rust/blob/d760df5aea483aae041c9a241e7acacf48f75035/src/librustc_codegen_ssa/mir/place.rs
5 use rustc_target::abi::{Int, TagEncoding, Variants};
9 pub(crate) fn codegen_set_discriminant<'tcx>(
10 fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
12 variant_index: VariantIdx,
14 let layout = place.layout();
15 if layout.for_variant(fx, variant_index).abi.is_uninhabited() {
18 match layout.variants {
19 Variants::Single { index } => {
20 assert_eq!(index, variant_index);
25 tag_encoding: TagEncoding::Direct,
28 let ptr = place.place_field(fx, mir::Field::new(tag_field));
31 .discriminant_for_variant(fx.tcx, variant_index)
34 let discr = CValue::const_val(fx, ptr.layout(), to);
35 ptr.write_cvalue(fx, discr);
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);
59 pub(crate) fn codegen_get_discriminant<'tcx>(
60 fx: &mut FunctionCx<'_, 'tcx, impl Backend>,
62 dest_layout: TyAndLayout<'tcx>,
64 let layout = value.layout();
66 if layout.abi == Abi::Uninhabited {
67 return trap_unreachable_ret_value(
70 "[panic] Tried to get discriminant for uninhabited type.",
74 let (tag_scalar, tag_field, tag_encoding) = match &layout.variants {
75 Variants::Single { index } => {
76 let discr_val = layout
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);
87 } => (tag, *tag_field, tag_encoding),
90 let cast_to = fx.clif_type(dest_layout.ty).unwrap();
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);
96 // Decode the discriminant (specifically if it's niche-encoded).
98 TagEncoding::Direct => {
99 let signed = match tag_scalar.value {
100 Int(_, signed) => signed,
103 let val = clif_intcast(fx, tag, cast_to, signed);
104 CValue::by_val(val, dest_layout)
111 // Rebase from niche values to discriminants, and check
112 // whether the result is in range for the niche variants.
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 {
126 // FIXME handle niche_start > i64::MAX
129 .iadd_imm(tag, -i64::try_from(niche_start).unwrap())
131 let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32();
135 IntCC::UnsignedLessThanOrEqual,
137 i128::from(relative_max),
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.
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)
155 clif_intcast(fx, relative_discr, cast_to, false)
159 .iadd_imm(relative_discr, i64::from(niche_variants.start().as_u32()))
162 let dataful_variant = fx
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)