//! Handling of enum discriminants //! //! Adapted from //! () use rustc_target::abi::{Int, TagEncoding, Variants}; use crate::prelude::*; pub(crate) fn codegen_set_discriminant<'tcx>( fx: &mut FunctionCx<'_, '_, 'tcx>, place: CPlace<'tcx>, variant_index: VariantIdx, ) { let layout = place.layout(); if layout.for_variant(fx, variant_index).abi.is_uninhabited() { return; } match layout.variants { Variants::Single { index } => { assert_eq!(index, variant_index); } Variants::Multiple { tag: _, tag_field, tag_encoding: TagEncoding::Direct, variants: _, } => { let ptr = place.place_field(fx, FieldIdx::new(tag_field)); let to = layout.ty.discriminant_for_variant(fx.tcx, variant_index).unwrap().val; let to = if ptr.layout().abi.is_signed() { ty::ScalarInt::try_from_int( ptr.layout().size.sign_extend(to) as i128, ptr.layout().size, ) .unwrap() } else { ty::ScalarInt::try_from_uint(to, ptr.layout().size).unwrap() }; let discr = CValue::const_val(fx, ptr.layout(), to); ptr.write_cvalue(fx, discr); } Variants::Multiple { tag: _, tag_field, tag_encoding: TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start }, variants: _, } => { if variant_index != untagged_variant { let niche = place.place_field(fx, FieldIdx::new(tag_field)); let niche_type = fx.clif_type(niche.layout().ty).unwrap(); let niche_value = variant_index.as_u32() - niche_variants.start().as_u32(); let niche_value = (niche_value as u128).wrapping_add(niche_start); let niche_value = match niche_type { types::I128 => { let lsb = fx.bcx.ins().iconst(types::I64, niche_value as u64 as i64); let msb = fx.bcx.ins().iconst(types::I64, (niche_value >> 64) as u64 as i64); fx.bcx.ins().iconcat(lsb, msb) } ty => fx.bcx.ins().iconst(ty, niche_value as i64), }; let niche_llval = CValue::by_val(niche_value, niche.layout()); niche.write_cvalue(fx, niche_llval); } } } } pub(crate) fn codegen_get_discriminant<'tcx>( fx: &mut FunctionCx<'_, '_, 'tcx>, dest: CPlace<'tcx>, value: CValue<'tcx>, dest_layout: TyAndLayout<'tcx>, ) { let layout = value.layout(); if layout.abi.is_uninhabited() { return; } let (tag_scalar, tag_field, tag_encoding) = match &layout.variants { Variants::Single { index } => { let discr_val = layout .ty .discriminant_for_variant(fx.tcx, *index) .map_or(u128::from(index.as_u32()), |discr| discr.val); let discr_val = if dest_layout.abi.is_signed() { ty::ScalarInt::try_from_int( dest_layout.size.sign_extend(discr_val) as i128, dest_layout.size, ) .unwrap() } else { ty::ScalarInt::try_from_uint(discr_val, dest_layout.size).unwrap() }; let res = CValue::const_val(fx, dest_layout, discr_val); dest.write_cvalue(fx, res); return; } Variants::Multiple { tag, tag_field, tag_encoding, variants: _ } => { (tag, *tag_field, tag_encoding) } }; let cast_to = fx.clif_type(dest_layout.ty).unwrap(); // Read the tag/niche-encoded discriminant from memory. let tag = value.value_field(fx, FieldIdx::new(tag_field)); let tag = tag.load_scalar(fx); // Decode the discriminant (specifically if it's niche-encoded). match *tag_encoding { TagEncoding::Direct => { let signed = match tag_scalar.primitive() { Int(_, signed) => signed, _ => false, }; let val = clif_intcast(fx, tag, cast_to, signed); let res = CValue::by_val(val, dest_layout); dest.write_cvalue(fx, res); } TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start } => { let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32(); // We have a subrange `niche_start..=niche_end` inside `range`. // If the value of the tag is inside this subrange, it's a // "niche value", an increment of the discriminant. Otherwise it // indicates the untagged variant. // A general algorithm to extract the discriminant from the tag // is: // relative_tag = tag - niche_start // is_niche = relative_tag <= (ule) relative_max // discr = if is_niche { // cast(relative_tag) + niche_variants.start() // } else { // untagged_variant // } // However, we will likely be able to emit simpler code. let (is_niche, tagged_discr, delta) = if relative_max == 0 { // Best case scenario: only one tagged variant. This will // likely become just a comparison and a jump. // The algorithm is: // is_niche = tag == niche_start // discr = if is_niche { // niche_start // } else { // untagged_variant // } let is_niche = codegen_icmp_imm(fx, IntCC::Equal, tag, niche_start as i128); let tagged_discr = fx.bcx.ins().iconst(cast_to, niche_variants.start().as_u32() as i64); (is_niche, tagged_discr, 0) } else { // The special cases don't apply, so we'll have to go with // the general algorithm. let niche_start = match fx.bcx.func.dfg.value_type(tag) { types::I128 => { let lsb = fx.bcx.ins().iconst(types::I64, niche_start as u64 as i64); let msb = fx.bcx.ins().iconst(types::I64, (niche_start >> 64) as u64 as i64); fx.bcx.ins().iconcat(lsb, msb) } ty => fx.bcx.ins().iconst(ty, niche_start as i64), }; let relative_discr = fx.bcx.ins().isub(tag, niche_start); let cast_tag = clif_intcast(fx, relative_discr, cast_to, false); let is_niche = crate::common::codegen_icmp_imm( fx, IntCC::UnsignedLessThanOrEqual, relative_discr, i128::from(relative_max), ); (is_niche, cast_tag, niche_variants.start().as_u32() as u128) }; let tagged_discr = if delta == 0 { tagged_discr } else { let delta = match cast_to { types::I128 => { let lsb = fx.bcx.ins().iconst(types::I64, delta as u64 as i64); let msb = fx.bcx.ins().iconst(types::I64, (delta >> 64) as u64 as i64); fx.bcx.ins().iconcat(lsb, msb) } ty => fx.bcx.ins().iconst(ty, delta as i64), }; fx.bcx.ins().iadd(tagged_discr, delta) }; let untagged_variant = if cast_to == types::I128 { let zero = fx.bcx.ins().iconst(types::I64, 0); let untagged_variant = fx.bcx.ins().iconst(types::I64, i64::from(untagged_variant.as_u32())); fx.bcx.ins().iconcat(untagged_variant, zero) } else { fx.bcx.ins().iconst(cast_to, i64::from(untagged_variant.as_u32())) }; let discr = fx.bcx.ins().select(is_niche, tagged_discr, untagged_variant); let res = CValue::by_val(discr, dest_layout); dest.write_cvalue(fx, res); } } }