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Diffstat (limited to 'compiler/rustc_codegen_cranelift/src/discriminant.rs')
| -rw-r--r-- | compiler/rustc_codegen_cranelift/src/discriminant.rs | 176 |
1 files changed, 176 insertions, 0 deletions
diff --git a/compiler/rustc_codegen_cranelift/src/discriminant.rs b/compiler/rustc_codegen_cranelift/src/discriminant.rs new file mode 100644 index 000000000..f619bb5ed --- /dev/null +++ b/compiler/rustc_codegen_cranelift/src/discriminant.rs @@ -0,0 +1,176 @@ +//! Handling of enum discriminants +//! +//! Adapted from <https://github.com/rust-lang/rust/blob/d760df5aea483aae041c9a241e7acacf48f75035/src/librustc_codegen_ssa/mir/place.rs> + +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, mir::Field::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 { dataful_variant, ref niche_variants, niche_start }, + variants: _, + } => { + if variant_index != dataful_variant { + let niche = place.place_field(fx, mir::Field::new(tag_field)); + let niche_value = variant_index.as_u32() - niche_variants.start().as_u32(); + let niche_value = ty::ScalarInt::try_from_uint( + u128::from(niche_value).wrapping_add(niche_start), + niche.layout().size, + ) + .unwrap(); + let niche_llval = CValue::const_val(fx, niche.layout(), niche_value); + niche.write_cvalue(fx, niche_llval); + } + } + } +} + +pub(crate) fn codegen_get_discriminant<'tcx>( + fx: &mut FunctionCx<'_, '_, 'tcx>, + value: CValue<'tcx>, + dest_layout: TyAndLayout<'tcx>, +) -> CValue<'tcx> { + let layout = value.layout(); + + if layout.abi == Abi::Uninhabited { + let true_ = fx.bcx.ins().iconst(types::I32, 1); + fx.bcx.ins().trapnz(true_, TrapCode::UnreachableCodeReached); + // Return a dummy value + return CValue::by_ref(Pointer::const_addr(fx, 0), dest_layout); + } + + 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() + }; + return CValue::const_val(fx, dest_layout, discr_val); + } + 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, mir::Field::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); + CValue::by_val(val, dest_layout) + } + TagEncoding::Niche { dataful_variant, ref niche_variants, niche_start } => { + // Rebase from niche values to discriminants, and check + // whether the result is in range for the niche variants. + + // We first compute the "relative discriminant" (wrt `niche_variants`), + // that is, if `n = niche_variants.end() - niche_variants.start()`, + // we remap `niche_start..=niche_start + n` (which may wrap around) + // to (non-wrap-around) `0..=n`, to be able to check whether the + // discriminant corresponds to a niche variant with one comparison. + // We also can't go directly to the (variant index) discriminant + // and check that it is in the range `niche_variants`, because + // that might not fit in the same type, on top of needing an extra + // comparison (see also the comment on `let niche_discr`). + let relative_discr = if niche_start == 0 { + tag + } else { + 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), + }; + fx.bcx.ins().isub(tag, niche_start) + }; + let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32(); + let is_niche = { + codegen_icmp_imm( + fx, + IntCC::UnsignedLessThanOrEqual, + relative_discr, + i128::from(relative_max), + ) + }; + + // NOTE(eddyb) this addition needs to be performed on the final + // type, in case the niche itself can't represent all variant + // indices (e.g. `u8` niche with more than `256` variants, + // but enough uninhabited variants so that the remaining variants + // fit in the niche). + // In other words, `niche_variants.end - niche_variants.start` + // is representable in the niche, but `niche_variants.end` + // might not be, in extreme cases. + let niche_discr = { + let relative_discr = if relative_max == 0 { + // HACK(eddyb) since we have only one niche, we know which + // one it is, and we can avoid having a dynamic value here. + fx.bcx.ins().iconst(cast_to, 0) + } else { + clif_intcast(fx, relative_discr, cast_to, false) + }; + fx.bcx.ins().iadd_imm(relative_discr, i64::from(niche_variants.start().as_u32())) + }; + + let dataful_variant = fx.bcx.ins().iconst(cast_to, i64::from(dataful_variant.as_u32())); + let discr = fx.bcx.ins().select(is_niche, niche_discr, dataful_variant); + CValue::by_val(discr, dest_layout) + } + } +} |