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|
//! Handling of enum discriminants
//!
//! Adapted from <https://github.com/rust-lang/rust/blob/31c0645b9d2539f47eecb096142474b29dc542f7/compiler/rustc_codegen_ssa/src/mir/place.rs>
//! (<https://github.com/rust-lang/rust/pull/104535>)
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);
}
}
}
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