diff options
Diffstat (limited to 'compiler/rustc_codegen_cranelift/src/discriminant.rs')
| -rw-r--r-- | compiler/rustc_codegen_cranelift/src/discriminant.rs | 207 |
1 files changed, 161 insertions, 46 deletions
diff --git a/compiler/rustc_codegen_cranelift/src/discriminant.rs b/compiler/rustc_codegen_cranelift/src/discriminant.rs index 97b395bcd..3cbf313ad 100644 --- a/compiler/rustc_codegen_cranelift/src/discriminant.rs +++ b/compiler/rustc_codegen_cranelift/src/discriminant.rs @@ -1,6 +1,7 @@ //! Handling of enum discriminants //! -//! Adapted from <https://github.com/rust-lang/rust/blob/d760df5aea483aae041c9a241e7acacf48f75035/src/librustc_codegen_ssa/mir/place.rs> +//! 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}; @@ -47,13 +48,19 @@ pub(crate) fn codegen_set_discriminant<'tcx>( } => { if variant_index != untagged_variant { let niche = place.place_field(fx, mir::Field::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 = 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); + 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); } } @@ -96,6 +103,7 @@ pub(crate) fn codegen_get_discriminant<'tcx>( } }; + let cast_to_size = dest_layout.layout.size(); let cast_to = fx.clif_type(dest_layout.ty).unwrap(); // Read the tag/niche-encoded discriminant from memory. @@ -114,21 +122,128 @@ pub(crate) fn codegen_get_discriminant<'tcx>( dest.write_cvalue(fx, res); } TagEncoding::Niche { untagged_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 + let tag_size = tag_scalar.size(fx); + let max_unsigned = tag_size.unsigned_int_max(); + let max_signed = tag_size.signed_int_max() as u128; + let min_signed = max_signed + 1; + let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32(); + let niche_end = niche_start.wrapping_add(relative_max as u128) & max_unsigned; + let range = tag_scalar.valid_range(fx); + + let sle = |lhs: u128, rhs: u128| -> bool { + // Signed and unsigned comparisons give the same results, + // except that in signed comparisons an integer with the + // sign bit set is less than one with the sign bit clear. + // Toggle the sign bit to do a signed comparison. + (lhs ^ min_signed) <= (rhs ^ min_signed) + }; + + // 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. + + // Find the least and greatest values in `range`, considered + // both as signed and unsigned. + let (low_unsigned, high_unsigned) = + if range.start <= range.end { (range.start, range.end) } else { (0, max_unsigned) }; + let (low_signed, high_signed) = if sle(range.start, range.end) { + (range.start, range.end) } else { + (min_signed, max_signed) + }; + + let niches_ule = niche_start <= niche_end; + let niches_sle = sle(niche_start, niche_end); + let cast_smaller = cast_to_size <= tag_size; + + // In the algorithm above, we can change + // cast(relative_tag) + niche_variants.start() + // into + // cast(tag + (niche_variants.start() - niche_start)) + // if either the casted type is no larger than the original + // type, or if the niche values are contiguous (in either the + // signed or unsigned sense). + let can_incr = cast_smaller || niches_ule || niches_sle; + + let data_for_boundary_niche = || -> Option<(IntCC, u128)> { + if !can_incr { + None + } else if niche_start == low_unsigned { + Some((IntCC::UnsignedLessThanOrEqual, niche_end)) + } else if niche_end == high_unsigned { + Some((IntCC::UnsignedGreaterThanOrEqual, niche_start)) + } else if niche_start == low_signed { + Some((IntCC::SignedLessThanOrEqual, niche_end)) + } else if niche_end == high_signed { + Some((IntCC::SignedGreaterThanOrEqual, niche_start)) + } else { + None + } + }; + + 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 if let Some((predicate, constant)) = data_for_boundary_niche() { + // The niche values are either the lowest or the highest in + // `range`. We can avoid the first subtraction in the + // algorithm. + // The algorithm is now this: + // is_niche = tag <= niche_end + // discr = if is_niche { + // cast(tag + (niche_variants.start() - niche_start)) + // } else { + // untagged_variant + // } + // (the first line may instead be tag >= niche_start, + // and may be a signed or unsigned comparison) + // The arithmetic must be done before the cast, so we can + // have the correct wrapping behavior. See issue #104519 for + // the consequences of getting this wrong. + let is_niche = codegen_icmp_imm(fx, predicate, tag, constant as i128); + let delta = (niche_variants.start().as_u32() as u128).wrapping_sub(niche_start); + let incr_tag = if delta == 0 { + tag + } else { + let delta = match fx.bcx.func.dfg.value_type(tag) { + 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(tag, delta) + }; + + let cast_tag = clif_intcast(fx, incr_tag, cast_to, !niches_ule); + + (is_niche, cast_tag, 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); @@ -138,40 +253,40 @@ pub(crate) fn codegen_get_discriminant<'tcx>( } 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( + 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) }; - // 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) + 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_imm(relative_discr, i64::from(niche_variants.start().as_u32())) + fx.bcx.ins().iadd(tagged_discr, delta) }; - let untagged_variant = - fx.bcx.ins().iconst(cast_to, i64::from(untagged_variant.as_u32())); - let discr = fx.bcx.ins().select(is_niche, niche_discr, untagged_variant); + 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); } |