rustc_codegen_ssa: Better code generation for niche discriminants.
In some cases we can avoid arithmetic before checking whether a niche represents an untagged variant. This is relevant to #101872
This commit is contained in:
Michael Benfield
parent
5b82ea74b7
commit
51918dcc51
@ -209,7 +209,9 @@ pub fn codegen_get_discr<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
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bx: &mut Bx,
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cast_to: Ty<'tcx>,
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) -> V {
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let cast_to = bx.cx().immediate_backend_type(bx.cx().layout_of(cast_to));
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let cast_to_layout = bx.cx().layout_of(cast_to);
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let cast_to_size = cast_to_layout.layout.size();
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let cast_to = bx.cx().immediate_backend_type(cast_to_layout);
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if self.layout.abi.is_uninhabited() {
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return bx.cx().const_undef(cast_to);
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}
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@ -229,7 +231,8 @@ pub fn codegen_get_discr<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
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// Read the tag/niche-encoded discriminant from memory.
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let tag = self.project_field(bx, tag_field);
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let tag = bx.load_operand(tag);
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let tag_op = bx.load_operand(tag);
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let tag_imm = tag_op.immediate();
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// Decode the discriminant (specifically if it's niche-encoded).
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match *tag_encoding {
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@ -242,68 +245,161 @@ pub fn codegen_get_discr<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
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Int(_, signed) => !tag_scalar.is_bool() && signed,
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_ => false,
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};
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bx.intcast(tag.immediate(), cast_to, signed)
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bx.intcast(tag_imm, cast_to, signed)
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}
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TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start } => {
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// Rebase from niche values to discriminants, and check
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// whether the result is in range for the niche variants.
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let niche_llty = bx.cx().immediate_backend_type(tag.layout);
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let tag = tag.immediate();
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// We first compute the "relative discriminant" (wrt `niche_variants`),
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// that is, if `n = niche_variants.end() - niche_variants.start()`,
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// we remap `niche_start..=niche_start + n` (which may wrap around)
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// to (non-wrap-around) `0..=n`, to be able to check whether the
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// discriminant corresponds to a niche variant with one comparison.
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// We also can't go directly to the (variant index) discriminant
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// and check that it is in the range `niche_variants`, because
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// that might not fit in the same type, on top of needing an extra
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// comparison (see also the comment on `let niche_discr`).
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let relative_discr = if niche_start == 0 {
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// Avoid subtracting `0`, which wouldn't work for pointers.
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// FIXME(eddyb) check the actual primitive type here.
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tag
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// Cast to an integer so we don't have to treat a pointer as a
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// special case.
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let (tag, tag_llty) = if tag_scalar.primitive().is_ptr() {
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let t = bx.type_isize();
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let tag = bx.ptrtoint(tag_imm, t);
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(tag, t)
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} else {
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bx.sub(tag, bx.cx().const_uint_big(niche_llty, niche_start))
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(tag_imm, bx.cx().immediate_backend_type(tag_op.layout))
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};
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let tag_size = tag_scalar.size(bx.cx());
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let max_unsigned = tag_size.unsigned_int_max();
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let max_signed = tag_size.signed_int_max() as u128;
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let min_signed = max_signed + 1;
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let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32();
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let is_niche = if relative_max == 0 {
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// Avoid calling `const_uint`, which wouldn't work for pointers.
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// Also use canonical == 0 instead of non-canonical u<= 0.
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// FIXME(eddyb) check the actual primitive type here.
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bx.icmp(IntPredicate::IntEQ, relative_discr, bx.cx().const_null(niche_llty))
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let niche_end = niche_start.wrapping_add(relative_max as u128) & max_unsigned;
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let range = tag_scalar.valid_range(bx.cx());
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let sle = |lhs: u128, rhs: u128| -> bool {
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// Signed and unsigned comparisons give the same results,
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// except that in signed comparisons an integer with the
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// sign bit set is less than one with the sign bit clear.
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// Toggle the sign bit to do a signed comparison.
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(lhs ^ min_signed) <= (rhs ^ min_signed)
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};
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// We have a subrange `niche_start..=niche_end` inside `range`.
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// If the value of the tag is inside this subrange, it's a
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// "niche value", an increment of the discriminant. Otherwise it
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// indicates the untagged variant.
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// A general algorithm to extract the discriminant from the tag
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// is:
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// relative_tag = tag - niche_start
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// is_niche = relative_tag <= (ule) relative_max
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// discr = if is_niche {
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// cast(relative_tag) + niche_variants.start()
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// } else {
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// untagged_variant
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// }
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// However, we will likely be able to emit simpler code.
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// Find the least and greatest values in `range`, considered
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// both as signed and unsigned.
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let (low_unsigned, high_unsigned) = if range.start <= range.end {
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(range.start, range.end)
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} else {
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let relative_max = bx.cx().const_uint(niche_llty, relative_max as u64);
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bx.icmp(IntPredicate::IntULE, relative_discr, relative_max)
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(0, max_unsigned)
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};
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let (low_signed, high_signed) = if sle(range.start, range.end) {
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(range.start, range.end)
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} else {
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(min_signed, max_signed)
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};
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// NOTE(eddyb) this addition needs to be performed on the final
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// type, in case the niche itself can't represent all variant
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// indices (e.g. `u8` niche with more than `256` variants,
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// but enough uninhabited variants so that the remaining variants
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// fit in the niche).
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// In other words, `niche_variants.end - niche_variants.start`
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// is representable in the niche, but `niche_variants.end`
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// might not be, in extreme cases.
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let niche_discr = {
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let relative_discr = if relative_max == 0 {
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// HACK(eddyb) since we have only one niche, we know which
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// one it is, and we can avoid having a dynamic value here.
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bx.cx().const_uint(cast_to, 0)
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let niches_ule = niche_start <= niche_end;
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let niches_sle = sle(niche_start, niche_end);
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let cast_smaller = cast_to_size <= tag_size;
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// In the algorithm above, we can change
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// cast(relative_tag) + niche_variants.start()
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// into
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// cast(tag) + (niche_variants.start() - niche_start)
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// if either the casted type is no larger than the original
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// type, or if the niche values are contiguous (in either the
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// signed or unsigned sense).
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let can_incr_after_cast = cast_smaller || niches_ule || niches_sle;
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let data_for_boundary_niche = || -> Option<(IntPredicate, u128)> {
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if !can_incr_after_cast {
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None
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} else if niche_start == low_unsigned {
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Some((IntPredicate::IntULE, niche_end))
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} else if niche_end == high_unsigned {
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Some((IntPredicate::IntUGE, niche_start))
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} else if niche_start == low_signed {
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Some((IntPredicate::IntSLE, niche_end))
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} else if niche_end == high_signed {
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Some((IntPredicate::IntSGE, niche_start))
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} else {
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bx.intcast(relative_discr, cast_to, false)
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};
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bx.add(
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relative_discr,
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bx.cx().const_uint(cast_to, niche_variants.start().as_u32() as u64),
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)
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None
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}
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};
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bx.select(
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let (is_niche, tagged_discr, delta) = if relative_max == 0 {
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// Best case scenario: only one tagged variant. This will
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// likely become just a comparison and a jump.
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// The algorithm is:
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// is_niche = tag == niche_start
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// discr = if is_niche {
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// niche_start
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// } else {
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// untagged_variant
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// }
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let niche_start = bx.cx().const_uint_big(tag_llty, niche_start);
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let is_niche = bx.icmp(IntPredicate::IntEQ, tag, niche_start);
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let tagged_discr =
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bx.cx().const_uint(cast_to, niche_variants.start().as_u32() as u64);
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(is_niche, tagged_discr, 0)
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} else if let Some((predicate, constant)) = data_for_boundary_niche() {
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// The niche values are either the lowest or the highest in
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// `range`. We can avoid the first subtraction in the
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// algorithm.
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// The algorithm is now this:
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// is_niche = tag <= niche_end
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// discr = if is_niche {
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// cast(tag) + (niche_variants.start() - niche_start)
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// } else {
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// untagged_variant
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// }
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// (the first line may instead be tag >= niche_start,
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// and may be a signed or unsigned comparison)
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let is_niche =
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bx.icmp(predicate, tag, bx.cx().const_uint_big(tag_llty, constant));
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let cast_tag = if cast_smaller {
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bx.intcast(tag, cast_to, false)
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} else if niches_ule {
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bx.zext(tag, cast_to)
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} else {
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bx.sext(tag, cast_to)
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};
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let delta = (niche_variants.start().as_u32() as u128).wrapping_sub(niche_start);
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(is_niche, cast_tag, delta)
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} else {
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// The special cases don't apply, so we'll have to go with
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// the general algorithm.
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let relative_discr = bx.sub(tag, bx.cx().const_uint_big(tag_llty, niche_start));
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let cast_tag = bx.intcast(relative_discr, cast_to, false);
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let is_niche = bx.icmp(
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IntPredicate::IntULE,
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relative_discr,
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bx.cx().const_uint(tag_llty, relative_max as u64),
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);
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(is_niche, cast_tag, niche_variants.start().as_u32() as u128)
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};
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let tagged_discr = if delta == 0 {
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tagged_discr
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} else {
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bx.add(tagged_discr, bx.cx().const_uint_big(cast_to, delta))
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};
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let discr = bx.select(
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is_niche,
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niche_discr,
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tagged_discr,
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bx.cx().const_uint(cast_to, untagged_variant.as_u32() as u64),
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)
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);
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// In principle we could insert assumes on the possible range of `discr`, but
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// currently in LLVM this seems to be a pessimization.
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discr
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}
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}
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}
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112
src/test/codegen/enum-match.rs
Normal file
112
src/test/codegen/enum-match.rs
Normal file
@ -0,0 +1,112 @@
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// compile-flags: -Copt-level=1
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// only-x86_64
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#![crate_type = "lib"]
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// Check each of the 3 cases for `codegen_get_discr`.
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// Case 0: One tagged variant.
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pub enum Enum0 {
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A(bool),
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B,
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}
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// CHECK: define i8 @match0{{.*}}
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// CHECK-NEXT: start:
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// CHECK-NEXT: %1 = icmp eq i8 %0, 2
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// CHECK-NEXT: %2 = and i8 %0, 1
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// CHECK-NEXT: %.0 = select i1 %1, i8 13, i8 %2
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#[no_mangle]
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pub fn match0(e: Enum0) -> u8 {
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use Enum0::*;
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match e {
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A(b) => b as u8,
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B => 13,
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}
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}
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// Case 1: Niche values are on a boundary for `range`.
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pub enum Enum1 {
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A(bool),
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B,
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C,
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}
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// CHECK: define i8 @match1{{.*}}
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// CHECK-NEXT: start:
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// CHECK-NEXT: %1 = icmp ugt i8 %0, 1
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// CHECK-NEXT: %2 = zext i8 %0 to i64
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// CHECK-NEXT: %3 = add nsw i64 %2, -1
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// CHECK-NEXT: %_2 = select i1 %1, i64 %3, i64 0
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// CHECK-NEXT: switch i64 %_2, label {{.*}} [
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#[no_mangle]
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pub fn match1(e: Enum1) -> u8 {
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use Enum1::*;
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match e {
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A(b) => b as u8,
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B => 13,
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C => 100,
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}
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}
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// Case 2: Special cases don't apply.
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pub enum X {
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_2=2, _3, _4, _5, _6, _7, _8, _9, _10, _11,
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_12, _13, _14, _15, _16, _17, _18, _19, _20,
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_21, _22, _23, _24, _25, _26, _27, _28, _29,
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_30, _31, _32, _33, _34, _35, _36, _37, _38,
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_39, _40, _41, _42, _43, _44, _45, _46, _47,
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_48, _49, _50, _51, _52, _53, _54, _55, _56,
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_57, _58, _59, _60, _61, _62, _63, _64, _65,
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_66, _67, _68, _69, _70, _71, _72, _73, _74,
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_75, _76, _77, _78, _79, _80, _81, _82, _83,
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_84, _85, _86, _87, _88, _89, _90, _91, _92,
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_93, _94, _95, _96, _97, _98, _99, _100, _101,
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_102, _103, _104, _105, _106, _107, _108, _109,
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_110, _111, _112, _113, _114, _115, _116, _117,
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_118, _119, _120, _121, _122, _123, _124, _125,
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_126, _127, _128, _129, _130, _131, _132, _133,
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_134, _135, _136, _137, _138, _139, _140, _141,
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_142, _143, _144, _145, _146, _147, _148, _149,
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_150, _151, _152, _153, _154, _155, _156, _157,
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_158, _159, _160, _161, _162, _163, _164, _165,
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_166, _167, _168, _169, _170, _171, _172, _173,
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_174, _175, _176, _177, _178, _179, _180, _181,
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_182, _183, _184, _185, _186, _187, _188, _189,
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_190, _191, _192, _193, _194, _195, _196, _197,
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_198, _199, _200, _201, _202, _203, _204, _205,
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_206, _207, _208, _209, _210, _211, _212, _213,
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_214, _215, _216, _217, _218, _219, _220, _221,
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_222, _223, _224, _225, _226, _227, _228, _229,
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_230, _231, _232, _233, _234, _235, _236, _237,
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_238, _239, _240, _241, _242, _243, _244, _245,
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_246, _247, _248, _249, _250, _251, _252, _253,
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}
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pub enum Enum2 {
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A(X),
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B,
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C,
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D,
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E,
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}
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// CHECK: define i8 @match2{{.*}}
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// CHECK-NEXT: start:
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// CHECK-NEXT: %1 = add i8 %0, 2
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// CHECK-NEXT: %2 = zext i8 %1 to i64
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// CHECK-NEXT: %3 = icmp ult i8 %1, 4
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// CHECK-NEXT: %4 = add nuw nsw i64 %2, 1
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// CHECK-NEXT: %_2 = select i1 %3, i64 %4, i64 0
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// CHECK-NEXT: switch i64 %_2, label {{.*}} [
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#[no_mangle]
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pub fn match2(e: Enum2) -> u8 {
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use Enum2::*;
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match e {
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A(b) => b as u8,
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B => 13,
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C => 100,
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D => 200,
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E => 250,
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}
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}
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114
src/test/ui/enum-discriminant/get_discr.rs
Normal file
114
src/test/ui/enum-discriminant/get_discr.rs
Normal file
@ -0,0 +1,114 @@
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// run-pass
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// Now that there are several variations on the code generated in
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// `codegen_get_discr`, let's make sure the various cases yield the correct
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// result.
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// To get the discriminant of an E<X1> value, there are no shortcuts - we must
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// do the full algorithm.
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#[repr(u8)]
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pub enum X1 {
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_1 = 1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16,
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_17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28, _29, _30, _31, _32,
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_33, _34, _35, _36, _37, _38, _39, _40, _41, _42, _43, _44, _45, _46, _47, _48,
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_49, _50, _51, _52, _53, _54, _55, _56, _57, _58, _59, _60, _61, _62, _63, _64,
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_65, _66, _67, _68, _69, _70, _71, _72, _73, _74, _75, _76, _77, _78, _79, _80,
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_81, _82, _83, _84, _85, _86, _87, _88, _89, _90, _91, _92, _93, _94, _95, _96,
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_97, _98, _99, _100, _101, _102, _103, _104, _105, _106, _107, _108, _109, _110, _111, _112,
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_113, _114, _115, _116, _117, _118, _119, _120, _121, _122, _123, _124, _125, _126, _127, _128,
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_129, _130, _131, _132, _133, _134, _135, _136, _137, _138, _139, _140, _141, _142, _143, _144,
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_145, _146, _147, _148, _149, _150, _151, _152, _153, _154, _155, _156, _157, _158, _159, _160,
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_161, _162, _163, _164, _165, _166, _167, _168, _169, _170, _171, _172, _173, _174, _175, _176,
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_177, _178, _179, _180, _181, _182, _183, _184, _185, _186, _187, _188, _189, _190, _191, _192,
|
||||
_193, _194, _195, _196, _197, _198, _199, _200, _201, _202, _203, _204, _205, _206, _207, _208,
|
||||
_209, _210, _211, _212, _213, _214, _215, _216, _217, _218, _219, _220, _221, _222, _223, _224,
|
||||
_225, _226, _227, _228, _229, _230, _231, _232, _233, _234, _235, _236, _237, _238, _239, _240,
|
||||
_241, _242, _243, _244, _245, _246, _247, _248, _249, _250, _251, _252, _253, _254,
|
||||
}
|
||||
|
||||
#[repr(i8)]
|
||||
pub enum X2 {
|
||||
_1 = -1, _2 = 0, _3 = 1,
|
||||
}
|
||||
|
||||
#[repr(i8)]
|
||||
pub enum X3 {
|
||||
_1 = -128, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16,
|
||||
_17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28, _29, _30, _31, _32,
|
||||
_33, _34, _35, _36, _37, _38, _39, _40, _41, _42, _43, _44, _45, _46, _47, _48,
|
||||
_49, _50, _51, _52, _53, _54, _55, _56, _57, _58, _59, _60, _61, _62, _63, _64,
|
||||
_65, _66, _67, _68, _69, _70, _71, _72, _73, _74, _75, _76, _77, _78, _79, _80,
|
||||
_81, _82, _83, _84, _85, _86, _87, _88, _89, _90, _91, _92, _93, _94, _95, _96,
|
||||
_97, _98, _99, _100, _101, _102, _103, _104, _105, _106, _107, _108, _109, _110, _111, _112,
|
||||
_113, _114, _115, _116, _117, _118, _119, _120, _121, _122, _123, _124, _125, _126, _127, _128,
|
||||
_129, _130, _131, _132, _133, _134, _135, _136, _137, _138, _139, _140, _141, _142, _143, _144,
|
||||
_145, _146, _147, _148, _149, _150, _151, _152, _153, _154, _155, _156, _157, _158, _159, _160,
|
||||
_161, _162, _163, _164, _165, _166, _167, _168, _169, _170, _171, _172, _173, _174, _175, _176,
|
||||
_177, _178, _179, _180, _181, _182, _183, _184, _185, _186, _187, _188, _189, _190, _191, _192,
|
||||
_193, _194, _195, _196, _197, _198, _199, _200, _201, _202, _203, _204, _205, _206, _207, _208,
|
||||
_209, _210, _211, _212, _213, _214, _215, _216, _217, _218, _219, _220, _221, _222, _223, _224,
|
||||
_225, _226, _227, _228, _229, _230, _231, _232, _233, _234, _235, _236, _237, _238, _239, _240,
|
||||
_241, _242, _243, _244, _245, _246, _247, _248, _249, _250, _251, _252, _253, _254,
|
||||
}
|
||||
|
||||
#[repr(i8)]
|
||||
pub enum X4 {
|
||||
_1 = -126, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16,
|
||||
_17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28, _29, _30, _31, _32,
|
||||
_33, _34, _35, _36, _37, _38, _39, _40, _41, _42, _43, _44, _45, _46, _47, _48,
|
||||
_49, _50, _51, _52, _53, _54, _55, _56, _57, _58, _59, _60, _61, _62, _63, _64,
|
||||
_65, _66, _67, _68, _69, _70, _71, _72, _73, _74, _75, _76, _77, _78, _79, _80,
|
||||
_81, _82, _83, _84, _85, _86, _87, _88, _89, _90, _91, _92, _93, _94, _95, _96,
|
||||
_97, _98, _99, _100, _101, _102, _103, _104, _105, _106, _107, _108, _109, _110, _111, _112,
|
||||
_113, _114, _115, _116, _117, _118, _119, _120, _121, _122, _123, _124, _125, _126, _127, _128,
|
||||
_129, _130, _131, _132, _133, _134, _135, _136, _137, _138, _139, _140, _141, _142, _143, _144,
|
||||
_145, _146, _147, _148, _149, _150, _151, _152, _153, _154, _155, _156, _157, _158, _159, _160,
|
||||
_161, _162, _163, _164, _165, _166, _167, _168, _169, _170, _171, _172, _173, _174, _175, _176,
|
||||
_177, _178, _179, _180, _181, _182, _183, _184, _185, _186, _187, _188, _189, _190, _191, _192,
|
||||
_193, _194, _195, _196, _197, _198, _199, _200, _201, _202, _203, _204, _205, _206, _207, _208,
|
||||
_209, _210, _211, _212, _213, _214, _215, _216, _217, _218, _219, _220, _221, _222, _223, _224,
|
||||
_225, _226, _227, _228, _229, _230, _231, _232, _233, _234, _235, _236, _237, _238, _239, _240,
|
||||
_241, _242, _243, _244, _245, _246, _247, _248, _249, _250, _251, _252, _253, _254,
|
||||
}
|
||||
|
||||
pub enum E<X> {
|
||||
A(X),
|
||||
B,
|
||||
C,
|
||||
}
|
||||
|
||||
pub fn match_e<X>(e: E<X>) -> u8 {
|
||||
use E::*;
|
||||
match e {
|
||||
A(_) => 0,
|
||||
B => 1,
|
||||
C => 2,
|
||||
}
|
||||
}
|
||||
|
||||
fn main() {
|
||||
assert_eq!(match_e(E::A(X1::_1)), 0);
|
||||
assert_eq!(match_e(E::A(X1::_2)), 0);
|
||||
assert_eq!(match_e(E::A(X1::_254)), 0);
|
||||
assert_eq!(match_e(E::<X1>::B), 1);
|
||||
assert_eq!(match_e(E::<X1>::C), 2);
|
||||
assert_eq!(match_e(E::A(X2::_1)), 0);
|
||||
assert_eq!(match_e(E::A(X2::_2)), 0);
|
||||
assert_eq!(match_e(E::A(X2::_3)), 0);
|
||||
assert_eq!(match_e(E::<X2>::B), 1);
|
||||
assert_eq!(match_e(E::<X2>::C), 2);
|
||||
assert_eq!(match_e(E::A(X3::_1)), 0);
|
||||
assert_eq!(match_e(E::A(X3::_2)), 0);
|
||||
assert_eq!(match_e(E::A(X3::_254)), 0);
|
||||
assert_eq!(match_e(E::<X3>::B), 1);
|
||||
assert_eq!(match_e(E::<X3>::C), 2);
|
||||
assert_eq!(match_e(E::A(X4::_1)), 0);
|
||||
assert_eq!(match_e(E::A(X4::_2)), 0);
|
||||
assert_eq!(match_e(E::A(X4::_254)), 0);
|
||||
assert_eq!(match_e(E::<X4>::B), 1);
|
||||
assert_eq!(match_e(E::<X4>::C), 2);
|
||||
assert_eq!(match_e(E::A(false)), 0);
|
||||
assert_eq!(match_e(E::A(true)), 0);
|
||||
assert_eq!(match_e(E::<bool>::B), 1);
|
||||
assert_eq!(match_e(E::<bool>::C), 2);
|
||||
}
|
||||
Loading…
Reference in New Issue
Block a user