diff options
Diffstat (limited to 'compiler/rustc_codegen_ssa/src/mir/place.rs')
| -rw-r--r-- | compiler/rustc_codegen_ssa/src/mir/place.rs | 549 |
1 files changed, 549 insertions, 0 deletions
diff --git a/compiler/rustc_codegen_ssa/src/mir/place.rs b/compiler/rustc_codegen_ssa/src/mir/place.rs new file mode 100644 index 000000000..268c4d765 --- /dev/null +++ b/compiler/rustc_codegen_ssa/src/mir/place.rs @@ -0,0 +1,549 @@ +use super::operand::OperandValue; +use super::{FunctionCx, LocalRef}; + +use crate::common::IntPredicate; +use crate::glue; +use crate::traits::*; +use crate::MemFlags; + +use rustc_middle::mir; +use rustc_middle::mir::tcx::PlaceTy; +use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf, TyAndLayout}; +use rustc_middle::ty::{self, Ty}; +use rustc_target::abi::{Abi, Align, FieldsShape, Int, TagEncoding}; +use rustc_target::abi::{VariantIdx, Variants}; + +#[derive(Copy, Clone, Debug)] +pub struct PlaceRef<'tcx, V> { + /// A pointer to the contents of the place. + pub llval: V, + + /// This place's extra data if it is unsized, or `None` if null. + pub llextra: Option<V>, + + /// The monomorphized type of this place, including variant information. + pub layout: TyAndLayout<'tcx>, + + /// The alignment we know for this place. + pub align: Align, +} + +impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> { + pub fn new_sized(llval: V, layout: TyAndLayout<'tcx>) -> PlaceRef<'tcx, V> { + assert!(!layout.is_unsized()); + PlaceRef { llval, llextra: None, layout, align: layout.align.abi } + } + + pub fn new_sized_aligned( + llval: V, + layout: TyAndLayout<'tcx>, + align: Align, + ) -> PlaceRef<'tcx, V> { + assert!(!layout.is_unsized()); + PlaceRef { llval, llextra: None, layout, align } + } + + // FIXME(eddyb) pass something else for the name so no work is done + // unless LLVM IR names are turned on (e.g. for `--emit=llvm-ir`). + pub fn alloca<Bx: BuilderMethods<'a, 'tcx, Value = V>>( + bx: &mut Bx, + layout: TyAndLayout<'tcx>, + ) -> Self { + assert!(!layout.is_unsized(), "tried to statically allocate unsized place"); + let tmp = bx.alloca(bx.cx().backend_type(layout), layout.align.abi); + Self::new_sized(tmp, layout) + } + + /// Returns a place for an indirect reference to an unsized place. + // FIXME(eddyb) pass something else for the name so no work is done + // unless LLVM IR names are turned on (e.g. for `--emit=llvm-ir`). + pub fn alloca_unsized_indirect<Bx: BuilderMethods<'a, 'tcx, Value = V>>( + bx: &mut Bx, + layout: TyAndLayout<'tcx>, + ) -> Self { + assert!(layout.is_unsized(), "tried to allocate indirect place for sized values"); + let ptr_ty = bx.cx().tcx().mk_mut_ptr(layout.ty); + let ptr_layout = bx.cx().layout_of(ptr_ty); + Self::alloca(bx, ptr_layout) + } + + pub fn len<Cx: ConstMethods<'tcx, Value = V>>(&self, cx: &Cx) -> V { + if let FieldsShape::Array { count, .. } = self.layout.fields { + if self.layout.is_unsized() { + assert_eq!(count, 0); + self.llextra.unwrap() + } else { + cx.const_usize(count) + } + } else { + bug!("unexpected layout `{:#?}` in PlaceRef::len", self.layout) + } + } +} + +impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> { + /// Access a field, at a point when the value's case is known. + pub fn project_field<Bx: BuilderMethods<'a, 'tcx, Value = V>>( + self, + bx: &mut Bx, + ix: usize, + ) -> Self { + let field = self.layout.field(bx.cx(), ix); + let offset = self.layout.fields.offset(ix); + let effective_field_align = self.align.restrict_for_offset(offset); + + let mut simple = || { + let llval = match self.layout.abi { + _ if offset.bytes() == 0 => { + // Unions and newtypes only use an offset of 0. + // Also handles the first field of Scalar, ScalarPair, and Vector layouts. + self.llval + } + Abi::ScalarPair(a, b) + if offset == a.size(bx.cx()).align_to(b.align(bx.cx()).abi) => + { + // Offset matches second field. + let ty = bx.backend_type(self.layout); + bx.struct_gep(ty, self.llval, 1) + } + Abi::Scalar(_) | Abi::ScalarPair(..) | Abi::Vector { .. } if field.is_zst() => { + // ZST fields are not included in Scalar, ScalarPair, and Vector layouts, so manually offset the pointer. + let byte_ptr = bx.pointercast(self.llval, bx.cx().type_i8p()); + bx.gep(bx.cx().type_i8(), byte_ptr, &[bx.const_usize(offset.bytes())]) + } + Abi::Scalar(_) | Abi::ScalarPair(..) => { + // All fields of Scalar and ScalarPair layouts must have been handled by this point. + // Vector layouts have additional fields for each element of the vector, so don't panic in that case. + bug!( + "offset of non-ZST field `{:?}` does not match layout `{:#?}`", + field, + self.layout + ); + } + _ => { + let ty = bx.backend_type(self.layout); + bx.struct_gep(ty, self.llval, bx.cx().backend_field_index(self.layout, ix)) + } + }; + PlaceRef { + // HACK(eddyb): have to bitcast pointers until LLVM removes pointee types. + llval: bx.pointercast(llval, bx.cx().type_ptr_to(bx.cx().backend_type(field))), + llextra: if bx.cx().type_has_metadata(field.ty) { self.llextra } else { None }, + layout: field, + align: effective_field_align, + } + }; + + // Simple cases, which don't need DST adjustment: + // * no metadata available - just log the case + // * known alignment - sized types, `[T]`, `str` or a foreign type + // * packed struct - there is no alignment padding + match field.ty.kind() { + _ if self.llextra.is_none() => { + debug!( + "unsized field `{}`, of `{:?}` has no metadata for adjustment", + ix, self.llval + ); + return simple(); + } + _ if !field.is_unsized() => return simple(), + ty::Slice(..) | ty::Str | ty::Foreign(..) => return simple(), + ty::Adt(def, _) => { + if def.repr().packed() { + // FIXME(eddyb) generalize the adjustment when we + // start supporting packing to larger alignments. + assert_eq!(self.layout.align.abi.bytes(), 1); + return simple(); + } + } + _ => {} + } + + // We need to get the pointer manually now. + // We do this by casting to a `*i8`, then offsetting it by the appropriate amount. + // We do this instead of, say, simply adjusting the pointer from the result of a GEP + // because the field may have an arbitrary alignment in the LLVM representation + // anyway. + // + // To demonstrate: + // + // struct Foo<T: ?Sized> { + // x: u16, + // y: T + // } + // + // The type `Foo<Foo<Trait>>` is represented in LLVM as `{ u16, { u16, u8 }}`, meaning that + // the `y` field has 16-bit alignment. + + let meta = self.llextra; + + let unaligned_offset = bx.cx().const_usize(offset.bytes()); + + // Get the alignment of the field + let (_, unsized_align) = glue::size_and_align_of_dst(bx, field.ty, meta); + + // Bump the unaligned offset up to the appropriate alignment + let offset = round_up_const_value_to_alignment(bx, unaligned_offset, unsized_align); + + debug!("struct_field_ptr: DST field offset: {:?}", offset); + + // Cast and adjust pointer. + let byte_ptr = bx.pointercast(self.llval, bx.cx().type_i8p()); + let byte_ptr = bx.gep(bx.cx().type_i8(), byte_ptr, &[offset]); + + // Finally, cast back to the type expected. + let ll_fty = bx.cx().backend_type(field); + debug!("struct_field_ptr: Field type is {:?}", ll_fty); + + PlaceRef { + llval: bx.pointercast(byte_ptr, bx.cx().type_ptr_to(ll_fty)), + llextra: self.llextra, + layout: field, + align: effective_field_align, + } + } + + /// Obtain the actual discriminant of a value. + #[instrument(level = "trace", skip(bx))] + pub fn codegen_get_discr<Bx: BuilderMethods<'a, 'tcx, Value = V>>( + self, + bx: &mut Bx, + cast_to: Ty<'tcx>, + ) -> V { + let cast_to = bx.cx().immediate_backend_type(bx.cx().layout_of(cast_to)); + if self.layout.abi.is_uninhabited() { + return bx.cx().const_undef(cast_to); + } + let (tag_scalar, tag_encoding, tag_field) = match self.layout.variants { + Variants::Single { index } => { + let discr_val = self + .layout + .ty + .discriminant_for_variant(bx.cx().tcx(), index) + .map_or(index.as_u32() as u128, |discr| discr.val); + return bx.cx().const_uint_big(cast_to, discr_val); + } + Variants::Multiple { tag, ref tag_encoding, tag_field, .. } => { + (tag, tag_encoding, tag_field) + } + }; + + // Read the tag/niche-encoded discriminant from memory. + let tag = self.project_field(bx, tag_field); + let tag = bx.load_operand(tag); + + // Decode the discriminant (specifically if it's niche-encoded). + match *tag_encoding { + TagEncoding::Direct => { + let signed = match tag_scalar.primitive() { + // We use `i1` for bytes that are always `0` or `1`, + // e.g., `#[repr(i8)] enum E { A, B }`, but we can't + // let LLVM interpret the `i1` as signed, because + // then `i1 1` (i.e., `E::B`) is effectively `i8 -1`. + Int(_, signed) => !tag_scalar.is_bool() && signed, + _ => false, + }; + bx.intcast(tag.immediate(), cast_to, signed) + } + 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. + let niche_llty = bx.cx().immediate_backend_type(tag.layout); + let tag = tag.immediate(); + + // 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 { + // Avoid subtracting `0`, which wouldn't work for pointers. + // FIXME(eddyb) check the actual primitive type here. + tag + } else { + bx.sub(tag, bx.cx().const_uint_big(niche_llty, niche_start)) + }; + let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32(); + let is_niche = if relative_max == 0 { + // Avoid calling `const_uint`, which wouldn't work for pointers. + // Also use canonical == 0 instead of non-canonical u<= 0. + // FIXME(eddyb) check the actual primitive type here. + bx.icmp(IntPredicate::IntEQ, relative_discr, bx.cx().const_null(niche_llty)) + } else { + let relative_max = bx.cx().const_uint(niche_llty, relative_max as u64); + bx.icmp(IntPredicate::IntULE, relative_discr, 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. + bx.cx().const_uint(cast_to, 0) + } else { + bx.intcast(relative_discr, cast_to, false) + }; + bx.add( + relative_discr, + bx.cx().const_uint(cast_to, niche_variants.start().as_u32() as u64), + ) + }; + + bx.select( + is_niche, + niche_discr, + bx.cx().const_uint(cast_to, dataful_variant.as_u32() as u64), + ) + } + } + } + + /// Sets the discriminant for a new value of the given case of the given + /// representation. + pub fn codegen_set_discr<Bx: BuilderMethods<'a, 'tcx, Value = V>>( + &self, + bx: &mut Bx, + variant_index: VariantIdx, + ) { + if self.layout.for_variant(bx.cx(), variant_index).abi.is_uninhabited() { + // We play it safe by using a well-defined `abort`, but we could go for immediate UB + // if that turns out to be helpful. + bx.abort(); + return; + } + match self.layout.variants { + Variants::Single { index } => { + assert_eq!(index, variant_index); + } + Variants::Multiple { tag_encoding: TagEncoding::Direct, tag_field, .. } => { + let ptr = self.project_field(bx, tag_field); + let to = + self.layout.ty.discriminant_for_variant(bx.tcx(), variant_index).unwrap().val; + bx.store( + bx.cx().const_uint_big(bx.cx().backend_type(ptr.layout), to), + ptr.llval, + ptr.align, + ); + } + Variants::Multiple { + tag_encoding: + TagEncoding::Niche { dataful_variant, ref niche_variants, niche_start }, + tag_field, + .. + } => { + if variant_index != dataful_variant { + if bx.cx().sess().target.arch == "arm" + || bx.cx().sess().target.arch == "aarch64" + { + // FIXME(#34427): as workaround for LLVM bug on ARM, + // use memset of 0 before assigning niche value. + let fill_byte = bx.cx().const_u8(0); + let size = bx.cx().const_usize(self.layout.size.bytes()); + bx.memset(self.llval, fill_byte, size, self.align, MemFlags::empty()); + } + + let niche = self.project_field(bx, tag_field); + let niche_llty = bx.cx().immediate_backend_type(niche.layout); + let niche_value = variant_index.as_u32() - niche_variants.start().as_u32(); + let niche_value = (niche_value as u128).wrapping_add(niche_start); + // FIXME(eddyb): check the actual primitive type here. + let niche_llval = if niche_value == 0 { + // HACK(eddyb): using `c_null` as it works on all types. + bx.cx().const_null(niche_llty) + } else { + bx.cx().const_uint_big(niche_llty, niche_value) + }; + OperandValue::Immediate(niche_llval).store(bx, niche); + } + } + } + } + + pub fn project_index<Bx: BuilderMethods<'a, 'tcx, Value = V>>( + &self, + bx: &mut Bx, + llindex: V, + ) -> Self { + // Statically compute the offset if we can, otherwise just use the element size, + // as this will yield the lowest alignment. + let layout = self.layout.field(bx, 0); + let offset = if let Some(llindex) = bx.const_to_opt_uint(llindex) { + layout.size.checked_mul(llindex, bx).unwrap_or(layout.size) + } else { + layout.size + }; + + PlaceRef { + llval: bx.inbounds_gep( + bx.cx().backend_type(self.layout), + self.llval, + &[bx.cx().const_usize(0), llindex], + ), + llextra: None, + layout, + align: self.align.restrict_for_offset(offset), + } + } + + pub fn project_downcast<Bx: BuilderMethods<'a, 'tcx, Value = V>>( + &self, + bx: &mut Bx, + variant_index: VariantIdx, + ) -> Self { + let mut downcast = *self; + downcast.layout = self.layout.for_variant(bx.cx(), variant_index); + + // Cast to the appropriate variant struct type. + let variant_ty = bx.cx().backend_type(downcast.layout); + downcast.llval = bx.pointercast(downcast.llval, bx.cx().type_ptr_to(variant_ty)); + + downcast + } + + pub fn storage_live<Bx: BuilderMethods<'a, 'tcx, Value = V>>(&self, bx: &mut Bx) { + bx.lifetime_start(self.llval, self.layout.size); + } + + pub fn storage_dead<Bx: BuilderMethods<'a, 'tcx, Value = V>>(&self, bx: &mut Bx) { + bx.lifetime_end(self.llval, self.layout.size); + } +} + +impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> { + #[instrument(level = "trace", skip(self, bx))] + pub fn codegen_place( + &mut self, + bx: &mut Bx, + place_ref: mir::PlaceRef<'tcx>, + ) -> PlaceRef<'tcx, Bx::Value> { + let cx = self.cx; + let tcx = self.cx.tcx(); + + let mut base = 0; + let mut cg_base = match self.locals[place_ref.local] { + LocalRef::Place(place) => place, + LocalRef::UnsizedPlace(place) => bx.load_operand(place).deref(cx), + LocalRef::Operand(..) => { + if place_ref.has_deref() { + base = 1; + let cg_base = self.codegen_consume( + bx, + mir::PlaceRef { projection: &place_ref.projection[..0], ..place_ref }, + ); + cg_base.deref(bx.cx()) + } else { + bug!("using operand local {:?} as place", place_ref); + } + } + }; + for elem in place_ref.projection[base..].iter() { + cg_base = match *elem { + mir::ProjectionElem::Deref => bx.load_operand(cg_base).deref(bx.cx()), + mir::ProjectionElem::Field(ref field, _) => { + cg_base.project_field(bx, field.index()) + } + mir::ProjectionElem::Index(index) => { + let index = &mir::Operand::Copy(mir::Place::from(index)); + let index = self.codegen_operand(bx, index); + let llindex = index.immediate(); + cg_base.project_index(bx, llindex) + } + mir::ProjectionElem::ConstantIndex { offset, from_end: false, min_length: _ } => { + let lloffset = bx.cx().const_usize(offset as u64); + cg_base.project_index(bx, lloffset) + } + mir::ProjectionElem::ConstantIndex { offset, from_end: true, min_length: _ } => { + let lloffset = bx.cx().const_usize(offset as u64); + let lllen = cg_base.len(bx.cx()); + let llindex = bx.sub(lllen, lloffset); + cg_base.project_index(bx, llindex) + } + mir::ProjectionElem::Subslice { from, to, from_end } => { + let mut subslice = cg_base.project_index(bx, bx.cx().const_usize(from as u64)); + let projected_ty = + PlaceTy::from_ty(cg_base.layout.ty).projection_ty(tcx, *elem).ty; + subslice.layout = bx.cx().layout_of(self.monomorphize(projected_ty)); + + if subslice.layout.is_unsized() { + assert!(from_end, "slice subslices should be `from_end`"); + subslice.llextra = Some(bx.sub( + cg_base.llextra.unwrap(), + bx.cx().const_usize((from as u64) + (to as u64)), + )); + } + + // Cast the place pointer type to the new + // array or slice type (`*[%_; new_len]`). + subslice.llval = bx.pointercast( + subslice.llval, + bx.cx().type_ptr_to(bx.cx().backend_type(subslice.layout)), + ); + + subslice + } + mir::ProjectionElem::Downcast(_, v) => cg_base.project_downcast(bx, v), + }; + } + debug!("codegen_place(place={:?}) => {:?}", place_ref, cg_base); + cg_base + } + + pub fn monomorphized_place_ty(&self, place_ref: mir::PlaceRef<'tcx>) -> Ty<'tcx> { + let tcx = self.cx.tcx(); + let place_ty = place_ref.ty(self.mir, tcx); + self.monomorphize(place_ty.ty) + } +} + +fn round_up_const_value_to_alignment<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>( + bx: &mut Bx, + value: Bx::Value, + align: Bx::Value, +) -> Bx::Value { + // In pseudo code: + // + // if value & (align - 1) == 0 { + // value + // } else { + // (value & !(align - 1)) + align + // } + // + // Usually this is written without branches as + // + // (value + align - 1) & !(align - 1) + // + // But this formula cannot take advantage of constant `value`. E.g. if `value` is known + // at compile time to be `1`, this expression should be optimized to `align`. However, + // optimization only holds if `align` is a power of two. Since the optimizer doesn't know + // that `align` is a power of two, it cannot perform this optimization. + // + // Instead we use + // + // value + (-value & (align - 1)) + // + // Since `align` is used only once, the expression can be optimized. For `value = 0` + // its optimized to `0` even in debug mode. + // + // NB: The previous version of this code used + // + // (value + align - 1) & -align + // + // Even though `-align == !(align - 1)`, LLVM failed to optimize this even for + // `value = 0`. Bug report: https://bugs.llvm.org/show_bug.cgi?id=48559 + let one = bx.const_usize(1); + let align_minus_1 = bx.sub(align, one); + let neg_value = bx.neg(value); + let offset = bx.and(neg_value, align_minus_1); + bx.add(value, offset) +} |