Don't alloca just to look at a discriminant
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| Original file line number | Diff line number | Diff line change |
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@@ -3,16 +3,17 @@ use std::fmt; | |
| use arrayvec::ArrayVec; | ||
| use either::Either; | ||
| use rustc_abi as abi; | ||
| use rustc_abi::{Align, BackendRepr, FIRST_VARIANT, Primitive, Size, TagEncoding, Variants}; | ||
| use rustc_middle::mir::interpret::{Pointer, Scalar, alloc_range}; | ||
| use rustc_middle::mir::{self, ConstValue}; | ||
| use rustc_middle::ty::Ty; | ||
| use rustc_middle::ty::layout::{LayoutOf, TyAndLayout}; | ||
| use rustc_middle::{bug, span_bug}; | ||
| use tracing::{debug, instrument}; | ||
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| use super::place::{PlaceRef, PlaceValue}; | ||
| use super::{FunctionCx, LocalRef}; | ||
| use crate::common::IntPredicate; | ||
| use crate::traits::*; | ||
| use crate::{MemFlags, size_of_val}; | ||
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@@ -415,6 +416,149 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> { | |
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| OperandRef { val, layout: field } | ||
| } | ||
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| /// Obtain the actual discriminant of a value. | ||
| #[instrument(level = "trace", skip(fx, bx))] | ||
| pub fn codegen_get_discr<Bx: BuilderMethods<'a, 'tcx, Value = V>>( | ||
| self, | ||
| fx: &mut FunctionCx<'a, 'tcx, Bx>, | ||
| bx: &mut Bx, | ||
| cast_to: Ty<'tcx>, | ||
| ) -> V { | ||
| let dl = &bx.tcx().data_layout; | ||
| let cast_to_layout = bx.cx().layout_of(cast_to); | ||
| let cast_to = bx.cx().immediate_backend_type(cast_to_layout); | ||
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| // We check uninhabitedness separately because a type like | ||
| // `enum Foo { Bar(i32, !) }` is still reported as `Variants::Single`, | ||
| // *not* as `Variants::Empty`. | ||
| if self.layout.is_uninhabited() { | ||
| return bx.cx().const_poison(cast_to); | ||
| } | ||
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| let (tag_scalar, tag_encoding, tag_field) = match self.layout.variants { | ||
| Variants::Empty => unreachable!("we already handled uninhabited types"), | ||
|
There was a problem hiding this comment. question: Can we just move the poison return here, instead of doing the if? Or is there a case where There was a problem hiding this comment. I was wondering that myself, TBH, but wasn't sure so left it as it was. Exploring a bit more now (and thinking of the whole "is pub enum Foo<T> {
Hmm(i32, T)
}
pub type Bar = Foo<std::convert::Infallible>;That uninhabited: true,
variants: Single {
index: 0,
},so it's uninhabited but still single-variant. There was a problem hiding this comment. rust/compiler/rustc_abi/src/layout.rs Lines 25 to 29 in 8536f20 |
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| Variants::Single { index } => { | ||
| let discr_val = | ||
| if let Some(discr) = self.layout.ty.discriminant_for_variant(bx.tcx(), index) { | ||
| discr.val | ||
| } else { | ||
| // This arm is for types which are neither enums nor coroutines, | ||
| // and thus for which the only possible "variant" should be the first one. | ||
| assert_eq!(index, FIRST_VARIANT); | ||
| // There's thus no actual discriminant to return, so we return | ||
| // what it would have been if this was a single-variant enum. | ||
| 0 | ||
| }; | ||
| return bx.cx().const_uint_big(cast_to, discr_val); | ||
| } | ||
| Variants::Multiple { tag, ref tag_encoding, tag_field, .. } => { | ||
| (tag, tag_encoding, tag_field) | ||
| } | ||
| }; | ||
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| // Read the tag/niche-encoded discriminant from memory. | ||
| let tag_op = match self.val { | ||
| OperandValue::ZeroSized => bug!(), | ||
| OperandValue::Immediate(_) | OperandValue::Pair(_, _) => { | ||
| self.extract_field(fx, bx, tag_field) | ||
| } | ||
| OperandValue::Ref(place) => { | ||
| let tag = place.with_type(self.layout).project_field(bx, tag_field); | ||
| bx.load_operand(tag) | ||
| } | ||
| }; | ||
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Comment on lines
+461
to
+470
There was a problem hiding this comment. This |
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| let tag_imm = tag_op.immediate(); | ||
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| // 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`. | ||
| Primitive::Int(_, signed) => !tag_scalar.is_bool() && signed, | ||
| _ => false, | ||
| }; | ||
| bx.intcast(tag_imm, cast_to, signed) | ||
| } | ||
| TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start } => { | ||
| // Cast to an integer so we don't have to treat a pointer as a | ||
| // special case. | ||
| let (tag, tag_llty) = match tag_scalar.primitive() { | ||
| // FIXME(erikdesjardins): handle non-default addrspace ptr sizes | ||
| Primitive::Pointer(_) => { | ||
| let t = bx.type_from_integer(dl.ptr_sized_integer()); | ||
| let tag = bx.ptrtoint(tag_imm, t); | ||
| (tag, t) | ||
| } | ||
| _ => (tag_imm, bx.cx().immediate_backend_type(tag_op.layout)), | ||
| }; | ||
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| let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32(); | ||
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| // 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 niche_start = bx.cx().const_uint_big(tag_llty, niche_start); | ||
| let is_niche = bx.icmp(IntPredicate::IntEQ, tag, niche_start); | ||
| let tagged_discr = | ||
| bx.cx().const_uint(cast_to, niche_variants.start().as_u32() as u64); | ||
| (is_niche, tagged_discr, 0) | ||
| } else { | ||
| // The special cases don't apply, so we'll have to go with | ||
| // the general algorithm. | ||
| let relative_discr = bx.sub(tag, bx.cx().const_uint_big(tag_llty, niche_start)); | ||
| let cast_tag = bx.intcast(relative_discr, cast_to, false); | ||
| let is_niche = bx.icmp( | ||
| IntPredicate::IntULE, | ||
| relative_discr, | ||
| bx.cx().const_uint(tag_llty, relative_max as u64), | ||
| ); | ||
| (is_niche, cast_tag, niche_variants.start().as_u32() as u128) | ||
| }; | ||
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| let tagged_discr = if delta == 0 { | ||
| tagged_discr | ||
| } else { | ||
| bx.add(tagged_discr, bx.cx().const_uint_big(cast_to, delta)) | ||
| }; | ||
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| let discr = bx.select( | ||
| is_niche, | ||
| tagged_discr, | ||
| bx.cx().const_uint(cast_to, untagged_variant.as_u32() as u64), | ||
| ); | ||
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| // In principle we could insert assumes on the possible range of `discr`, but | ||
| // currently in LLVM this seems to be a pessimization. | ||
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| discr | ||
| } | ||
| } | ||
| } | ||
| } | ||
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| impl<'a, 'tcx, V: CodegenObject> OperandValue<V> { | ||
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Annot: this is dead code, since
LowerIntrinsicsturns all the calls to this into MIR.(And by deleting it there was only the one caller of
codegen_get_discrto update.)