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Add a distinct OperandValue::ZeroSized variant for ZSTs

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These tend to have special handling in a bunch of places anyway, so the variant helps remember that.  And I think it's easier to grok than non-Scalar Aggregates sometimes being `Immediates` (like I got wrong and caused 109992).  As a minor bonus, it means we don't need to generate poison LLVM values for them to pass around in `OperandValue::Immediate`s.
This commit is contained in:
Scott McMurray 2023-05-07 03:00:41 -07:00
parent 9af3865dec
commit bf36193ef6
11 changed files with 157 additions and 76 deletions
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49
compiler/rustc_codegen_ssa/src/mir/operand.rs
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@ -45,6 +45,14 @@ pub enum OperandValue<V> {
/// as returned by [`LayoutTypeMethods::scalar_pair_element_backend_type`]
/// with `immediate: true`.
Pair(V, V),
/// A value taking no bytes, and which therefore needs no LLVM value at all.
///
/// If you ever need a `V` to pass to something, get a fresh poison value
/// from [`ConstMethods::const_poison`].
///
/// An `OperandValue` *must* be this variant for any type for which
/// `is_zst` on its `Layout` returns `true`.
ZeroSized,
}
/// An `OperandRef` is an "SSA" reference to a Rust value, along with
@ -71,15 +79,9 @@ impl<V: CodegenObject> fmt::Debug for OperandRef<'_, V> {
}
impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
pub fn new_zst<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
bx: &mut Bx,
layout: TyAndLayout<'tcx>,
) -> OperandRef<'tcx, V> {
pub fn zero_sized(layout: TyAndLayout<'tcx>) -> OperandRef<'tcx, V> {
assert!(layout.is_zst());
OperandRef {
val: OperandValue::Immediate(bx.const_poison(bx.immediate_backend_type(layout))),
layout,
}
OperandRef { val: OperandValue::ZeroSized, layout }
}
pub fn from_const<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
@ -97,7 +99,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
let llval = bx.scalar_to_backend(x, scalar, bx.immediate_backend_type(layout));
OperandValue::Immediate(llval)
}
ConstValue::ZeroSized => return OperandRef::new_zst(bx, layout),
ConstValue::ZeroSized => return OperandRef::zero_sized(layout),
ConstValue::Slice { data, start, end } => {
let Abi::ScalarPair(a_scalar, _) = layout.abi else {
bug!("from_const: invalid ScalarPair layout: {:#?}", layout);
@ -178,7 +180,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
);
OperandRef { val: OperandValue::Pair(a_val, b_val), layout }
}
_ if layout.is_zst() => OperandRef::new_zst(bx, layout),
_ if layout.is_zst() => OperandRef::zero_sized(layout),
_ => {
// Neither a scalar nor scalar pair. Load from a place
let init = bx.const_data_from_alloc(alloc);
@ -216,6 +218,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
OperandValue::Immediate(llptr) => (llptr, None),
OperandValue::Pair(llptr, llextra) => (llptr, Some(llextra)),
OperandValue::Ref(..) => bug!("Deref of by-Ref operand {:?}", self),
OperandValue::ZeroSized => bug!("Deref of ZST operand {:?}", self),
};
let layout = cx.layout_of(projected_ty);
PlaceRef { llval: llptr, llextra, layout, align: layout.align.abi }
@ -273,9 +276,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
let mut val = match (self.val, self.layout.abi) {
// If the field is ZST, it has no data.
_ if field.is_zst() => {
return OperandRef::new_zst(bx, field);
}
_ if field.is_zst() => OperandValue::ZeroSized,
// Newtype of a scalar, scalar pair or vector.
(OperandValue::Immediate(_) | OperandValue::Pair(..), _)
@ -306,6 +307,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
};
match (&mut val, field.abi) {
(OperandValue::ZeroSized, _) => {}
(
OperandValue::Immediate(llval),
Abi::Scalar(_) | Abi::ScalarPair(..) | Abi::Vector { .. },
@ -359,8 +361,8 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
/// Returns an `OperandValue` that's generally UB to use in any way.
///
/// Depending on the `layout`, returns an `Immediate` or `Pair` containing
/// poison value(s), or a `Ref` containing a poison pointer.
/// Depending on the `layout`, returns `ZeroSized` for ZSTs, an `Immediate` or
/// `Pair` containing poison value(s), or a `Ref` containing a poison pointer.
///
/// Supports sized types only.
pub fn poison<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
@ -368,7 +370,9 @@ impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
layout: TyAndLayout<'tcx>,
) -> OperandValue<V> {
assert!(layout.is_sized());
if bx.cx().is_backend_immediate(layout) {
if layout.is_zst() {
OperandValue::ZeroSized
} else if bx.cx().is_backend_immediate(layout) {
let ibty = bx.cx().immediate_backend_type(layout);
OperandValue::Immediate(bx.const_poison(ibty))
} else if bx.cx().is_backend_scalar_pair(layout) {
@ -421,12 +425,11 @@ impl<'a, 'tcx, V: CodegenObject> OperandValue<V> {
flags: MemFlags,
) {
debug!("OperandRef::store: operand={:?}, dest={:?}", self, dest);
// Avoid generating stores of zero-sized values, because the only way to have a zero-sized
// value is through `undef`, and store itself is useless.
if dest.layout.is_zst() {
return;
}
match self {
OperandValue::ZeroSized => {
// Avoid generating stores of zero-sized values, because the only way to have a zero-sized
// value is through `undef`/`poison`, and the store itself is useless.
}
OperandValue::Ref(r, None, source_align) => {
if flags.contains(MemFlags::NONTEMPORAL) {
// HACK(nox): This is inefficient but there is no nontemporal memcpy.
@ -527,7 +530,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
// checks in `codegen_consume` and `extract_field`.
let elem = o.layout.field(bx.cx(), 0);
if elem.is_zst() {
o = OperandRef::new_zst(bx, elem);
o = OperandRef::zero_sized(elem);
} else {
return None;
}
@ -561,7 +564,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
// ZSTs don't require any actual memory access.
if layout.is_zst() {
return OperandRef::new_zst(bx, layout);
return OperandRef::zero_sized(layout);
}
if let Some(o) = self.maybe_codegen_consume_direct(bx, place_ref) {