bjoernager/rust
bjoernager/rust
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Use poison instead of undef

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In cases where it is legal, we should prefer poison values over
undef values.

This replaces undef with poison for aggregate construction and
for uninhabited types. There are more places where we can likely
use poison, but I wanted to stay conservative to start with.

In particular the aggregate case is important for newer LLVM
versions, which are not able to handle an undef base value during
early optimization due to poison-propagation concerns.
This commit is contained in:
Nikita Popov 2023-03-16 14:56:02 +01:00
parent cd6c574af3
commit 30331828cb
9 changed files with 17 additions and 6 deletions
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4
compiler/rustc_codegen_ssa/src/mir/operand.rs
View file

@ -60,7 +60,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
) -> OperandRef<'tcx, V> {
assert!(layout.is_zst());
OperandRef {
val: OperandValue::Immediate(bx.const_undef(bx.immediate_backend_type(layout))),
val: OperandValue::Immediate(bx.const_poison(bx.immediate_backend_type(layout))),
layout,
}
}
@ -145,7 +145,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
let llty = bx.cx().backend_type(self.layout);
debug!("Operand::immediate_or_packed_pair: packing {:?} into {:?}", self, llty);
// Reconstruct the immediate aggregate.
let mut llpair = bx.cx().const_undef(llty);
let mut llpair = bx.cx().const_poison(llty);
let imm_a = bx.from_immediate(a);
let imm_b = bx.from_immediate(b);
llpair = bx.insert_value(llpair, imm_a, 0);

2
compiler/rustc_codegen_ssa/src/mir/place.rs
View file

@ -214,7 +214,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
let cast_to_size = cast_to_layout.layout.size();
let cast_to = bx.cx().immediate_backend_type(cast_to_layout);
if self.layout.abi.is_uninhabited() {
return bx.cx().const_undef(cast_to);
return bx.cx().const_poison(cast_to);
}
let (tag_scalar, tag_encoding, tag_field) = match self.layout.variants {
Variants::Single { index } => {

2
compiler/rustc_codegen_ssa/src/mir/rvalue.rs
View file

@ -295,7 +295,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
assert!(bx.cx().is_backend_immediate(cast));
let ll_t_out = bx.cx().immediate_backend_type(cast);
if operand.layout.abi.is_uninhabited() {
let val = OperandValue::Immediate(bx.cx().const_undef(ll_t_out));
let val = OperandValue::Immediate(bx.cx().const_poison(ll_t_out));
return OperandRef { val, layout: cast };
}
let r_t_in =

1
compiler/rustc_codegen_ssa/src/traits/consts.rs
View file

@ -8,6 +8,7 @@ pub trait ConstMethods<'tcx>: BackendTypes {
// Constant constructors
fn const_null(&self, t: Self::Type) -> Self::Value;
fn const_undef(&self, t: Self::Type) -> Self::Value;
fn const_poison(&self, t: Self::Type) -> Self::Value;
fn const_int(&self, t: Self::Type, i: i64) -> Self::Value;
fn const_uint(&self, t: Self::Type, i: u64) -> Self::Value;
fn const_uint_big(&self, t: Self::Type, u: u128) -> Self::Value;