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Auto merge of #119675 - cjgillot:set-no-discriminant, r=tmiasko

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Skip threading over no-op SetDiscriminant.

Fixes https://github.com/rust-lang/rust/issues/119674
This commit is contained in:
bors 2024-01-07 15:34:05 +00:00
commit 75c68cfd2b
4 changed files with 150 additions and 2 deletions
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22
compiler/rustc_mir_transform/src/jump_threading.rs
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@ -43,6 +43,7 @@ use rustc_middle::mir::visit::Visitor;
use rustc_middle::mir::*;
use rustc_middle::ty::{self, ScalarInt, Ty, TyCtxt};
use rustc_mir_dataflow::value_analysis::{Map, PlaceIndex, State, TrackElem};
use rustc_target::abi::{TagEncoding, Variants};
use crate::cost_checker::CostChecker;
@ -391,8 +392,25 @@ impl<'tcx, 'a> TOFinder<'tcx, 'a> {
StatementKind::SetDiscriminant { box place, variant_index } => {
let discr_target = self.map.find_discr(place.as_ref())?;
let enum_ty = place.ty(self.body, self.tcx).ty;
let discr = discriminant_for_variant(enum_ty, *variant_index)?;
self.process_operand(bb, discr_target, &discr, state)?;
// `SetDiscriminant` may be a no-op if the assigned variant is the untagged variant
// of a niche encoding. If we cannot ensure that we write to the discriminant, do
// nothing.
let enum_layout = self.tcx.layout_of(self.param_env.and(enum_ty)).ok()?;
let writes_discriminant = match enum_layout.variants {
Variants::Single { index } => {
assert_eq!(index, *variant_index);
true
}
Variants::Multiple { tag_encoding: TagEncoding::Direct, .. } => true,
Variants::Multiple {
tag_encoding: TagEncoding::Niche { untagged_variant, .. },
..
} => *variant_index != untagged_variant,
};
if writes_discriminant {
let discr = discriminant_for_variant(enum_ty, *variant_index)?;
self.process_operand(bb, discr_target, &discr, state)?;
}
}
// If we expect `lhs ?= true`, we have an opportunity if we assume `lhs == true`.
StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(