bjoernager/rust
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Tell LLVM about impossible niche tags

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This commit is contained in:
Scott McMurray 2025-03-28 20:50:28 -07:00
parent c2110769cd
commit 1f06a6a252
3 changed files with 465 additions and 19 deletions
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28
compiler/rustc_codegen_ssa/src/mir/operand.rs
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@ -9,6 +9,7 @@ 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 rustc_session::config::OptLevel;
use tracing::{debug, instrument};
use super::place::{PlaceRef, PlaceValue};
@ -496,6 +497,18 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
_ => (tag_imm, bx.cx().immediate_backend_type(tag_op.layout)),
};
// Layout ensures that we only get here for cases where the discriminant
// value and the variant index match, since that's all `Niche` can encode.
// But for emphasis and debugging, let's double-check one anyway.
debug_assert_eq!(
self.layout
.ty
.discriminant_for_variant(bx.tcx(), untagged_variant)
.unwrap()
.val,
u128::from(untagged_variant.as_u32()),
);
let relative_max = niche_variants.end().as_u32() - niche_variants.start().as_u32();
// We have a subrange `niche_start..=niche_end` inside `range`.
@ -537,6 +550,21 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
relative_discr,
bx.cx().const_uint(tag_llty, relative_max as u64),
);
// Thanks to parameter attributes and load metadata, LLVM already knows
// the general valid range of the tag. It's possible, though, for there
// to be an impossible value *in the middle*, which those ranges don't
// communicate, so it's worth an `assume` to let the optimizer know.
if niche_variants.contains(&untagged_variant)
&& bx.cx().sess().opts.optimize != OptLevel::No
{
let impossible =
u64::from(untagged_variant.as_u32() - niche_variants.start().as_u32());
let impossible = bx.cx().const_uint(tag_llty, impossible);
let ne = bx.icmp(IntPredicate::IntNE, relative_discr, impossible);
bx.assume(ne);
}
(is_niche, cast_tag, niche_variants.start().as_u32() as u128)
};