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rustc_codegen_ssa: Fix for codegen_get_discr

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When doing the optimized implementation of getting the discriminant, the
arithmetic needs to be done in the tag type so wrapping behavior works
correctly.

Fixes #104519
This commit is contained in:
Michael Benfield 2022-11-17 15:03:02 +00:00
parent fd3bfb3551
commit 31c0645b9d
3 changed files with 58 additions and 16 deletions
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31
compiler/rustc_codegen_ssa/src/mir/place.rs
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@ -309,14 +309,14 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
// In the algorithm above, we can change
// cast(relative_tag) + niche_variants.start()
// into
// cast(tag) + (niche_variants.start() - niche_start)
// cast(tag + (niche_variants.start() - niche_start))
// if either the casted type is no larger than the original
// type, or if the niche values are contiguous (in either the
// signed or unsigned sense).
let can_incr_after_cast = cast_smaller || niches_ule || niches_sle;
let can_incr = cast_smaller || niches_ule || niches_sle;
let data_for_boundary_niche = || -> Option<(IntPredicate, u128)> {
if !can_incr_after_cast {
if !can_incr {
None
} else if niche_start == low_unsigned {
Some((IntPredicate::IntULE, niche_end))
@ -353,24 +353,33 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
// The algorithm is now this:
// is_niche = tag <= niche_end
// discr = if is_niche {
// cast(tag) + (niche_variants.start() - niche_start)
// cast(tag + (niche_variants.start() - niche_start))
// } else {
// untagged_variant
// }
// (the first line may instead be tag >= niche_start,
// and may be a signed or unsigned comparison)
// The arithmetic must be done before the cast, so we can
// have the correct wrapping behavior. See issue #104519 for
// the consequences of getting this wrong.
let is_niche =
bx.icmp(predicate, tag, bx.cx().const_uint_big(tag_llty, constant));
let cast_tag = if cast_smaller {
bx.intcast(tag, cast_to, false)
} else if niches_ule {
bx.zext(tag, cast_to)
let delta = (niche_variants.start().as_u32() as u128).wrapping_sub(niche_start);
let incr_tag = if delta == 0 {
tag
} else {
bx.sext(tag, cast_to)
bx.add(tag, bx.cx().const_uint_big(tag_llty, delta))
};
let delta = (niche_variants.start().as_u32() as u128).wrapping_sub(niche_start);
(is_niche, cast_tag, delta)
let cast_tag = if cast_smaller {
bx.intcast(incr_tag, cast_to, false)
} else if niches_ule {
bx.zext(incr_tag, cast_to)
} else {
bx.sext(incr_tag, cast_to)
};
(is_niche, cast_tag, 0)
} else {
// The special cases don't apply, so we'll have to go with
// the general algorithm.